Huge performance difference of the command find with and without using %M option to show permissions





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3















On my CentOS 7.6, I have created a folder (called many_files) with 3,000,000 files, by running:



for i in {1..3000000}; do echo $i>$i; done;


I am using the command find to write the information about files in this directory into a file. This works surprisingly fast:



$ time find many_files -printf '%i %y %pn'>info_file

real 0m6.970s
user 0m3.812s
sys 0m0.904s


Now if I add %M to get the permissions:



$ time find many_files -printf '%i %y %M %pn'>info_file

real 2m30.677s
user 0m5.148s
sys 0m37.338s


The command takes much longer. This is very surprising to me, since in a C program we can use struct stat to get inode and permission information of a file and in the kernel the struct inode saves both these information.



My Questions:




  1. What causes this behavior?

  2. Is there a faster way to get file permissions for so many files?










share|improve this question

























  • The second question is the wrong question to ask. The real question is what you are doing with the output. If you are piping it somewhere for later processing of files based on the permissions, then you are probably doing it in a roundabout way. Instead you may want to use -perm with find to pick out the files with the permissions you're looking for.

    – Kusalananda
    4 hours ago




















3















On my CentOS 7.6, I have created a folder (called many_files) with 3,000,000 files, by running:



for i in {1..3000000}; do echo $i>$i; done;


I am using the command find to write the information about files in this directory into a file. This works surprisingly fast:



$ time find many_files -printf '%i %y %pn'>info_file

real 0m6.970s
user 0m3.812s
sys 0m0.904s


Now if I add %M to get the permissions:



$ time find many_files -printf '%i %y %M %pn'>info_file

real 2m30.677s
user 0m5.148s
sys 0m37.338s


The command takes much longer. This is very surprising to me, since in a C program we can use struct stat to get inode and permission information of a file and in the kernel the struct inode saves both these information.



My Questions:




  1. What causes this behavior?

  2. Is there a faster way to get file permissions for so many files?










share|improve this question

























  • The second question is the wrong question to ask. The real question is what you are doing with the output. If you are piping it somewhere for later processing of files based on the permissions, then you are probably doing it in a roundabout way. Instead you may want to use -perm with find to pick out the files with the permissions you're looking for.

    – Kusalananda
    4 hours ago
















3












3








3








On my CentOS 7.6, I have created a folder (called many_files) with 3,000,000 files, by running:



for i in {1..3000000}; do echo $i>$i; done;


I am using the command find to write the information about files in this directory into a file. This works surprisingly fast:



$ time find many_files -printf '%i %y %pn'>info_file

real 0m6.970s
user 0m3.812s
sys 0m0.904s


Now if I add %M to get the permissions:



$ time find many_files -printf '%i %y %M %pn'>info_file

real 2m30.677s
user 0m5.148s
sys 0m37.338s


The command takes much longer. This is very surprising to me, since in a C program we can use struct stat to get inode and permission information of a file and in the kernel the struct inode saves both these information.



My Questions:




  1. What causes this behavior?

  2. Is there a faster way to get file permissions for so many files?










share|improve this question
















On my CentOS 7.6, I have created a folder (called many_files) with 3,000,000 files, by running:



for i in {1..3000000}; do echo $i>$i; done;


I am using the command find to write the information about files in this directory into a file. This works surprisingly fast:



$ time find many_files -printf '%i %y %pn'>info_file

real 0m6.970s
user 0m3.812s
sys 0m0.904s


Now if I add %M to get the permissions:



$ time find many_files -printf '%i %y %M %pn'>info_file

real 2m30.677s
user 0m5.148s
sys 0m37.338s


The command takes much longer. This is very surprising to me, since in a C program we can use struct stat to get inode and permission information of a file and in the kernel the struct inode saves both these information.



My Questions:




  1. What causes this behavior?

  2. Is there a faster way to get file permissions for so many files?







linux files permissions find performance






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 2 hours ago









Jeff Schaller

45k1164147




45k1164147










asked 4 hours ago









BahramBahram

212




212













  • The second question is the wrong question to ask. The real question is what you are doing with the output. If you are piping it somewhere for later processing of files based on the permissions, then you are probably doing it in a roundabout way. Instead you may want to use -perm with find to pick out the files with the permissions you're looking for.

    – Kusalananda
    4 hours ago





















  • The second question is the wrong question to ask. The real question is what you are doing with the output. If you are piping it somewhere for later processing of files based on the permissions, then you are probably doing it in a roundabout way. Instead you may want to use -perm with find to pick out the files with the permissions you're looking for.

    – Kusalananda
    4 hours ago



















The second question is the wrong question to ask. The real question is what you are doing with the output. If you are piping it somewhere for later processing of files based on the permissions, then you are probably doing it in a roundabout way. Instead you may want to use -perm with find to pick out the files with the permissions you're looking for.

– Kusalananda
4 hours ago







The second question is the wrong question to ask. The real question is what you are doing with the output. If you are piping it somewhere for later processing of files based on the permissions, then you are probably doing it in a roundabout way. Instead you may want to use -perm with find to pick out the files with the permissions you're looking for.

– Kusalananda
4 hours ago












2 Answers
2






active

oldest

votes


















4














The first version requires only to readdir(3)/getdents(2) the directory, when run on a filesystem supporting this feature (ext4: filetype feature displayed with tune2fs -l /dev/xxx, xfs: ftype=1 displayed with xfs_info /mount/point ...).



The second version in addition also requires to stat(2) each file, requiring an additional inode lookup, and thus more seeks on the filesystem and device, possibly quite slower if it's a rotating disk and cache wasn't kept. This stat is not required when looking only for name, inode and filetype because the directory entry is enough:




  The linux_dirent structure is declared as follows:

struct linux_dirent {
unsigned long d_ino; /* Inode number */
unsigned long d_off; /* Offset to next linux_dirent */
unsigned short d_reclen; /* Length of this linux_dirent */
char d_name; /* Filename (null-terminated) */
/* length is actually (d_reclen - 2 -
offsetof(struct linux_dirent, d_name)) */
/*
char pad; // Zero padding byte
char d_type; // File type (only since Linux
// 2.6.4); offset is (d_reclen - 1)
*/
}



the same informations are available to readdir(3):




struct dirent {
ino_t d_ino; /* Inode number */
off_t d_off; /* Not an offset; see below */
unsigned short d_reclen; /* Length of this record */
unsigned char d_type; /* Type of file; not supported
by all filesystem types */
char d_name[256]; /* Null-terminated filename */
};



Suspected but confirmed by comparing (on a smaller sample...) the two outputs of:



strace -o v1 find many_files -printf '%i %y %pn'>info_file
strace -o v2 find many_files -printf '%i %y %M %pn'>info_file


Which on my Linux amd64 kernel 5.0.x just shows as main difference:



[...]



 getdents(4, /* 0 entries */, 32768)     = 0
close(4) = 0
fcntl(5, F_DUPFD_CLOEXEC, 0) = 4
-write(1, "25499894 d many_filesn25502410 f"..., 4096) = 4096
-write(1, "iles/844n25502253 f many_files/8"..., 4096) = 4096
-write(1, "096 f many_files/686n25502095 f "..., 4096) = 4096
-write(1, "es/529n25501938 f many_files/528"..., 4096) = 4096
-write(1, "1 f many_files/371n25501780 f ma"..., 4096) = 4096
-write(1, "/214n25497527 f many_files/213n2"..., 4096) = 4096
-brk(0x55b29a933000) = 0x55b29a933000
+newfstatat(5, "1000", {st_mode=S_IFREG|0644, st_size=5, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "999", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "998", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "997", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "996", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "995", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "994", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "993", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "992", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "991", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "990", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0


[...]



+newfstatat(5, "891", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+write(1, "25499894 d drwxr-xr-x many_files"..., 4096) = 4096
+newfstatat(5, "890", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0


[...]






share|improve this answer


























  • Unfortunately, the d_type field of a dir entry is a non-standard feature, only present on Linux and BSD, as mentioned in the readdir(3) manpage. (Though on Linux it is implemented on most filesystems that matter).

    – mosvy
    2 hours ago













  • @mosvy That's ok, the question is tagged CentOS. But yes I understand that on other *nix, results may differ

    – A.B
    2 hours ago













  • Hum actually xfs (CentOS' default) support isn't quite clear...

    – A.B
    2 hours ago











  • added how to check if the filetype feature is present on xfs, in case xfs is in use.

    – A.B
    2 hours ago











  • I think it's supported on xfs -- when I was making a testcase for a glibc glob(3) that only triggered when the d_type field was absent, I had to use either minixfs or use the GLOB_ALTDIRFUNC.

    – mosvy
    2 hours ago



















0














For your 1st question:



I think your problem is not with how quickly the information is accessed, but the output bottleneck.



You are writing the output to info_file.



When you add %M to the find command, you are now outputting more text due to the permissions. 10 additional characters per line of output. That is 30,000,000 more characters.



This is more data that has to go through the STDOUT redirect to info_file and get written to disk. More data to push == longer time to write and complete.



In a situation with a single file or a small number of files, it would not be noticeable to a human; time may give you some variation to measure but it might be too slight to notice.



In your question you are working with 3,000,000 files, so obviously it takes longer to write out the permissions output.



2nd question



I have no idea. Do you have a practical use case for needing to collect permissions for 3,000,000 files, or is this an academic exercise?






share|improve this answer





















  • 1





    info_file has size 94M after the first command and 125M after the second one. An extra 31M shouldn't cause the command to run 20 times slower!

    – Bahram
    4 hours ago






  • 2





    ... it would be easy to test whether this is the case, by replacing %M with a fixed string like -rw-rw-r--

    – steeldriver
    3 hours ago











  • I don't think it's a matter of raw "disk space", I think it has to do with processing each line of output with an extra 10 characters. Depending on exactly what is coming out, that could mean an increase of 30% or more, per line.

    – 0xSheepdog
    3 hours ago












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2 Answers
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active

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2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









4














The first version requires only to readdir(3)/getdents(2) the directory, when run on a filesystem supporting this feature (ext4: filetype feature displayed with tune2fs -l /dev/xxx, xfs: ftype=1 displayed with xfs_info /mount/point ...).



The second version in addition also requires to stat(2) each file, requiring an additional inode lookup, and thus more seeks on the filesystem and device, possibly quite slower if it's a rotating disk and cache wasn't kept. This stat is not required when looking only for name, inode and filetype because the directory entry is enough:




  The linux_dirent structure is declared as follows:

struct linux_dirent {
unsigned long d_ino; /* Inode number */
unsigned long d_off; /* Offset to next linux_dirent */
unsigned short d_reclen; /* Length of this linux_dirent */
char d_name; /* Filename (null-terminated) */
/* length is actually (d_reclen - 2 -
offsetof(struct linux_dirent, d_name)) */
/*
char pad; // Zero padding byte
char d_type; // File type (only since Linux
// 2.6.4); offset is (d_reclen - 1)
*/
}



the same informations are available to readdir(3):




struct dirent {
ino_t d_ino; /* Inode number */
off_t d_off; /* Not an offset; see below */
unsigned short d_reclen; /* Length of this record */
unsigned char d_type; /* Type of file; not supported
by all filesystem types */
char d_name[256]; /* Null-terminated filename */
};



Suspected but confirmed by comparing (on a smaller sample...) the two outputs of:



strace -o v1 find many_files -printf '%i %y %pn'>info_file
strace -o v2 find many_files -printf '%i %y %M %pn'>info_file


Which on my Linux amd64 kernel 5.0.x just shows as main difference:



[...]



 getdents(4, /* 0 entries */, 32768)     = 0
close(4) = 0
fcntl(5, F_DUPFD_CLOEXEC, 0) = 4
-write(1, "25499894 d many_filesn25502410 f"..., 4096) = 4096
-write(1, "iles/844n25502253 f many_files/8"..., 4096) = 4096
-write(1, "096 f many_files/686n25502095 f "..., 4096) = 4096
-write(1, "es/529n25501938 f many_files/528"..., 4096) = 4096
-write(1, "1 f many_files/371n25501780 f ma"..., 4096) = 4096
-write(1, "/214n25497527 f many_files/213n2"..., 4096) = 4096
-brk(0x55b29a933000) = 0x55b29a933000
+newfstatat(5, "1000", {st_mode=S_IFREG|0644, st_size=5, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "999", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "998", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "997", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "996", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "995", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "994", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "993", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "992", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "991", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "990", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0


[...]



+newfstatat(5, "891", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+write(1, "25499894 d drwxr-xr-x many_files"..., 4096) = 4096
+newfstatat(5, "890", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0


[...]






share|improve this answer


























  • Unfortunately, the d_type field of a dir entry is a non-standard feature, only present on Linux and BSD, as mentioned in the readdir(3) manpage. (Though on Linux it is implemented on most filesystems that matter).

    – mosvy
    2 hours ago













  • @mosvy That's ok, the question is tagged CentOS. But yes I understand that on other *nix, results may differ

    – A.B
    2 hours ago













  • Hum actually xfs (CentOS' default) support isn't quite clear...

    – A.B
    2 hours ago











  • added how to check if the filetype feature is present on xfs, in case xfs is in use.

    – A.B
    2 hours ago











  • I think it's supported on xfs -- when I was making a testcase for a glibc glob(3) that only triggered when the d_type field was absent, I had to use either minixfs or use the GLOB_ALTDIRFUNC.

    – mosvy
    2 hours ago
















4














The first version requires only to readdir(3)/getdents(2) the directory, when run on a filesystem supporting this feature (ext4: filetype feature displayed with tune2fs -l /dev/xxx, xfs: ftype=1 displayed with xfs_info /mount/point ...).



The second version in addition also requires to stat(2) each file, requiring an additional inode lookup, and thus more seeks on the filesystem and device, possibly quite slower if it's a rotating disk and cache wasn't kept. This stat is not required when looking only for name, inode and filetype because the directory entry is enough:




  The linux_dirent structure is declared as follows:

struct linux_dirent {
unsigned long d_ino; /* Inode number */
unsigned long d_off; /* Offset to next linux_dirent */
unsigned short d_reclen; /* Length of this linux_dirent */
char d_name; /* Filename (null-terminated) */
/* length is actually (d_reclen - 2 -
offsetof(struct linux_dirent, d_name)) */
/*
char pad; // Zero padding byte
char d_type; // File type (only since Linux
// 2.6.4); offset is (d_reclen - 1)
*/
}



the same informations are available to readdir(3):




struct dirent {
ino_t d_ino; /* Inode number */
off_t d_off; /* Not an offset; see below */
unsigned short d_reclen; /* Length of this record */
unsigned char d_type; /* Type of file; not supported
by all filesystem types */
char d_name[256]; /* Null-terminated filename */
};



Suspected but confirmed by comparing (on a smaller sample...) the two outputs of:



strace -o v1 find many_files -printf '%i %y %pn'>info_file
strace -o v2 find many_files -printf '%i %y %M %pn'>info_file


Which on my Linux amd64 kernel 5.0.x just shows as main difference:



[...]



 getdents(4, /* 0 entries */, 32768)     = 0
close(4) = 0
fcntl(5, F_DUPFD_CLOEXEC, 0) = 4
-write(1, "25499894 d many_filesn25502410 f"..., 4096) = 4096
-write(1, "iles/844n25502253 f many_files/8"..., 4096) = 4096
-write(1, "096 f many_files/686n25502095 f "..., 4096) = 4096
-write(1, "es/529n25501938 f many_files/528"..., 4096) = 4096
-write(1, "1 f many_files/371n25501780 f ma"..., 4096) = 4096
-write(1, "/214n25497527 f many_files/213n2"..., 4096) = 4096
-brk(0x55b29a933000) = 0x55b29a933000
+newfstatat(5, "1000", {st_mode=S_IFREG|0644, st_size=5, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "999", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "998", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "997", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "996", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "995", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "994", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "993", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "992", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "991", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "990", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0


[...]



+newfstatat(5, "891", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+write(1, "25499894 d drwxr-xr-x many_files"..., 4096) = 4096
+newfstatat(5, "890", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0


[...]






share|improve this answer


























  • Unfortunately, the d_type field of a dir entry is a non-standard feature, only present on Linux and BSD, as mentioned in the readdir(3) manpage. (Though on Linux it is implemented on most filesystems that matter).

    – mosvy
    2 hours ago













  • @mosvy That's ok, the question is tagged CentOS. But yes I understand that on other *nix, results may differ

    – A.B
    2 hours ago













  • Hum actually xfs (CentOS' default) support isn't quite clear...

    – A.B
    2 hours ago











  • added how to check if the filetype feature is present on xfs, in case xfs is in use.

    – A.B
    2 hours ago











  • I think it's supported on xfs -- when I was making a testcase for a glibc glob(3) that only triggered when the d_type field was absent, I had to use either minixfs or use the GLOB_ALTDIRFUNC.

    – mosvy
    2 hours ago














4












4








4







The first version requires only to readdir(3)/getdents(2) the directory, when run on a filesystem supporting this feature (ext4: filetype feature displayed with tune2fs -l /dev/xxx, xfs: ftype=1 displayed with xfs_info /mount/point ...).



The second version in addition also requires to stat(2) each file, requiring an additional inode lookup, and thus more seeks on the filesystem and device, possibly quite slower if it's a rotating disk and cache wasn't kept. This stat is not required when looking only for name, inode and filetype because the directory entry is enough:




  The linux_dirent structure is declared as follows:

struct linux_dirent {
unsigned long d_ino; /* Inode number */
unsigned long d_off; /* Offset to next linux_dirent */
unsigned short d_reclen; /* Length of this linux_dirent */
char d_name; /* Filename (null-terminated) */
/* length is actually (d_reclen - 2 -
offsetof(struct linux_dirent, d_name)) */
/*
char pad; // Zero padding byte
char d_type; // File type (only since Linux
// 2.6.4); offset is (d_reclen - 1)
*/
}



the same informations are available to readdir(3):




struct dirent {
ino_t d_ino; /* Inode number */
off_t d_off; /* Not an offset; see below */
unsigned short d_reclen; /* Length of this record */
unsigned char d_type; /* Type of file; not supported
by all filesystem types */
char d_name[256]; /* Null-terminated filename */
};



Suspected but confirmed by comparing (on a smaller sample...) the two outputs of:



strace -o v1 find many_files -printf '%i %y %pn'>info_file
strace -o v2 find many_files -printf '%i %y %M %pn'>info_file


Which on my Linux amd64 kernel 5.0.x just shows as main difference:



[...]



 getdents(4, /* 0 entries */, 32768)     = 0
close(4) = 0
fcntl(5, F_DUPFD_CLOEXEC, 0) = 4
-write(1, "25499894 d many_filesn25502410 f"..., 4096) = 4096
-write(1, "iles/844n25502253 f many_files/8"..., 4096) = 4096
-write(1, "096 f many_files/686n25502095 f "..., 4096) = 4096
-write(1, "es/529n25501938 f many_files/528"..., 4096) = 4096
-write(1, "1 f many_files/371n25501780 f ma"..., 4096) = 4096
-write(1, "/214n25497527 f many_files/213n2"..., 4096) = 4096
-brk(0x55b29a933000) = 0x55b29a933000
+newfstatat(5, "1000", {st_mode=S_IFREG|0644, st_size=5, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "999", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "998", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "997", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "996", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "995", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "994", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "993", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "992", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "991", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "990", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0


[...]



+newfstatat(5, "891", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+write(1, "25499894 d drwxr-xr-x many_files"..., 4096) = 4096
+newfstatat(5, "890", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0


[...]






share|improve this answer















The first version requires only to readdir(3)/getdents(2) the directory, when run on a filesystem supporting this feature (ext4: filetype feature displayed with tune2fs -l /dev/xxx, xfs: ftype=1 displayed with xfs_info /mount/point ...).



The second version in addition also requires to stat(2) each file, requiring an additional inode lookup, and thus more seeks on the filesystem and device, possibly quite slower if it's a rotating disk and cache wasn't kept. This stat is not required when looking only for name, inode and filetype because the directory entry is enough:




  The linux_dirent structure is declared as follows:

struct linux_dirent {
unsigned long d_ino; /* Inode number */
unsigned long d_off; /* Offset to next linux_dirent */
unsigned short d_reclen; /* Length of this linux_dirent */
char d_name; /* Filename (null-terminated) */
/* length is actually (d_reclen - 2 -
offsetof(struct linux_dirent, d_name)) */
/*
char pad; // Zero padding byte
char d_type; // File type (only since Linux
// 2.6.4); offset is (d_reclen - 1)
*/
}



the same informations are available to readdir(3):




struct dirent {
ino_t d_ino; /* Inode number */
off_t d_off; /* Not an offset; see below */
unsigned short d_reclen; /* Length of this record */
unsigned char d_type; /* Type of file; not supported
by all filesystem types */
char d_name[256]; /* Null-terminated filename */
};



Suspected but confirmed by comparing (on a smaller sample...) the two outputs of:



strace -o v1 find many_files -printf '%i %y %pn'>info_file
strace -o v2 find many_files -printf '%i %y %M %pn'>info_file


Which on my Linux amd64 kernel 5.0.x just shows as main difference:



[...]



 getdents(4, /* 0 entries */, 32768)     = 0
close(4) = 0
fcntl(5, F_DUPFD_CLOEXEC, 0) = 4
-write(1, "25499894 d many_filesn25502410 f"..., 4096) = 4096
-write(1, "iles/844n25502253 f many_files/8"..., 4096) = 4096
-write(1, "096 f many_files/686n25502095 f "..., 4096) = 4096
-write(1, "es/529n25501938 f many_files/528"..., 4096) = 4096
-write(1, "1 f many_files/371n25501780 f ma"..., 4096) = 4096
-write(1, "/214n25497527 f many_files/213n2"..., 4096) = 4096
-brk(0x55b29a933000) = 0x55b29a933000
+newfstatat(5, "1000", {st_mode=S_IFREG|0644, st_size=5, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "999", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "998", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "997", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "996", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "995", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "994", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "993", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "992", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "991", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+newfstatat(5, "990", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0


[...]



+newfstatat(5, "891", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0
+write(1, "25499894 d drwxr-xr-x many_files"..., 4096) = 4096
+newfstatat(5, "890", {st_mode=S_IFREG|0644, st_size=4, ...}, AT_SYMLINK_NOFOLLOW) = 0


[...]







share|improve this answer














share|improve this answer



share|improve this answer








edited 2 hours ago

























answered 3 hours ago









A.BA.B

6,00711030




6,00711030













  • Unfortunately, the d_type field of a dir entry is a non-standard feature, only present on Linux and BSD, as mentioned in the readdir(3) manpage. (Though on Linux it is implemented on most filesystems that matter).

    – mosvy
    2 hours ago













  • @mosvy That's ok, the question is tagged CentOS. But yes I understand that on other *nix, results may differ

    – A.B
    2 hours ago













  • Hum actually xfs (CentOS' default) support isn't quite clear...

    – A.B
    2 hours ago











  • added how to check if the filetype feature is present on xfs, in case xfs is in use.

    – A.B
    2 hours ago











  • I think it's supported on xfs -- when I was making a testcase for a glibc glob(3) that only triggered when the d_type field was absent, I had to use either minixfs or use the GLOB_ALTDIRFUNC.

    – mosvy
    2 hours ago



















  • Unfortunately, the d_type field of a dir entry is a non-standard feature, only present on Linux and BSD, as mentioned in the readdir(3) manpage. (Though on Linux it is implemented on most filesystems that matter).

    – mosvy
    2 hours ago













  • @mosvy That's ok, the question is tagged CentOS. But yes I understand that on other *nix, results may differ

    – A.B
    2 hours ago













  • Hum actually xfs (CentOS' default) support isn't quite clear...

    – A.B
    2 hours ago











  • added how to check if the filetype feature is present on xfs, in case xfs is in use.

    – A.B
    2 hours ago











  • I think it's supported on xfs -- when I was making a testcase for a glibc glob(3) that only triggered when the d_type field was absent, I had to use either minixfs or use the GLOB_ALTDIRFUNC.

    – mosvy
    2 hours ago

















Unfortunately, the d_type field of a dir entry is a non-standard feature, only present on Linux and BSD, as mentioned in the readdir(3) manpage. (Though on Linux it is implemented on most filesystems that matter).

– mosvy
2 hours ago







Unfortunately, the d_type field of a dir entry is a non-standard feature, only present on Linux and BSD, as mentioned in the readdir(3) manpage. (Though on Linux it is implemented on most filesystems that matter).

– mosvy
2 hours ago















@mosvy That's ok, the question is tagged CentOS. But yes I understand that on other *nix, results may differ

– A.B
2 hours ago







@mosvy That's ok, the question is tagged CentOS. But yes I understand that on other *nix, results may differ

– A.B
2 hours ago















Hum actually xfs (CentOS' default) support isn't quite clear...

– A.B
2 hours ago





Hum actually xfs (CentOS' default) support isn't quite clear...

– A.B
2 hours ago













added how to check if the filetype feature is present on xfs, in case xfs is in use.

– A.B
2 hours ago





added how to check if the filetype feature is present on xfs, in case xfs is in use.

– A.B
2 hours ago













I think it's supported on xfs -- when I was making a testcase for a glibc glob(3) that only triggered when the d_type field was absent, I had to use either minixfs or use the GLOB_ALTDIRFUNC.

– mosvy
2 hours ago





I think it's supported on xfs -- when I was making a testcase for a glibc glob(3) that only triggered when the d_type field was absent, I had to use either minixfs or use the GLOB_ALTDIRFUNC.

– mosvy
2 hours ago













0














For your 1st question:



I think your problem is not with how quickly the information is accessed, but the output bottleneck.



You are writing the output to info_file.



When you add %M to the find command, you are now outputting more text due to the permissions. 10 additional characters per line of output. That is 30,000,000 more characters.



This is more data that has to go through the STDOUT redirect to info_file and get written to disk. More data to push == longer time to write and complete.



In a situation with a single file or a small number of files, it would not be noticeable to a human; time may give you some variation to measure but it might be too slight to notice.



In your question you are working with 3,000,000 files, so obviously it takes longer to write out the permissions output.



2nd question



I have no idea. Do you have a practical use case for needing to collect permissions for 3,000,000 files, or is this an academic exercise?






share|improve this answer





















  • 1





    info_file has size 94M after the first command and 125M after the second one. An extra 31M shouldn't cause the command to run 20 times slower!

    – Bahram
    4 hours ago






  • 2





    ... it would be easy to test whether this is the case, by replacing %M with a fixed string like -rw-rw-r--

    – steeldriver
    3 hours ago











  • I don't think it's a matter of raw "disk space", I think it has to do with processing each line of output with an extra 10 characters. Depending on exactly what is coming out, that could mean an increase of 30% or more, per line.

    – 0xSheepdog
    3 hours ago
















0














For your 1st question:



I think your problem is not with how quickly the information is accessed, but the output bottleneck.



You are writing the output to info_file.



When you add %M to the find command, you are now outputting more text due to the permissions. 10 additional characters per line of output. That is 30,000,000 more characters.



This is more data that has to go through the STDOUT redirect to info_file and get written to disk. More data to push == longer time to write and complete.



In a situation with a single file or a small number of files, it would not be noticeable to a human; time may give you some variation to measure but it might be too slight to notice.



In your question you are working with 3,000,000 files, so obviously it takes longer to write out the permissions output.



2nd question



I have no idea. Do you have a practical use case for needing to collect permissions for 3,000,000 files, or is this an academic exercise?






share|improve this answer





















  • 1





    info_file has size 94M after the first command and 125M after the second one. An extra 31M shouldn't cause the command to run 20 times slower!

    – Bahram
    4 hours ago






  • 2





    ... it would be easy to test whether this is the case, by replacing %M with a fixed string like -rw-rw-r--

    – steeldriver
    3 hours ago











  • I don't think it's a matter of raw "disk space", I think it has to do with processing each line of output with an extra 10 characters. Depending on exactly what is coming out, that could mean an increase of 30% or more, per line.

    – 0xSheepdog
    3 hours ago














0












0








0







For your 1st question:



I think your problem is not with how quickly the information is accessed, but the output bottleneck.



You are writing the output to info_file.



When you add %M to the find command, you are now outputting more text due to the permissions. 10 additional characters per line of output. That is 30,000,000 more characters.



This is more data that has to go through the STDOUT redirect to info_file and get written to disk. More data to push == longer time to write and complete.



In a situation with a single file or a small number of files, it would not be noticeable to a human; time may give you some variation to measure but it might be too slight to notice.



In your question you are working with 3,000,000 files, so obviously it takes longer to write out the permissions output.



2nd question



I have no idea. Do you have a practical use case for needing to collect permissions for 3,000,000 files, or is this an academic exercise?






share|improve this answer















For your 1st question:



I think your problem is not with how quickly the information is accessed, but the output bottleneck.



You are writing the output to info_file.



When you add %M to the find command, you are now outputting more text due to the permissions. 10 additional characters per line of output. That is 30,000,000 more characters.



This is more data that has to go through the STDOUT redirect to info_file and get written to disk. More data to push == longer time to write and complete.



In a situation with a single file or a small number of files, it would not be noticeable to a human; time may give you some variation to measure but it might be too slight to notice.



In your question you are working with 3,000,000 files, so obviously it takes longer to write out the permissions output.



2nd question



I have no idea. Do you have a practical use case for needing to collect permissions for 3,000,000 files, or is this an academic exercise?







share|improve this answer














share|improve this answer



share|improve this answer








edited 4 hours ago

























answered 4 hours ago









0xSheepdog0xSheepdog

1,72511024




1,72511024








  • 1





    info_file has size 94M after the first command and 125M after the second one. An extra 31M shouldn't cause the command to run 20 times slower!

    – Bahram
    4 hours ago






  • 2





    ... it would be easy to test whether this is the case, by replacing %M with a fixed string like -rw-rw-r--

    – steeldriver
    3 hours ago











  • I don't think it's a matter of raw "disk space", I think it has to do with processing each line of output with an extra 10 characters. Depending on exactly what is coming out, that could mean an increase of 30% or more, per line.

    – 0xSheepdog
    3 hours ago














  • 1





    info_file has size 94M after the first command and 125M after the second one. An extra 31M shouldn't cause the command to run 20 times slower!

    – Bahram
    4 hours ago






  • 2





    ... it would be easy to test whether this is the case, by replacing %M with a fixed string like -rw-rw-r--

    – steeldriver
    3 hours ago











  • I don't think it's a matter of raw "disk space", I think it has to do with processing each line of output with an extra 10 characters. Depending on exactly what is coming out, that could mean an increase of 30% or more, per line.

    – 0xSheepdog
    3 hours ago








1




1





info_file has size 94M after the first command and 125M after the second one. An extra 31M shouldn't cause the command to run 20 times slower!

– Bahram
4 hours ago





info_file has size 94M after the first command and 125M after the second one. An extra 31M shouldn't cause the command to run 20 times slower!

– Bahram
4 hours ago




2




2





... it would be easy to test whether this is the case, by replacing %M with a fixed string like -rw-rw-r--

– steeldriver
3 hours ago





... it would be easy to test whether this is the case, by replacing %M with a fixed string like -rw-rw-r--

– steeldriver
3 hours ago













I don't think it's a matter of raw "disk space", I think it has to do with processing each line of output with an extra 10 characters. Depending on exactly what is coming out, that could mean an increase of 30% or more, per line.

– 0xSheepdog
3 hours ago





I don't think it's a matter of raw "disk space", I think it has to do with processing each line of output with an extra 10 characters. Depending on exactly what is coming out, that could mean an increase of 30% or more, per line.

– 0xSheepdog
3 hours ago


















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