Re: xfs: Temporary extra disk space consumption?

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Mar 23, 2022 at 08:21:52PM +0900, Tetsuo Handa wrote:
> Hello.
> 
> I found that running a sample program shown below on xfs filesystem
> results in consuming extra disk space until close() is called.
> Is this expected result?
> 
> I don't care if temporarily consumed extra disk space is trivial. But since
> this amount as of returning from fsync() is as much as amount of written data,
> I worry that there might be some bug.
> 
> ---------- my_write_unlink.c ----------
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <unistd.h>
> 
> int main(int argc, char *argv[])
> {
> 	static char buffer[1048576];
> 	const char *filename = "my_testfile";
> 	const int fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0600);
> 	int i;
> 
> 	if (fd == EOF)
> 		return 1;
> 	printf("Before write().\n");
> 	system("/bin/df -m .");
> 	for (i = 0; i < 1024; i++)
> 		if (write(fd, buffer, sizeof(buffer)) != sizeof(buffer))
> 			return 1;
> 	if (fsync(fd))
> 		return 1;
> 	printf("Before close().\n");

If you run filefrag -v at this point and see blocks mapped into the file
after EOF, then the extra disk space consumption you see is most likely
speculative preallocation for extending writes.

--D

> 	system("/bin/df -m .");
> 	if (close(fd))
> 		return 1;
> 	printf("Before unlink().\n");
> 	system("/bin/df -m .");
> 	if (unlink(filename))
> 		return 1;
> 	printf("After unlink().\n");
> 	system("/bin/df -m .");
> 	return 0;
> }
> ---------- my_write_unlink.c ----------
> 
> ----------
> $ uname -r
> 5.17.0
> $ ./my_write_unlink
> Before write().
> Filesystem     1M-blocks   Used Available Use% Mounted on
> /dev/sda1         255875 130392    125483  51% /
> Before close().
> Filesystem     1M-blocks   Used Available Use% Mounted on
> /dev/sda1         255875 132443    123432  52% /
> Before unlink().
> Filesystem     1M-blocks   Used Available Use% Mounted on
> /dev/sda1         255875 131416    124459  52% /
> After unlink().
> Filesystem     1M-blocks   Used Available Use% Mounted on
> /dev/sda1         255875 130392    125483  51% /
> $ grep sda /proc/mounts
> /dev/sda1 / xfs rw,relatime,attr2,inode64,logbufs=8,logbsize=32k,noquota 0 0
> ----------
> 
> ----------
> $ uname -r
> 4.18.0-365.el8.x86_64
> $ ./my_write_unlink
> Before write().
> Filesystem     1M-blocks  Used Available Use% Mounted on
> /dev/sda1          20469  2743     17727  14% /
> Before close().
> Filesystem     1M-blocks  Used Available Use% Mounted on
> /dev/sda1          20469  4791     15679  24% /
> Before unlink().
> Filesystem     1M-blocks  Used Available Use% Mounted on
> /dev/sda1          20469  3767     16703  19% /
> After unlink().
> Filesystem     1M-blocks  Used Available Use% Mounted on
> /dev/sda1          20469  2743     17727  14% /
> $ grep sda /proc/mounts
> /dev/sda1 / xfs rw,seclabel,relatime,attr2,inode64,logbufs=8,logbsize=32k,noquota 0 0
> ----------
> 
> ----------
> $ uname -r
> 3.10.0-1160.59.1.el7.x86_64
> $ ./my_write_unlink
> Before write().
> Filesystem     1M-blocks  Used Available Use% Mounted on
> /dev/sda1          20469  2310     18160  12% /
> Before close().
> Filesystem     1M-blocks  Used Available Use% Mounted on
> /dev/sda1          20469  4358     16112  22% /
> Before unlink().
> Filesystem     1M-blocks  Used Available Use% Mounted on
> /dev/sda1          20469  3334     17136  17% /
> After unlink().
> Filesystem     1M-blocks  Used Available Use% Mounted on
> /dev/sda1          20469  2310     18160  12% /
> $ grep sda /proc/mounts
> /dev/sda1 / xfs rw,seclabel,relatime,attr2,inode64,noquota 0 0
> ----------