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
> ----------
