Hi Chris-
I noticed some data and metadata getting out of sync on disk, despite
wrapping my writes with btrfs transactions. After digging into it a bit,
it appears to be a larger problem with inode size/data getting written
during a regular commit.
I have a test program append a few bytes at a time to a few different
files, in a loop. I let it run until I see a btrfs transaction commit
(via a printk at the bottom of btrfs_commit_transaction). Then 'reboot -f
-n'. After remounting, all files exist but are 0 bytes, and debug-tree
shows a bunch of empty files. I would expect to see either the sizes when
the commit happend (a few hundred KB in my case), or no files at all;
there was actually no point in time when any of the files were 0 bytes.
Similarly, if I do the same but wait for a few commits to happen, after
remount the file sizes reflect the size from around the next-to-last
commit, not the last commit.
This is probably more information than you need, but my original test was
a bit more complicated, with weirder results. Append to each file, then
write it's size to an xattr on another file. Wrap both operations in a
transaction. Start it up, run 'sync', then reboot -f -n. When I remount
the size and xattr are out of sync by exactly one iteration: the xattr
reflects the size that resulted from _two_ writes back, not the
immediately preceeding write. If anything I would expect to see a larger
actual size than xattr value (for example if the start/end transaction
ioctls weren't working)...
sage
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main(int argc, char **argv)
{
while (1) {
int r, fd, pos, i = rand() % 10;
char a[20];
sprintf(a, "%d.log", i);
fd = open(a, O_CREAT|O_APPEND|O_WRONLY, 0600);
r = write(fd, "foobarfoo\n", 10);
pos = lseek(fd, 0, SEEK_CUR);
printf("write %s = %d, size = %d\n", a, r, pos);
close(fd);
}
}
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