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bees: work around btrfs fsync bug

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btrfs provides a flush on rename when the rename target exists, so the
fsync is not necessary.  In the initialization case (when the rename
target does not exist and the implicit flush does not occur), the file
may be empty or a hole after a crash.  Bees treats this case the same
as if the file did not exist.  Since this condition occurs for only the
first 15 minutes of the lifetime of a bees installation, it's not worth
bothering to fix.

If we attempt to fsync the file ourselves, on a crash with log replay,
btrfs will end up with a directory entry pointing to a non-existent inode.
This directory entry cannot be deleted or renamed except by deleting
the entire subvol.  On large filesystems this bug is triggered by nearly
every crash (verified on kernels up to 4.5.7).

Remove the fsync to avoid the btrfs bug, and accept the failure mode
that occurs in the first 15 minutes after a bees install.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell 2016-12-27 14:10:25 -05:00
parent c1e31004b6
commit 6e7137f282
1 changed files with 5 additions and 0 deletions
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5
src/bees.cc
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@ -394,8 +394,13 @@ BeesStringFile::write(string contents)
Fd ofd = openat_or_die(m_dir_fd, tmpname, FLAGS_CREATE_FILE, S_IRUSR | S_IWUSR);
BEESNOTE("writing " << tmpname << " in " << name_fd(m_dir_fd));
write_or_die(ofd, contents);
#if 0
// This triggers too many btrfs bugs. I wish I was kidding.
// Forget snapshots, balance, compression, and dedup:
// the system call you have to fear on btrfs is fsync().
BEESNOTE("fsyncing " << tmpname << " in " << name_fd(m_dir_fd));
DIE_IF_NON_ZERO(fsync(ofd));
#endif
}
BEESNOTE("renaming " << tmpname << " to " << m_name << " in FD " << name_fd(m_dir_fd));
BEESTRACE("renaming " << tmpname << " to " << m_name << " in FD " << name_fd(m_dir_fd));