Hm. That's almost, but not quite, right. Disabling dirdata during the
fsck run has no positive effect - fsck will still get upset about the
incorrectly placed entry. (And whether or not dirdata is enabled, fsck
will do the same thing. It doesn't know or care about the dirdata
setting as such.)
Steps #1 and #2 will not cause any problems until you run fsck, but
there's no way around the issue once you do run fsck. The .. dentry
must go back to the correct location to make fsck happy. If I remember
right, fsck creates the .. dentry and doesn't include the fid
(regardless of dirdata setting). This can overwrite another dentry if
one has been placed in the location normally reserved for the .. dentry
(which can happen if the dentry which was after the .. dentry is
deleted, thereby making a space large enough for a dentry+FID).
Furthermore, if you have a non-Htree directory where the .. dentry is
incorrectly placed (your steps 1 & 2), then you add files until it
shifts to become an HTree directory, THAT directory becomes corrupted in
a more severe manner that will cause your MDT to remount read only
and/or LBUG. (LU-2638 only fixes the .. dentry bug for HTree
directories themselves. It does not help with a corrupted directory
that then becomes an HTree directory.)
- Patrick
On 11/03/2015 11:17 AM, Mohr Jr, Richard Frank (Rick Mohr) wrote:
On Oct 27, 2015, at 1:46 PM, Patrick Farrell <p...@cray.com> wrote:
That's something of a time bomb - If one of those directories fsck wishes it
could correct is small and grows in number of files, you'll get the MDT going
read only (and a few odd LBUGs if you try to put it back).
I was looking back over the incident where I thought I had hit this bug, but
based on the lack of side effects that you mentioned, I am now starting to
think that I was mistaken. Nevertheless, I am trying to understand the bug a
little better in case I am still susceptible to it. I tried to summarize my
understanding below, and maybe you can tell me if I am correct.
For HTree directories, the problem is described in LU-2638. But since I am
running Lustre >2.4, I should not be affected by this bug.
For non-Tree directories, the problem is described in LU-5626. In order to
trigger the bug, the following steps must happen:
1) A non-HTree directory created under Lustre 1.8 (which does not have a FID
for its “..” entry) gets moved to a different parent directory.
2) Lustre tries to update the “..” entry in the directory, and if there is not
enough space in the existing entry, it creates a new “..” entry and adds the
FID.
3) Something happens to the MDT, and fsck needs to be run. When it runs, it
notices that “..” is no longer the second entry in the directory.
4) fsck tries to “fix” the problem by moving the “..” entry back to its
original position. With the FID in place, there is not enough space in the
original position, but fsck moves it anyway which causes the “..” entry to
overwrite part of the third entry in the directory.
If that is correct, then steps #1 and #2 can happen without causing any
problems. It is only at steps #3 and #4 that the corruption occurs, and as
long as dirdata is disabled before fsck is run, then there should not be any
problems.
Is that explanation accurate?
--
Rick Mohr
Senior HPC System Administrator
National Institute for Computational Sciences
http://www.nics.tennessee.edu
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