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Re: [Gluster-devel] Operation reordering and data corruption

From: Anand Avati
Subject: Re: [Gluster-devel] Operation reordering and data corruption
Date: Thu, 8 Sep 2011 23:14:16 +0530

On Thu, Sep 8, 2011 at 6:34 PM, Emmanuel Dreyfus <address@hidden> wrote:

I experience a nasty data corruption bug that I can track down to a bad
interraction between how the NetBSD kernel resize files and an operation
reordering ability glusterfs seems to have.

Consider the exchanges below that I logged at the FUSE level:

>> SETATTR size = 12178 -> 32394
>> WRITE size = 12178
<< WRITE size = 12178 -> 32394
>> GETATTR size = 32394
<< GETATTR size = 32394 -> 12178
<< SETATTR size = 32394 -> 32394

SETATTR si sent before WRITE, but WRITE completes first. Then GETATTR
kicks in and it seems SETATTR is not yet completed, since the old size
is obtained.

This is a real problem, because the NetBSD kernel will detect a file
shrink in the GETATTR vnode method, and it will truncate the file to
match the new size. The result is that the WRITE that was just done is
discarded, and data is replaced by a chunk of zeroes.

In what conditions glusterfs performs operation reordering? Is it a bug
or a feature? Linux does not truncate in the same situation?

GlusterFS does guarantee order of completing operation between two system calls if the second system call starts _after_ the return of the first system call. When two system calls are concurrently in progress there is no expected order of completion. There is no such order of completion in NFS either. Linux's expectation of ordering of system calls is this as well. From what you describe, it looks like NetBSD would have to implement synchronization in its VFS just the way Linux does (and to live within the expectations set by NFS and FUSE like filesystems).


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