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question about xfs_fsync on linux

To: xfs@xxxxxxxxxxx
Subject: question about xfs_fsync on linux
From: Chris Torek <chris.torek@xxxxxxxxxxxxx>
Date: Mon, 14 Jul 2008 16:13:40 -0600
Sender: xfs-bounce@xxxxxxxxxxx
The implementation of xfs_fsync() in 2.6.2x, for reasonably late x,
reads as follows in xfs_vnodeops.c:

        error = filemap_fdatawait(vn_to_inode(XFS_ITOV(ip))->i_mapping);

We have a customer who is seeing data not "make it" to disk on a
stress test that involves doing an fsync() or fdatasync() and then
deliberately rebooting the machine (to simulate a failure; note
that the underlying RAID has its own battery backup and this is
just one of many different parts of the stress-test).

Looking into this, I am now wondering if this call should read:

        error = filemap_write_and_wait(vn_to_inode(XFS_ITOV(ip))->i_mapping);

instead.  From a quick skim, it seems as though fdatawait does not
start dirty page pushes, but rather only wait for any that are
currently in progress.  The write-and-wait call starts them first,
which seems more appropriate for an fsync.  I must admit to being
relatively unfamiliar with all the innards of the Linux filemap
code though.


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