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xattr atomicy

To: xfs@xxxxxxxxxxx
Subject: xattr atomicy
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Fri, 13 Dec 2013 03:56:44 -0800
Delivered-to: xfs@xxxxxxxxxxx
User-agent: Mutt/1.5.21 (2010-09-15)
On the nfsv4 list it was recently discussed how atomic / transaction
xattr updates are.  It turns out none of that seems documented on the
syscall level, but for XFS we have an odd inconsistancy in that attr
updates generally are atomic and logged, except when we go out to
remote attributes in xfs_attr_rmtval_set, in which case attr updates
are no logged, and we do synchronous writes instead.

Besides the weird semantic difference that is impossible to explain to
users performance will also generally be bad with a synchronous buffer
write.  Is there any good reason to not log the buffer for the remote
attributes? Given that attribute are limited to 64kB it's not like
the value is larger than large directory blocks that we already
support.

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