xfs: improve sync behaviour in the face of aggressive dirtying
The following script from Wu Fengguang shows very bad behaviour in XFS
when aggressively dirtying data during a sync on XFS, with sync times
up to almost 10 times as long as ext4.
A large part of the issue is that XFS writes data out itself two times
in the ->sync_fs method, overriding the livelock protection in the core
writeback code, and another issue is the lock-less xfs_ioend_wait call,
which doesn't prevent new ioend from being queue up while waiting for
the count to reach zero.
This patch removes the XFS-internal sync calls and relies on the VFS
to do it's work just like all other filesystems do. Note that the
i_iocount wait which is rather suboptimal is simply removed here.
We already do it in ->write_inode, which keeps the current supoptimal
behaviour. We'll eventually need to remove that as well, but that's
material for a separate commit.