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Long sleep with i_mutex in xfs_flush_device(), affects NFS service

To: xfs@xxxxxxxxxxx, nfs@xxxxxxxxxxxxxxxxxxxxx
Subject: Long sleep with i_mutex in xfs_flush_device(), affects NFS service
From: Stephane Doyon <sdoyon@xxxxxxxxx>
Date: Tue, 26 Sep 2006 14:51:45 -0400 (EDT)
Sender: xfs-bounce@xxxxxxxxxxx

I'm seeing an unpleasant behavior when an XFS file system becomes full, particularly when accessed over NFS. Both XFS and the linux NFS client appear to be contributing to the problem.

When the file system becomes nearly full, we eventually call down to xfs_flush_device(), which sleeps for 0.5seconds, waiting for xfssyncd to do some work.

        xfs_iunlock(ip, XFS_ILOCK_EXCL);
before calling xfs_flush_device(), but i_mutex is still held, at least when we're being called from under xfs_write(). It seems like a fairly long time to hold a mutex. And I wonder whether it's really necessary to keep going through that again and again for every new request after we've hit NOSPC.

In particular this can cause a pileup when several threads are writing concurrently to the same file. Some specialized apps might do that, and nfsd threads do it all the time.

To reproduce locally, on a full file system:
for i in `seq 30`; do
  dd if=/dev/zero of=f bs=1 count=1 &
time that, it takes nearly exactly 15s.

The linux NFS client typically sends bunches of 16 requests, and so if the client is writing a single file, some NFS requests are therefore delayed by up to 8seconds, which is kind of long for NFS.

What's worse, when my linux NFS client writes out a file's pages, it does not react immediately on receiving a NOSPC error. It will remember and report the error later on close(), but it still tries and issues write requests for each page of the file. So even if there isn't a pileup on the i_mutex on the server, the NFS client still waits 0.5s for each 32K (typically) request. So on an NFS client on a gigabit network, on an already full filesystem, if I open and write a 10M file and close() it, it takes 2m40.083s for it to issue all the requests, get an NOSPC for each, and finally have my close() call return ENOSPC. That can stretch to several hours for gigabyte-sized files, which is how I noticed the problem.

I'm not too familiar with the NFS client code, but would it not be possible for it to give up when it encounters NOSPC? Or is there some reason why this wouldn't be desirable?

The rough workaround I have come up with for the problem is to have xfs_flush_space() skip calling xfs_flush_device() if we are within 2secs of having returned ENOSPC. I have verified that this workaround is effective, but I imagine there might be a cleaner solution.


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