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.
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
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
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.