NFS Help! Terrible performance with sync, fast performance with async

I've got a problem that I've spent quite a bit of time on, though I'm
not an expert at NFS. In summary, operations that require meta-data
changes (such as file/directory creations/deletions), perform extremely
slow over sync, but over 10x faster using async.

I have two systems, connected to a GigE switch using intel pro 1000 NICs
(jumbo frames is currently not enabled on any of the points). 

The NFS server is a dual-core opteron system with 1GB of RAM and 3x300
SAS disk RAID-5 on a Perc5/i controller with 256MB battery backed cache
(write cache is enabled). The file system is ext3. I've configured nfsd
to spawn 32 processes upon startup. I'm using defaults for export the
nfs shares, no changes to rsize or wsize.

The NFS client is a dual Xeon with 4GB of RAM and a single 7200rpm SATA
disk. Both systems are running RHEL WS 3 Update 8 and kernel
2.4.21-47.0.1.ELsmp. 

For testing, I'm using bonnie++. The following are some sample test
results that sum up the problem:

Test on NFS server directly (not NFS loopback)
-Sequential File Creation: 2976
-Sequential File Deletion: N/A
-Random File Creation: 3077
-Random File Deletion: 9922

NFS test with sync enabled
-Sequential File Creation: 39
-Sequential File Deletion: 79
-Random File Creation: 39
-Random File Delection: 65

NFS test with async enabled
-Sequential File Creation: 575
-Sequential File Deletion: 1718
-Random File Creation: 543
-Random File Deletion: 1228

Based on the local performance of the NFS server, it does not appear the
IO setup is the culprit. My understanding of the sync operation is a
commit happens which means the NFS server doesn't reply back until the
change has actually been committed to stable storage. There is something
happening behind the scenes though which is causing a huge delay before
the NFS server replies back the commit was complete.

This question is actually work related and I'm planning to put the NFS
server into production, but I'd rather not use async, even with a UPS
and dual PSU's on the server. With the newer nfs-utils, sync is the
default option as well so it seems like sync should perform relatively
well.

Another question is I don't quite understand how the data corruption
happens if a power loss occurs on an NFS server using async. Even with
sync, data transferred over the wire maybe loss if the nfs server gets
shut down before that data is committed. Can anyone go into more detail
on how the data corruption happens?

Thanks a bunch!

 

 

Thanks,

Chris Wornell
Network Administrator, Information Technology
Peerless Systems Corporation
http://www.peerless.com <http://www.peerless.com/> 
office: 310.727.5723
fax: 310.727.5715
mailto:cwornell-KWjJJj/cdYZWk0Htik3J/[email protected] <mailto:cwornell-KWjJJj/cdYZWk0Htik3J/[email protected]>

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