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On 2/12/13 1:25 PM, Jason Lock wrote:
Mail Issues - FreeBSD
Hello, my apologies if this may be the wrong forum but hoping that maybe someone might be able to provide some insight.
Have a very sporadic and strange issue with our mail servers running Dovecot on FreeBSD. There are three servers hosting Dovecot with FreeBSD as the underlying operating system. All three connect to a NAS server, again running FreeBSD and ZFS.
Speaking for NFS on FreeBSD, please note that the FreeBSD NFS client and server generally respects application's sync() operations which could cause delays, if the backing pool is not configured with any fast ZIL device, ideally a SSD. Normally a 'gstat -I 1s' on the NFS server will give you busy disks with small throughput, in this case, a ZIL would be helpful. Before buying a SSD, you may want to try disabling 'sync' on dataset (be careful though, that doing so would increase the chance of data loss in the event of power loss, make sure to revert the change with 'zfs inherit sync' on the datasets once you are done with experiments) and see if that helps, if so, then a SSD ZIL would be helpful; higher end systems can use a Fusion IO or ZeusRAM for that purpose.
Another possible cause is compression, at this time, FreeBSD's ZFS compression thread runs at kernel priority and that would cause the system to do ZFS compression before doing everything else. If you use compression=gzip, you will see stalls more often, for short term, please consider using compression=lzjb or have dovecot to do the compression; for mid term, please use my recent LZ4 port that would consume way less CPU and have better compression ratio than lzjb; for long term, we are going to implement a scheduling policy which is similar to Solaris's System Duty Cycle and have the compatibility shims to emulate that.
Lastly, try increasing your NFS iod numbers. The system default is very conservative and may become a bottleneck when you have a lot of users on one system.
Hope this helps... If you have further questions about FreeBSD please consider posting to freebsd-stable@freebsd.org .
Cheers, -----BEGIN PGP SIGNATURE-----
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