Matt Ingenthron wrote: > Hi all, > > Does anyone have any data to show how ZFS raidz with the on-disk cache > enabled for small, random IOs compares to a raid controller card with > cache in raid 5. > > I'm working on a very competitive RFP, and one thing that could give us > an advantage is the ability to remove this controller card. I've never > measured this or seen it measured-- any pointers would be useful. I > believe the IOs are 8KB, the application is MySQL. >
In general, low cost and fast are mutually exclusive. To get lots of database performance you tend to need lots of disks. Cache effects are secondary. Also, if you need performance, RAID-1 beats RAID-5. Anton B. Rang wrote: > For small random I/O operations I would expect a substantial performance > penalty for ZFS. The reason is that RAID-Z is more akin to RAID-3 than > RAID-5; each read and write operation touches all of the drives. RAID-5 > allows multiple I/O operations to proceed in parallel since each read and > write operation touches only 2 drives. > There are a lot of caveats glossed over here. The main RAID-5 penalty for writes is the read-modify write sequence which is often required, but usually nicely hidden by a RAID controller with nonvolatile cache. For raidz, it is a little more complex because it is possible that a write will only cause 2 iops or a single write (2+ physical writes) may contain many database blocks written sequentially. The net effect of these complexities is that write performance is very, very difficult to predict. Read performance is more consistently predictable, but only for the case where reads are aligned and caches are always missed (which suck anyway). > As always, benchmarking the application is best. :-) > Absolutely! -- richard _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
