The number of spindles gained is offset by the algorythms used to write the parity on the raid 5. On writes the 10 should out perform the 5. Also if a drive is lost there will be a big hit in performance until the drive is replaced in a raid5/6 configuration.
On Wed, Nov 10, 2010 at 4:10 AM, Stan Hoeppner <s...@hardwarefreak.com>wrote: > Rich put forth on 11/10/2010 1:52 AM: > > The only difference I would have on this server is I would make it a 10 > raid > > and not raid5. This is a much more higher performing with all the writes > to > > maildir. Its also better fault tolerance. > > I typically use RAID10 for most high load transaction heavy systems as > well. I rarely recommend it to others, as they usually have trouble > grasping that losing half the space of the disks is a good thing, > regardless of the additional redundancy and performance. :) > > Modern quality caching controllers, either PCI-X/e HBAs or SAN > controllers, with decent parity ASICs and 1GB RAM or more of cache, can > often get RAID5/6 relatively close to RAID10 in IOPs and throughput. > > The OP is currently planing on using a single mirror pair for his mail > store. Anything is going to be better than that. An 8 disk RAID5 will > have about 6-7x the IOPs of his mirror set WRT writes, maybe a little > less WRT reads if his RAID controller intelligently interleaves block > reads. > > An 8 disk RAID10 will give 4 spindles of IOPs compared to 7 spindles for > the RAID5 using the same 8 disks. Assuming the card has a decent parity > ASIC, the write performance should be similar, though it will be lower > for the RAID5. The read performance of the RAID5 will be quite a bit > higher due to the extra 3 spindles and no parity calculations on READ > operations. > > The one thing I really really like about RAID10 is the rebuild time. > Simply mirroring one disk during the rebuild is much faster than any > parity scheme. The only downside is it creates a huge IO hot spot on > the healthy drive of the failed pair. Configurable rebuild priority > helps mitigate this though. Regardless, rebuild times for RAID10 are > typically dramatically lower than RAID5/50/6/60. > > -- > Stan >
