It's irrelavent what controller, you still have to actualy write the parity blocks, which slows down your write speed because you have to write n+n/2 blocks. instead of just n blocks making the system write 50% more data. RAID 5 must write 50% more data to disk therefore it will always be slower. Alex. On 12/25/05, Michael Stone <mstone+postgres@xxxxxxxxx> wrote: > On Sat, Dec 24, 2005 at 05:45:20PM -0500, Ron wrote: > >Caches help, and the bigger the cache the better, but once you are > >doing enough writes fast enough (and that doesn't take much even with > >a few GBs of cache) the recalculate-checksums-and-write-new-ones > >overhead will decrease the write speed of real data. Bear in mind > >that the HD's _raw_ write speed hasn't been decreased. Those HD's > >are pounding away as fast as they can for you. Your _effective_ or > >_data level_ write speed is what decreases due to overhead. > > You're overgeneralizing. Assuming a large cache and a sequential write, > there's need be no penalty for raid 5. (For random writes you may > need to read unrelated blocks in order to calculate parity, but for > large sequential writes the parity blocks should all be read from > cache.) A modern cpu can calculate parity for raid 5 on the order of > gigabytes per second, and even crummy embedded processors can do > hundreds of megabytes per second. You may have run into some lousy > implementations, but you should be much more specific about what > hardware you're talking about instead of making sweeping > generalizations. > > >Side Note: people often forget the other big reason to use RAID 10 > >over RAID 5. RAID 5 is always only 2 HD failures from data > >loss. RAID 10 can lose up to 1/2 the HD's in the array w/o data loss > >unless you get unlucky and lose both members of a RAID 1 set. > > IOW, your RAID 10 is only 2 HD failures from data loss also. If that's > an issue you need to go with RAID 6 or add another disk to each mirror. > > Mike Stone > > ---------------------------(end of broadcast)--------------------------- > TIP 6: explain analyze is your friend >
