On Tue, Feb 05, 2008 at 11:54:27AM -0500, Justin Piszcz wrote: > > > On Tue, 5 Feb 2008, Keld Jørn Simonsen wrote: > > >On Thu, Jan 31, 2008 at 02:55:07AM +0100, Keld Jørn Simonsen wrote: > >>On Wed, Jan 30, 2008 at 11:36:39PM +0100, Janek Kozicki wrote: > >>>Keld Jørn Simonsen said: (by the date of Wed, 30 Jan 2008 23:00:07 > >>>+0100) > >>> > >> > >>All the raid10's will have double time for writing, and raid5 and raid6 > >>will also have double or triple writing times, given that you can do > >>striped writes on the raid0. > > > >For raid5 and raid6 I think this is even worse. My take is that for > >raid5 when you write something, you first read the chunk data involved, > >then you read the parity data, then you xor-subtract the data to be > >changed, and you xor-add the new data, and then write the new data chunk > >and the new parity chunk. In total 2 reads and 2 writes. The read/writes > >happen on the same chunks, so latency is minimized. But in essence it is > >still 4 IO operations, where it is only 2 writes on raid1/raid10, > >that is only half the speed for writing on raid5 compared to raid1/10. > > > >On raid6 this amounts to 6 IO operations, resulting in 1/3 of the > >writing speed of raid1/10. > > > >I note in passing that there is no difference between xor-subtract and > >xor-add. > > > >Also I assume that you can calculate the parities of both raid5 and > >raid6 given the old parities chunks and the old and new data chunk. > >If you have to calculate the new parities by reading all the component > >data chunks this is going to be really expensive, both in IO and CPU. > >For a 10 drive raid5 this would involve reading 9 data chunks, and > >making writes 5 times as expensive as raid1/10. > > > >best regards > >keld > >- > >To unsubscribe from this list: send the line "unsubscribe linux-raid" in > >the body of a message to majordomo@xxxxxxxxxxxxxxx > >More majordomo info at http://vger.kernel.org/majordomo-info.html > > > > On my benchmarks RAID5 gave the best overall speed with 10 raptors, > although I did not play with the various offsets/etc as much as I have > tweaked the RAID5. Could you give some figures? best regards keld - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
