Re: which raid level gives maximum overall speed? (raid-10,f2 vs. raid-0)

["Keld Jørn Simonsen" <keld@dkuug.dk>]

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
> >-
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> >
> 
> 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
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