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

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

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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