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Re: Help: very slow software RAID 5.



On Tue, 2007-09-18 at 18:26 +0200, Test wrote:
> This off course is very logical...
> 
> Raid5 writes to all 3 disks at about the same time plus it has to write 
> the crc/verification data which also causes some overhead.
> 
> so the average speed = 55+71+75 / 3 = 67...
> 
> So your speed measurement is correct...

I wouldn't have expected such poor performance for *reads*, which is
what the OP complained about specifically. Even the web link you
provided below states:

        "The read performance of RAID 5 is almost as good as RAID 0 for
        the same number of disks. Except for the parity blocks, the
        distribution of data over the drives follows the same pattern as
        RAID 0."

So RAID5 should, presumably, be able to split the reads over multiple
disks and achieve much better than disk-average performance when
reading.

> Check out the difference from your raid0 config...
> 
> Raid0 writes to all disks simultaneously (so if you write 100mb it is 3
> x 33,3mb on each disk)
> 
> If you add more disks your array does not necessarily have to become
> faster because of the overhead needed to be calculated...
> 
> 
> http://en.wikipedia.org/wiki/Standard_RAID_levels



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