Re: Software vs Hardware RAID 10?
Gabor Gombas <gombasg@sztaki.hu> writes:
> On Tue, Aug 28, 2007 at 02:16:52PM +0200, Goswin von Brederlow wrote:
>
>> On the other hand raid6 will make 2 disk failures survivable, any
>> two disks. If you can afford the cpu overhead for software raid6 I
>> would allways prefer raid6 over a raid10 with 4 disks. With 6 disks
>> too where raid6 also has more space and might be faster (at the cost
>> of cpu usage).
>
> IMHO the CPU usage difference is not that important. What is more
> important that now you have to send the same amount of data over the bus
> _3 times_ compared to the 2 times for RAID1/4/5/10. So if you really
> need RAID6 then a good HW RAID card becomes quite attractive.
That is not quite true. The truth is both better and worse.
If you write a single block then you have to read the 2 parity blocks
for the stripe, update them and write 3 blocks. So you have 5 times
the traffic on the bus. For anything doing single block writes this is
quite the killer. I would imagine a database like oracle to just die
with software raid6.
On the other hand anything else will be writing larger chunks of data,
in which case most of the time a complete stripe should get written
causing only <# of disks> writes for <# of disks - 2> data blocks.
> I also fail to see RAID6 can be faster than RAID10, unless your I/O
> pattern happens to interact rather badly with the on-disk layout of
> RAID10.
With 6 disks software raid1 and raid10 on some layouts will only read
from 3 disks. Raid6 on the other hand reads from 4 disks. So you get
an extra boost.
MfG
Goswin
PS: as I said initialy: Do test yourself. All this is highly dependend
on your use case.
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