Re: Adding more drives/saturating the bandwidth

[Goswin von Brederlow <goswin-v-b@web.de>]

Richard Scobie <richard@sauce.co.nz> writes:

> Goswin von Brederlow wrote:
>
>>
>> Now think about the same with 6 disk raid5. Suddenly you have partial
>> stripes. And the alignment on stripe boundaries is gone too. So now
>> you need to read 384k (I think) of data, compute or delta (whichever
>> requires less reads) the parity and write back 384k in 4 out of 6
>> cases and read 64k and write back 320k otherwise. So on average you
>> read 277.33k and write 362.66k (= 640k combined). That is twice the
>> previous bandwidth not to mention the delay for reading.
>>
>> So by adding a drive your throughput is suddenly halfed. Reading in
>> degraded mode suffers a slowdown too. CPU goes up too.
>>
>>
>> The performance of a raid is so much dependent on its access pattern
>> that imho one can not talk about a general case. But note that the
>> more drives you have the bigger a stripe becomes and you need larger
>> sequential writes to avoid reads.
>
> I take your point, but don't filesystems like XFS and ext4 play nice
> in this scenario by combining multiple sub-stripe writes into stripe
> sized writes out to disk?
>
> Regards,
>
> Richard

Some FS have a parameter to tune to the stripe size. If that actually
helps or not I leave for you to test.

But ask yourself: Have any a tool to retune after you've grown the raid?

MfG
        Goswin