Re: Adding more drives/saturating the bandwidth

[Richard Scobie <richard@sauce.co.nz>]

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