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?
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