On 30/12/2013 19:48, Phillip Susi wrote:
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I believe that using a single "chunk size" causes a lose-lose tradeoff
when creating raid 5/6/10 arrays.
I don't think your analysis is correct.
Firstly you are forgetting that multiple requests are issued
simultaneously to one disk by the kernel, and they can be served
out-of-order via NCQ / TCQ by the disks. The kernel does not wait for
sector N to be read before issuing the read for sector N+1, it issues a
lot of them together since it knows how much data it has to read (via
readahead, most of the times). The disk reorders read/write requests
according to its angular position, so you almost never pay for the
angular offset during a sequential read/write, not even when skipping
redundant data from one component disk of the RAID.
Secondly, for writes, I suspect you are assuming that a whole stipe has
to be read and rewritten in order for one small write to be performed,
but it is not so. For a 4k write in raid5, two 4k sectors are read, then
two 4k sectors are written, and this is completely independent from
chunk size. It already behaves mostly like your "groups", which are the
stripes actually.
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