i think that a good add to today read balance could be add disk
acess_time (1/rpm, 0 for 'non rotational', 1 for unknow)
head_distance * acess_time, could allow a better select of disks or
mix of 7200/10000/15000 rpm disks
maybe it can be read from disk information at assemble time, i don't
know if disk speed (rpm) can be read from device block (some disks
display information with hdparm and smartctl)
it's not a 1000% improvement, it's 1% at high read/write load

2011/5/4 Keld Jørn Simonsen [off-list ref]:

On Wed, May 04, 2011 at 09:42:40AM +0200, David Brown wrote:

quoted

raid10,far is better for sequential reads - it gives better-than-raid0
performance on average since it will do striped reads from the faster
outer tracks.  And for multi-threaded reads, it should also be a little
faster than other raid10 layouts (and raid1, which is much the same as
raid10,near).  Since it prefers to get the data from the outer half, you
get the benefits of short-stroking your disks - faster transfer speeds
and less head movement.

The cost of raid10,far is greater head movement for writes - but that is
not the OP's main concern.

yes, in theory this is so. But two reasons almost eliminates this in
practice. First, the processes do not wait for completion of the IO of
writes, the processes only deliver the data to the file buffer cache of
the kernel, which then periodically flushes the data to the disk drives.
Second, the flushing of the data is ordered so that the collected data
buffers are written as much sequentially as possible to the drives.
This goes for all Linux MD RAID1/RAID10 layouts. Given that random
writes are random over the whole set of drives, for any mirrored
raid1/raid10 layout, the flushing of the data is about the same.

best regards
keld
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Spadim Technology / SPAEmpresarial
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