Re: Creating new raid5 or 6 with --assume-clean

On Sun Dec 01, 2013 at 10:07:07PM -0500, Wakko Warner wrote:

This is a general question, but I'll give a specific.

If I create an array with assume clean and proceed to write to the device,
what happens with the parity?

EX:
mdadm -C /dev/md0 -n 4 -l 6 -e 1.0 -b internal -c 128 --assume-clean \
	sdc2 sdd2 sdf2 sde2

Then:
dd if=/dev/zero of=/dev/md0 bs=1M

When it finishes, will all the parity blocks be correct for raid 5 and
raid6?

Would changing md/stripe_cache_size up/down make a difference to the
parities?

Doing that specifically should result in all parity being correct, yes.
Providing the writes are not going to a single disk in a stripe (in
which case read-modify-write may be used to update the parity rather
than a full stripe read and parity recalculation) then any write should
result in the parity for that stripe being written correctly. I think
RMW is only used in RAID5 currently though.

Changing the stripe cache size won't affect the parity calculation at
all (except in terms of performance/throughput).

Regards,
    Robin
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