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

[Robin Hill <robin@robinhill.me.uk>]

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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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    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
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