Re: man mdadm regarding --assume-clean says it's OK to use with RAID1 or 10, when creating:

On Sun, August 23, 2009 3:48 am, Igor Podlesny wrote:

2009/8/23 Drew [off-list ref]:

quoted

quoted

       "... It can also be used when creating a RAID1 or RAID10 if
you want
to avoid  the  initial  resync ..."

       I'm wondering why namely those levels are chosen and not
RAID5/6 for e. g.?

       I think it's also safe to use with RAID5/6, isn't it?

RAID1/10 don't have parity calculations whereas RAID5/6 do.

	Well, and what does it change? I mean if we write some data to that
"assume clean" RAID5/6 the parity will be updated anyway.

Yes it will be updated.  But if it is updated with a
read-modify-write cycle then an old incorrect value will be updated
to a new incorrect value, which doesn't help you much.

The current RAID6 implementation never does read-modify-write for
P/Q update so you could get away with --assume-clean there, but I
don't promise that future implementations will never use r-m-w.
RAID5 definitely does use R-M-W for single-block writes on
arrays of 4 or more drives.

NeilBrown

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