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

	"... 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?

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       "... 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.

-- 
Drew

"Nothing in life is to be feared. It is only to be understood."
--Marie Curie
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2009/8/23 Drew [off-list ref]:

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.

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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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2009/8/23 NeilBrown [off-list ref]:
[...]

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.

	I see. Thank you for the answer. My assumption was due thinking that
parity is never being used when RAID5 is in "healthy" mode.
	
	But anyway, please clear it up: if I've made --asume-clean RAID5
array, formatted it with some FS (or even LVM), and put OS/data to it,
then (in idle at night, for e. g.) run "resync", would that be the
same valid result (overall data integrity) as if I had previously
created it without "--asume-clean" at all? (Let's suppose hard disks
were healthy in both cases :-)

	And Neil, using the chance to have your reply :-) I'd like to ask one
more question: is that true (I think and hope it is!) that MD doesn't
read/write full chunk size when it's just to read/write another block
of data and hence chunk size only affects data interleaving on disks.
	
	P. S. Is there someone on this list who can share his linux-RAID
archive in mbox format? From time to time I find interesting
discussions have been going but can't just hit "Reply". If you can
help me with that, please reply personally (not CC:ing to the list).
Thanks!

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