Subject: Re: kern/30674: RAIDframe should be able to create volumes without parity rewrite
To: None <,,>
From: Matthias Scheler <>
List: netbsd-bugs
Date: 07/06/2005 15:48:03
The following reply was made to PR kern/30674; it has been noted by GNATS.

From: Matthias Scheler <>
To: Greg Oster <>
Subject: Re: kern/30674: RAIDframe should be able to create volumes without parity rewrite
Date: Wed, 6 Jul 2005 16:47:30 +0100

 On Wed, Jul 06, 2005 at 09:19:19AM -0600, Greg Oster wrote:
 > Let's address the RAID 1 case first:
 > If you're just going to build a FFS on it, then one can get away with 
 > marking the parity as "good" because data will never be read until 
 > after it has been written.  Fine.
 > If the machine crashes or otherwise goes down without marking the
 > parity as "good", then you are back to square one -- you *HAVE* to
 > do the parity rebuild at that point,
 That is actually another disadvantage of RAIDframe. SVM doesn't manage
 "parity good" by a single bit. It uses a database which manages it
 on per "SVM meta cluster" base. The result is that Solaris only needs
 to sync a few MBs after a crash and not the whole volume.
 > There is, however, also a violation of the Principle of Least Astonishment.
 I don't ask for this being turned on by default. Solaris doesn't manul page
 doesn't even recomment. But it is nice to have that option if you know what
 your are doing.
 > If, for example, the components had random data on them before the 
 > RAID 1 set was created, and one does two "dd if=/dev/rraid0d | md5" 
 > with the parity marked as "good" (but not actually synced!) then one
 > might well yield different results.
 That is a very artifical case. The *really* interesting information is
 the checksum of the filesystem data on the RAID volume. And that will
 always match even if the mirror was created without an initial
 parity rewrite.
 > Let's now look at the RAID 5 case:
 I already guessed that it is different for RAID 5. So we can just leave
 that case out of the discussion.
 > I've heard the argument a couple of times, but I don't see it buying 
 > anything other than removing one parity rebuild...
 Which might save you hours of waiting and/or slow system performance.
 	Kind regards
 Matthias Scheler