On Apr 15, 2008, at 13:18, Bob Friesenhahn wrote:

> ZFS raidz1 and raidz2 are NOT directly equivalent to RAID5 and RAID6
> so the failure statistics would be different.  Regardless, single disk
> failure in a raidz1 substantially increases the risk that something
> won't be recoverable if there is a media failure while rebuilding.
> Since ZFS duplicates its own metadata blocks, it is most likely that
> some user data would be lost but the pool would otherwise recover.  If
> a second disk drive completely fails, then you are toast with raidz1.
>
> RAID5 and RAID6 rebuild the entire disk while raidz1 and raidz2 only
> rebuild existing data blocks so raidz1 and raidz2 are less likely to
> experience media failure if the pool is not full.

While the failure statistics may be different, I think any comparison  
would be "apples-to-apples".

RAID-5/6 would be the "worst case", with ZFS having slightly better  
numbers; it having a slight advantage because of checksuming (to head  
reduce the chance of undetected / unrecoverable read errors) and  
because you're only recreating used blocks instead of all blocks  
(less stress on the disks; shorter rebuild time). There's also the  
consistency advantage (no fsck time; no write hole).

In general you could probably calculate the numbers for RAID-5/6, and  
be fairly comfortable thinking that RAID-Z/2 is better than whatever  
you get. (For some quantity of "better".)
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss

Reply via email to