On Mon, Nov 23, 2009 at 9:25 AM, Tom Buskey <[email protected]> wrote:
> I think the RAID 5 write hole refers to the slowdown on writes with RAID > 5. In order to lose data, a 2nd drive needs to fail (as opposed to only 1 > drive on a RAID 0 or JBOD). > According to http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_5_performance: "In the event of a system failure while there are active writes, the parity of a stripe may become inconsistent with the data. If this is not detected and repaired before a disk or block fails, data loss may ensue as incorrect parity will be used to reconstruct the missing block in that stripe. This potential vulnerability is sometimes known as the *write hole*. Battery-backed cache and similar techniques are commonly used to reduce the window of opportunity for this to occur. The same issue occurs for RAID-6." > I think most software RAID only does mirrors for boot. RAID 1, not 5. > I have a Ubuntu 9.10 box that boots a RAID6 with GRUB2. I expect that is very new, eh? > > RAID5 will have faster read performance then RAID 1 or a single disk. It > might be faster for reads then RAID-0 (striping) also. > If the disks are a severe bottle neck, RAID5 can match RAID0 read speeds in theory. However, I've never seen this in practice. RAID5 cannot be faster than RAID0 unless something outside those definitions being at play. ZFS's RAIDZ ...RAIDZ2 ... RAIDZ3 which has 3 parity disks. > I know what you mean, but I'm just nit-picking here for clarification so as not to confuse the uninitiated: party disks are a thing of RAID3. RAID5/6/Z all use distributed parity, so no one disk is dedicated to parities. This is a big part of what makes rebuilds so slow on RAID5/6. The process is not as linear as a mirror or a RAID3 with dedicated parity drive. How does RAIDZ do on a rebuild? > > ... ZFS ... ZFS ... ZFS fanboy and I'm very disappointed it won't be > adopted in Linux due to its license. It's in FreeBSD (and FreeNAS). btrfs > looks like it has some nice improvements so I'm hoping to see it succeed > alongside ZFS. > Weeeee! From all the theory I've read and watched, ZFS is the end game. I'm still trying to figure out how to work it into cloud storage. Does FreeNAS some how enable ZFS over iSCSI? I can't wrap my mind around that, but the benefits of ZFS on the minimal overhead of iSCSI (vs. NFS) would be ideal, if impossible. I'm tempted to try Fuse+ZFS for our database servers, or even just to right to FreeBSD, but that would be a hard sell in my company and I don't even want to try it without some lab work to back it up, which is not in the cards in the near future.
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