On Mon, Nov 23, 2009 at 10:31 AM, Alan Johnson <a...@datdec.com> wrote:
> On Mon, Nov 23, 2009 at 9:25 AM, Tom Buskey <t...@buskey.name> 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." > Thanks for clarifying that for me. > > >> 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? > So your Ubuntu does software RAID6 on the boot disks with / and /boot? Or you have a hardware RAID card doing RAID6? > >> 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. > > I'm corrected. RAID 5 will always have to do parity and RAID 0 does not. > 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? > RAIDZ is a modified RAID5. 1 distributed set of parity . Lose 1 disk w/o failing RAIDZ2 is modified RAID6. 2 sets of distributed parity. Lose 2 disks 2/o failing RAIDZ3 is ummm. 3 sets of distributed parity. Lose 3 disks 2/o failing I once replaced my 120 GB drives with 500 GB drives to increase the pool. It didn't seem slow to me, but.. You'll have to google :-/ to get real numbers. I suspect the speed is similar to RAID 5/6 rebuilds >> ... 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. > ZFS will work on top of ISCSI SAN drives. Or you can share out a partion from a ZFS pool as an iSCSI target. zpool create raidz mypool c0t0d0 c0t1d0 c0t2d0 # create a RAIDZ from 3 disks # Create a home with 10GB, max, share it on NFS and compress the data as it comes in zfs create mypool/home zfs set quota=10G mypool/home zfs set compression=on mypool/home zfs set sharenfs=on mypool/home # Another one, but put it on iscsi zfs create mypool/iSC zfs set quota=10G mypool/iSC zfs set compression=on mypool/iSC zfs set shareiscsi=on mypool/iSC They really got the CLI stuff right! > > I'm tempted to try Fuse+ZFS for our database servers, or even just to right > to FreeBSD, but I wouldn't touch *anything* FUSE for production work. Well, I've used NTFS-3G because I had to. > 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. > Get VirtualBox and play with FreeBSD/FreeNAS/Solaris/OpenSolaris inside it.
_______________________________________________ gnhlug-discuss mailing list gnhlug-discuss@mail.gnhlug.org http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss/
