okay, thanks guys ! On Sat, Mar 6, 2010 at 1:02 AM, Goswin von Brederlow <goswin-v-b@xxxxxx> wrote: > Asdo <asdo@xxxxxxxxxxxxx> writes: > >> rj wrote: >>> Hi, >>> I wanted to know if md solves write hole issue for RAID5/RAID6? > > It is unsolvable without a crash persistant journal. Hardware raids have > battery backed cache for that but a fast disk would also work. But no > support for this in linux software raid. > >> No but the filesystem can do that, ext3/4 and XFS in particular, if >> well aligned on the RAID, if you are creating new files and not >> updating old files (i.e. doesn't work for databases). >> For full write hole avoidance you need to wait for Btrfs (and I don't >> know the state of raid-5/6 on btrfs), or use ZFS on Solaris or Freebsd. > > No raid5/6 in btrfs. Needs major restructuring of the on-disk data for > that. > > ZFS on the other hand uses what they call raid-x. Which is a raid5 with > copy-on-write semantic. Any write to a virtual block will write to a new > physical block and update the parity to a new physical block too. Only > once that was written is the stripe atomically changed to the new > physical location. So no hole there. > > There is also a zfs-fuse implementation for linux. > >>> Also, does md support RAID 50 level ? >>> >> Not directly, but you can create that manually by overlaying a raid 0 >> over 1+ raid5's. > > And by layering one raid over others you can get any level you like, > even 14065 if you like (and have enought disks). > > For raid50 I would suggest LVM over raid5 with striping though. Raid 0 > is somewhat pointless when compared with all the extra flexibility LVM > gives you on top of striping. > > MfG > Goswin > -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
