Asdo <asdo@shiftmail.org> writes:

quoted text
> 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.

quoted text
> 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.

quoted text
>> 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
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