RAID array

Posted Mar 19, 2008 21:51 UTC (Wed) by Cato (guest, #7643)
In reply to: RAID array by clugstj
Parent article: On the vger.kernel.org outage

RAID is nowhere near as reliable in practice as it appears on paper - one example is that you
frequently find, after a single disk (A) has failed, that another disk (B) 'fails during
recovery' - in reality, some sectors on disk B failed first but on a sector that did not get
read or written, so the bad sectors are only picked up during RAID recovery, which entails
reading the whole of disk B and others.  

See http://www.nber.org/sys-admin/linux-nas-raid.html for coverage of some of these issues and
how NetApp and others would cover them.  Personally I would only use ZFS, NetApp or something
similar that does a lot of media scrubbing, sector checksumming etc, in addition to basic
RAID.

RAID array

Posted Mar 19, 2008 22:20 UTC (Wed) by quotemstr (subscriber, #45331) [Link] (6 responses)

We use 3ware controllers at work, which can be set up to automatically and periodically verify
that no sectors ave failed. That feature has already saved us once.

RAID array

Posted Mar 19, 2008 23:38 UTC (Wed) by daniel (guest, #3181) [Link] (1 responses)

"Personally I would only use ZFS, NetApp or something similar that does a lot of media
scrubbing, sector checksumming etc, in addition to basic RAID."

Would that not be an admission of defeat for Linux?

RAID array

Posted Mar 20, 2008 22:29 UTC (Thu) by Cato (guest, #7643) [Link]

Yes, it means I could only use Linux currently for storage if I went for ZFS/FUSE.  This is a
significant weakness in the Linux story - perhaps btrfs will go some way to fixing this, but
overall it seems that Linux RAID and associated sector-checksumming filesystems are a long way
behind where it needs to be.  

I don't think this stuff is particularly hard to do, but it does require some focused work on
btrfs and on RAID improvements, and there are patent risks due to the Sun/NetApp patent
lawsuits, of course.

RAID array

Posted Mar 20, 2008 6:46 UTC (Thu) by pbrutsch (guest, #4987) [Link] (3 responses)

That is a very common feature, and most certaily not limited to 3ware!

I have Areca, Adaptec, and LSI Logic RAID cards that can all do the same.

RAID array

Posted Mar 20, 2008 12:23 UTC (Thu) by wurtel (guest, #7155) [Link] (1 responses)

Linux software raid (md) can also check an array for problems, Debian offers to run a cronjob
every month to do that, if you install mdadm. Coupled with SMART autotests you're pretty safe.

RAID array

Posted Mar 20, 2008 20:20 UTC (Thu) by hmh (subscriber, #3838) [Link]

If you want to be pretty safe, you'll need to test every *day*, not once a month.  I know this
from experience.

RAID array

Posted Mar 20, 2008 22:31 UTC (Thu) by Cato (guest, #7643) [Link]

This sounds like a useful media scrubbing feature.  However, sector checksumming seems like it
requires filesystem changes, and I've only seen that proposed in btrfs, which is in early
development stages.