Re: read-from-all-disks support for RAID1?

From: Jesper Juhl
Date: Sun Sep 11 2005 - 19:22:31 EST


On 9/12/05, Neil Brown <neilb@xxxxxxxxxxxxxxx> wrote:
> On Saturday September 10, buytenh@xxxxxxxxxxxxxx wrote:
> > (please CC on replies)
> >
> > Hi!
> >
> > I recently had a case where one disk in a two-disk RAID1 array went
> > subtly bad, effectively refusing to write to certain sectors without
> > reporting an error. Basically, parts of the disk went undetectably
> > read-only, causing file system corruption that wouldn't go away after
> > fsck, and all kinds of other fun.
>
> That really isn't something that a drive should do. If a write fails,
> you need to be told that it failed. If anything else happens, maybe
> you should consider boycotting that manufacturer, or at least buying
> more expensive drives (do I guess right that there were fairly
> cheap??).
>
>
> >
> > Would it be hard/wise to add an option for RAID1 mode to read from all
> > devices on a read, and report an error to syslog or simply return an
> > I/O error if there is a mismatch? (Or use majority voting and tell
> > people to use 3-disk RAID1 arrays from now on ;-)
> >
>
> No, I don't think so. The overhead would be substantial, so people
> would be very unlikely to use it.

There are situations where data integrity is far more important than speed.
On AIX I usually use the Mirror Write Consistency and Write Verify
options on my mirrored volumes that store data where integrity is more
important than speed.
I guess something like those options would also satisfy Lennert's
needs, but I don't know if it's currently possible with the Linux LVM
or elsewhere.

You can read a bit about the MWC and WV options in AIX at :
http://publib.boulder.ibm.com/infocenter/pseries/index.jsp?topic=/com.ibm.aix.doc/aixbman/prftungd/diskperf2.htm


--
Jesper Juhl <jesper.juhl@xxxxxxxxx>
Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please http://www.expita.com/nomime.html
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/