Re: Two RAID1 mirrors are faster than three

[Jakob Oestergaard <jakob@unthought.net>]

On Thu, May 15, 2003 at 11:51:14AM +0100, Anders Karlsson wrote:
...
> No, it should not cause problems as when you add the split-off copy back
> into the mirror, it is treated as 'stale' and will get synchronised with
> the original. 

Correct

If this was not the case, background resynchronization of standard
2-disk RAID-1 would be a really horrible feature (with half the reads
returning stale data from the new disk)...

> 
> I would be very surprised if the Linux software md driver worked any
> diffrently than this. Perhaps someone that knows it in-depth can add to
> the conversation?

Unless there's bugs in the driver, your description is correct  :)

> 
> With the facilities of LVM 'snapshots' now being available, this
> practice of splitting off one copy from a three-way mirror is perhaps
> becoming redundant, but people will likely take the approach of "if it
> ain't broken, don't fix it" and leave old backup methods as they are.
> So if you work in the sysadm field, you might well come across this
> practice.

The really good argument for N>2 disk RAID-1 is still the seek-time and
multiple-readers performance benefits which you won't be addressing with
LVM snapshots.

-- 
................................................................
:   jakob@unthought.net   : And I see the elder races,         :
:.........................: putrid forms of man                :
:   Jakob Østergaard      : See him rise and claim the earth,  :
:        OZ9ABN           : his downfall is at hand.           :
:.........................:............{Konkhra}...............: