[ILUG] Why RAID

[Niall O Broin]

On Wednesday 14 July 2004, bigbro at skynet.ie (Gareth Eason) wrote:

>RAID is NOT for backup - it's for resilience. I think that's been 
>adequately covered in this discussion.

Yes, it has. If you don't want/need the resilience provided by RAID that's
fine - but don't get upset if others do.

>    It's also for massive storage and high speed access / massive 
>bandwidth. Show me an alternate method of having a massive and 
>dynamically extensible storage platform without RAID4 / WAFL (a la 
>NetApp) or similar? Running out of space? Throw some more disks at it. 
>Problem solved. Without RAID you'd be looking at buying a bigger drive, 
>and dealing with the downtime as you installed it, configured it, copied 
>the data across from the old drive and made it live. Not really an 
>option in many cases.

Extensible filesystems and RAID are different things. With any implementation
of RAID I have used, you do NOT get what you describe here - if you add
another disk, the RAID software (host CPU or dedicated controller) has to
rebuild the RAID layout (striping etc.) across the complete set of drives,
which is a destructive process. If that were somehow possible you still need
to have some means of telling the higher layers that there is more space e.g.
LVM or its successor, EVMS AND you then need a filesystem which can be grown.

( For a nice desription of the various RAID levels, take a look at
http://linux.cudeso.be/raid.php )

The Netapp WAFL is another different thing, though a very wonderful thing, it
must be said, if your budget runs to it. AIUI this does allow exactly what
Gareth described - when you need more space on a filesystem, you just plug in
an extra disk, and this space automagically becomes available, and RAID
protected. This definitely counts as "sufficiently advanced technology" in my
book :-) The curious can find papers by Netapp people describing this - I
really must read them in detail one day.


>Rick ;-)  ) Google will be happy to find you the mathematical 
>probabilities of drive failures as number of drives in an array 
>increases

As a major user of commodity disk drives, Google could also tell you a lot
about their reliability, but I imagine it won't, as it plays its cards very
close to its chest about operational stuff for fear of the 800 pound gorilla.



Niall