building a RAID system

Jakob Oestergaard jakob at unthought.net
Fri Oct 10 07:56:39 EDT 2003


On Fri, Oct 10, 2003 at 10:34:41AM -0700, pesch at attglobal.net wrote:
> You write:
> 
> "The problem with offloading is, that while it made great sense in the
> days of 1 MHz CPUs, it really doesn't make a noticable difference in the
> load on your typical N GHz processor."
> 
> Did you have a maximum data storage size in mind? - or to put it differently: at what data size do you see the
> practical limit of SW RAID?

In this forum, I run small storage only. Around 150G for the most busy
server that I have.

Linux has problems with >2TB devices as far as I know, so that sort of
puts an upper limit to whatever you can do with SW/HW RAID there.

In between, it's just one order of magnitude  :)

More seriously - the SW RAID code is extremely simple, and it performs
two different tasks:
*) Reconstruction - which has time complexity T(n) for n bytes of data
*) Read/write - which has time complexity T(1) for n bytes of data

In other words - the more data you have, the longer a resync is going to
take - HW or SW makes no difference (except for a factor, which tends to
be rediculously large on cheap HW RAID cards but acceptable on more
expensive ones).  Reads and writes are not affected by the amount of
data, in the SW RAID layer (and hopefully not in the HW RAID layer
either).

The scalability limits you will run into are:
*) Number of disks you can attach to your box (HW RAID may hide this
   from you and may thus buy you some scalability there)
*) Filesystem limits/performance problems. HW/SW RAID makes no difference
*) Device size limits. HW/SW RAID makes no difference
*) Reconstruction time after unclean shutdown - SW performs much better
   than crap/cheap HW solutions, but I don't know about the expensive
   ones.

There are others on this list with much larger servers and less antique
hardware - guys, speak up - where does it begin to hurt?    :)

-- 
................................................................
:   jakob at 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}...............:
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