[clug] forget RAID?

[Michael Cohen]

Dale,
   The parity calculations are actually not significant on modern CPUs - most
   software raid implementations are actually much faster than most hardware
   implementations (which offload the parity to the hardware). 

   Raid 1 is a mirrored configuration which means that on reading consecutive
   blocks - you can actually read one block from each disk before waiting for
   the read to complete - hence raid 1 gives 2x increase on read speed.

   Writing on the other hand is still 1x because you still need to write both
   blocks to both speed - hence why raid 1 is great for reading - not so good
   for writing.

   Raid 5 OTOH is less good for reading because for a 3 disk investment you
   only get 2x speed reads (only 2 sectors can be done simultaneously). But
   writes are not quite so bad because you can start writing 2 disks at once
   and then write the parity a little later (so 1.5x speed increase).

   Michael.
  
On Wed, Feb 21, 2007 at 05:23:07PM +1100, Dale Shaw wrote:
> Hi Michael,
> 
> Just a minor point:
> 
> On 2/21/07, Michael Cohen <michael.cohen at netspeed.com.au> wrote:
> >For reading raid 1 is great, not so good for writing.
> 
> I think you'll find it's RAID-5 that suffers from heavy write
> activity. There are no parity calculations for RAID-1.
> 
> cheers,
> Dale
>