[ILUG] Dell PERCs and Linux

[Paul Jakma]

On Thu, 15 Jun 2000, David Murphy wrote:

> No, you forget:
> 1) If the disks are all on one SCSI bus, you have to issue the
> requests sequentially - and wait for 3 writes to be acknowledged,
> instead of 2.
> 

let's see. each chunk won't be more than 128kb/64kb, and the drive
will have at least 512kb of cache. so that can be go straight into
cache, and the drive can disconnect, controller can feed another
drive, the original drive can post the write later. 

> 2) If you have three SCSI controllers, one disk on each, then you
> still have to copy the data to two controllers instead of three.
> 

same for RAID1...

> 3) No matter what, you have to calculate the RAID5 parity info before
> you write it.
> 

ok.. i am assuming that the RAID card's CPU is fast enough, that this
is not a factor. Which is true for most newer cards, (eg ExtremeRAID
has a StrongARM @ 100MHz+) and for linux software RAID.

> > no, the 3disk RAID5 has double the read throughput of a 2 disk
> > RAID1.
> 
> How do you work that one out?

by observation. bonnie doesn't lie.

-- 
Paul Jakma	paul at clubi.ie
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