[ILUG] Dell PERCs and Linux

[David Murphy]

Quoting <Pine.LNX.4.21.0006151944140.22397-100000 at rossi.itg.ie>
by Paul Jakma <paulj at itg.ie>:

> > > but in RAID5 you write different data to each disk...

> > You have an extra column's worth of data to write - the parity
> > info.

> for which you have an extra disk. so forget that argument.

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.

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.

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

> > That's 50% more data for a 3-column RAID5 than for a mirror.

> no, the 3disk RAID5 has double the read throughput of a 2 disk
> RAID1.

How do you work that one out?

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