Re: best arrangement of 3 disks for (insert) performance - Dell

TDyson(at)sybex(dot)com ("Thom Dyson") writes:
> The Dell PERC controllers have a very strong reputation for terrible
> performance. If you search the archives of the Dell Linux Power
> Edge list (dell.com/linux) you will find many, many people who get
> better performance from software RAID, rather than the hw RAID on
> the PERC. Having said that, the 3/SC might be one of the better
> PERC controllers. I would spend and hour or two and benchmark hw
> vs. sw before I committed to either one.

I can't agree with that.

1. If you search the archives for messages dated a couple of years
ago, you can find lots of messages indicating terrible performance.

Drivers are not cast in concrete; there has been a LOT of change to
them since then.

2. The second MAJOR merit to hardware RAID is the ability to hot-swap
drives. Software RAID doesn't help with that at all.

3. The _immense_ performance improvement that can be gotten out of
these controllers comes from having fsync() turn into a near no-op
since changes can be committed to the 128K battery-backed cache REALLY
QUICKLY.

That is something you should avoid doing with software RAID in any
case where you actually care about your data.

That third part is where Big Wins come. It is the very same sort of
"big win from cacheing" that we saw, years ago, when we improved
system performance _immensely_ by adding a mere 16 bytes of cache by
buying serial controller cards with cacheing UUARTs. It is akin to
the way SCSI controllers got pretty big performance improvements by
adding 256 bytes of tagged command cache.
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Christopher Browne
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