[OT] Re: [CentOS] Centos 4.2 and Boot/Root on RAID? -- avoid blanket statements on RAID hardware

[Bryan J. Smith]

Les Mikesell <lesmikesell at gmail.com> wrote:
> Even if they permit drives to move between different models
> it still means you have to have a spare compatible one
> handy if your primary box dies.

Not always.  There are ways to map various hardware RAID-0
and 10 block-striped volumes in Linux MD or LVM (via
DeviceMapper), including 3Ware.  RAID-1 is no issue at all,
it's just a mirror.

RAID-5 is the only one that differs greatly.

> You can mount one partition out of an md RAID1 set directly
> as the underlying partition, bypassing any concern about
> compatibility of md versions if you need to recover data.
> LVM is a different story.

MD hasn't always been that good, but only in more recent
kernels.  I've been using 3Ware since late kernel 2.0.

> Nothing against the 3Ware cards - I agree they are very
> good, although you did forget to mention the various bugs
> they have had and fixed over those 6+ years.

Bugs have been limited to RAID-5 limitations.

First off, they should have never implemented RAID-5 on the
Escalade 6000.  It's ASIC was never designed for it.

Secondly, I have repeatedly stated that the ASIC+SRAM
approach  in even the 7000/8000 series is for non-blocking
I/O, and not good for a buffering/XOR operation like RAID-5
writes.  It's fine if you largely just read from the RAID-5
volume, but tanks on RAID-5 writes (although it's can be far
better than software RAID when rebuilding).

Lastly, I have _never_ been a proponent of the Escalade
9500S, and recomended people stick with the 8506 and use
RAID-10.  Their recent introduction of the Escalade 9550SX
series which adds an embedded PowerPC tells me that the ASIC
design, even with DRAM added (in the 9500S), would never be a
good performer for RAID-5 writes.  But I'm hopeful we'll see
great things out of the 9550SX series.

Until someone shows me an application that RAID-5 is faster
than RAID-10, I will stick with RAID-10.  With ATA drives as
cheap as they are, getting a few extra GBs is not worth the
write performance gains of RAID-10.  And RAID-10 load
balances reads better than RAID-5.


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