[OT] Re: [CentOS] Centos 4.2 and Boot/Root on RAID? -- avoid
blanket statements on RAID hardware
Bryan J. Smith
thebs413 at earthlink.net
Thu Oct 20 23:28:33 UTC 2005
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.
Bryan J. Smith | Sent from Yahoo Mail
mailto:b.j.smith at ieee.org | (please excuse any
http://thebs413.blogspot.com/ | missing headers)
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