RE: [gentoo-server] RAID

It depends.  I'd like to see SW RAID keep up with dual 2GB fibre
attached to an IBM FAStT600.  Then again, I haven't been given a budget
to test this with a Linux server.  :)

Also remember which RAID you're talking about.  If you need parity, see
my previous post about BAARF.

Finally, saying the CPU is not usually the bottleneck in a database
server is misleading at best.  Being mainly an Oracle DBA these days,
faster CPUs can *cause* CPU contention.  Like I said before, "it
depends".  And like everything else, YMMV.

Rich

-----Original Message-----
From: Sean Cook [mailto:scook@×××××.net] 
Sent: Wednesday, February 01, 2006 3:39 PM
To: gentoo-server@l.g.o
Subject: Re: [gentoo-server] RAID


-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

kashani wrote:
> Sean Cook wrote:
>  > Redhat In their manual:
>
>> "With today's fast CPUs, Software RAID performance can excel against
>> Hardware RAID."
>>
>> As I stated before, it depends on where your bottleneck is... if you
>> are not cpu bound, software raid is great! and will boost IO through
>> put on comparable hardware.  If you are already CPU bound, forget
>> software raid, it will degrade your system to a crawl...
>
>
> Badly done tests circa 1998 without any sort of methodology, mention
of
> cluster sizes, etc is proof than any idiot can make a computer slower.
>
>     I'll argue that a fully supported hard raid card is always
superior
> to a software raid by it's very nature, having local I/O cache and a
> dedicated chip. However there are definitely workloads where a
software
> raid is good enough that spending money on a hardware raid card is
> pointless. I can not imagine a case where all things being equal that
> software raid would be measurably faster.
>
>     In the event that removing your RAID card makes your disk 5x
faster
> I'd also recommend removing the admin who setup the original system as
> well. :-)
>
> kashani

My post did say "Back in the Day" and it was around 1999 that we did
this however, then I was running unstable raid tools and they have come
a long way, and I was running 2.2 kernel.

However, the only people I am aware of that say the performance is
better on hardware raid are hardware raid manufacturers and you... You
don't work for LSI do you?  Most of the linux software folks agree with
me or take a mildly more conservative tone.

eg:

Redhat Enterprise Linux (circa 2005-2006)
http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/pdf/rhel-isa
-en.pdf

Often the excess CPU power available for software RAID parity
calculations greatly exceeds the processing power present on a RAID
controller card. Therefore, some software RAID implementations
actually have the capability for higher performance than hardware RAID
implementations.



Mysql:

Hardware Versus Software

Some operating systems can perform software RAID. Rather than buying a
dedicated RAID controller, the operating system's kernel splits the I/O
among multiple disks. Many users shy away from using these features
because they've long been considered slow or buggy.

In reality, software RAID is quite stable and performs rather well. The
performance differences between hardware and software RAID tend not to
be significant until they're under quite a bit of load. For smaller and
medium-sized workloads, there's little discernible difference between
them. Yes, the server's CPU must do a bit more work when using software
RAID, but modern CPUs are so fast that the RAID operations consume a
small fraction of the available CPU time. And, as we stressed earlier,
the CPU is usually not the bottleneck in a database server anyway.

-- 
gentoo-server@g.o mailing list