Re: Cheapest way to acheive RAID5 ?

[David Greaves <david@xxxxxxxxxxxx>]

aashram wrote:

> I have setup a raid5 using slackware and 2 4bay firewire enclosure.
> with software raid5 there is a performance hit. A considerable one.

compared to?
RAID5 is not the choice for performance - but it does offer parallel
throughput and should be better than a single drive.

> Something to consider when using as a video server.
> In fact in documentation I have read raid5 is not recommended
> for large video files, they recommend raid 4.

How many video streams are you serving?
And what quality?
DVD typically runs at 8000kb/s - avi's often run as low as 1mb/s
I'd not expect *any* modern disk subsystem to have problems handling
that kind of throughput ;)
(If, OTOH, you want to serve video from a cable company head end then
yes, you should reconsider...)

raid5, IMHO, is a good choice for video storage - it protects against
annoying hardware failure but, at the end of the day, if you lose it,
it's only video.

>
> Either how there is a performance hit. I am still working with mine
> to get it working as it should.

Ah - now that's a *lot* more likely.
have you tuned things like readahead in the various layers you're using?
(I posted a script to do this to the linux-raid list months ago if that
would help)

> I would partition 4 drives to about 100mb or so and play with creating
> raids with different settings (chunk etc) and get used to the working
> with it.

you can configure readahead on the raw devices, the md device and the
lvm device. I think setting all to zero and having the lvm device with a
huge readahead gave me the best *video* file handling performance
(usually mult-gb files)
look at bonnie+ to do the perfomance tests.

>
> For linux I used mdadm which is a great little tool and alot more
> reliable than raidtools.
>
> I am just about to install slackware or debian on to my dell poweredge
> and use that as the fileserver as my pentium III with 512mb ram is
> struggling
> with raid5 during some actions.

PIII - hmmm - take a look at the cpu - raid5 does need a reasonable amount.
Try running top and watching how much the md0_raid5 process takes when
you do some heavy IO.
Also bear in mind that if you're backing up using rsync then it
typically runs over ssh which encrypts traffic and even nfsd uses a
reasonable amount of cpu.

HTH
David

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