Subject: Re: RaidFrame poor performance
To: Jochen Keil <J.Keil@gmx.de>
From: Greg Oster <email@example.com>
Date: 01/20/2005 13:49:48
Jochen Keil writes:
> Greg Oster wrote:
> > Jochen Keil writes:
> >>At the moment i'm doing a benchmark on my 4 disk R5 setup and later i
> >>will do so on a 3 disk R5. What i'm wondering about is your comment
> >>about matching stripe size and file system block size.
> >>Default FS block size is 16kb for partitions above 1024MB. I'm using
> >>this R5 Layout:
> >> START layout
> >> # sectPerSU SUsPerParityUnit SUsPerReconUnit RAID_level
> >> 32 1 1 5
> >>I learned that 32 blocks equal to 16KB. As far as i can see my block
> >>size matches my stripe size so this is the optimum.
> > No. In this case, with "4 data disks", a full stripe will hold
> > 4*32=128 blocks (64K) of data. (That "32" is per-component.)
> > So a block size of 64K will probably perform the best with this
> > configuration.
> In case i misunderstood something: are you talking about four data disk
> plus one disk for parity which would sum up to five disks or do you mean
> four disks overall?
Sorry.. I was mixed up.. what I said is true for a 5-disk RAID 5 set.
For your 3-disk RAID 5 set ("2 data disks") a full stripe would hold
2*32 = 64blocks = 32K of data, and a 32K block size will probably
perform best with that one. You will probably see even better
performance on a 3-disk set with: "64 1 1 5" as the layout
parameters, and then using a 64K block size.
> To make my point more clear: wd0+wd1+wd2+wd3+wd4 or
> wd0+wd1+wd2+wd3? I meant wd0+wd1+wd2 for the three disk R5 setup and
> wd0+wd1+wd2+wd3 for the four disk R5 setup. I'm very sorry if this
> didn't came out very clear.
Your post was clear -- my respose to it wasn't :-}
The first point I was wanting to make was that the block size you were
using was not sufficiently large enough to cover a full stripe (for
either the 3-component or 4-component cases). The second thing I
wanted to do was suggest alternate block sizes. Hopefully this post
does a better job than the first :-}
> According to your and Daniels email a good R5 setup would be like that:
> 4 data disks + 1 parity + 1 hot spare. If i choose a stripe size of 32
> blocks the best FS block size i can choose would be 4*(32/2)K=64K.
> Is this a correct conclusion?
> If there is some interest i'll send my benchmark result to the list.
Even if there is no interest on the list, I'd still like to see the
benchmark results :)
> > Later...
> > Greg Oster
> Thank you very much in advance and with best regards,
> Jochen Keil