Subject: Re: RaidFrame poor performance
To: Jochen Keil <>
From: Greg Oster <>
List: current-users
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


Greg Oster