[CentOS] 3Ware 9550SX and latency/system responsiveness

Ross S. W. Walker rwalker at medallion.com
Fri Sep 14 15:02:53 UTC 2007

Simon Banton wrote:
> At 15:43 +0200 14/9/07, Sebastian Walter wrote:
> >Simon Banton wrote:
> >  > No, I haven't. This is 3ware hardware RAID-1 on two disks with a
> >>  single LVM ext3 / partition - I'm afraid I don't know how 
> to go about
> >>  discovering the chunk size to plug into Ross's calcs.
> >
> >You can see the chunk size either in the raid's BIOS tool (Alt-3 at
> >startup) or, if installed, in the 3dm CLI (defaults to 64k, I think).
> Hmmm, from what I can see in the tw_cli documentation, stripe size 
> (and hence, presumably, chunk size) doesn't apply to RAID 1.

Some cards do, some cards don't. I suppose the idea is by chunking the
data in a RAID 1 the logic on the card can be the same between a RAID1
and a RAID10 (just a 2 drive RAID10) and therefore can save R&D money
and NVRAM costs due to smaller firmware.

> (apologies is the formatting goes awry):
> Stripe consists of the logical unit stripe size to be used. The 
> following table illustrates the supported and applicable stripes on 
> unit types and controller models. Stripe size units are in K (kilo 
> bytes).

I'm going to guess that it is not configurable, but 3ware uses 64k
chunk size on RAID1 simply for the cost savings mentioned earlier.

It will have 0 performance impact either way.

> I'm focused now on swapping the card for a fresh one to see if it 
> makes any difference, as per Ross's suggestion.

Well I should have said try on another identical machine too in case
there is something off on this one, and since you have 2 others and
time to kill until a different model machine arrives...


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