ADSM-L

Re: Using FILE instead of DISK devclass to avoid disk under-utilization

2006-10-26 16:45:09
Subject: Re: Using FILE instead of DISK devclass to avoid disk under-utilization
From: Stef Coene <stef.coene AT DOCUM DOT ORG>
To: ADSM-L AT VM.MARIST DOT EDU
Date: Thu, 26 Oct 2006 22:43:33 +0200
On Thursday 26 October 2006 21:29, Allen S. Rout wrote:
> >> On Wed, 25 Oct 2006 23:34:31 -0500, Roger Deschner <rogerd AT UIC DOT EDU>
> >> said:
> >
> > You probably want to avoid RAID5 for disk storage pools, whether
> > sequential or random. That can really slow client backups, because
> > RAID5 is quite slow for writing. RAID5 is really only good for
> > read-mostly applications, so at least you'll migrate quickly. You
> > probably want RAID10 instead, a striped set of mirrored pairs. (Make
> > sure your RAID10 implementation is NOT a mirrored pair of striped
> > sets, which is quite unsafe!) RAID10 is a less efficient use of raw
> > disk space, but both faster and safer than RAID5.
I do not agree.  New SAN boxes with write cache (with battery's) are smart.
Very smart.  New boxes are even better and need less ram to have the same
performance.
I did some tests on raw devices and I saw NO (repeat NO) performance
difference between raid 5 and raid 10.

Sorry for the layout, but this is a copy/paste from an open office document.
These are tests done on DS6000 with 75 GB, 1500 RPM disks, 8 disks in a raid
and show the difference between raid 10 and raid 5:
raid10 -> raid 5                                                                
raid    type    Q       Bytes   Ops     Rate (MB/s)     IOPS    Latency %CPU
10      random  10      101,55% 101,55% 101,55% 101,55% 100,00% 101,64%
10      seq     10      103,90% 103,90% 103,91% 103,90% 100,00% 103,74%
10      random  5       99,26%  99,26%  99,24%  99,25%  100,00% 99,37%
10      seq     5       99,23%  99,23%  99,19%  99,19%  100,00% 98,83%
10      random  1       101,46% 101,46% 101,46% 101,46% 100,00% 101,39%
10      seq     1       95,51%  95,51%  95,51%  95,51%  100,00% 96,48%
average 100,15% 100,15% 100,14% 100,14% 100,00% 100,24%

Just forget averything you think to know about raid 5 vs raid 10 IF you have a
good SAN box with write cache.  I don't know how it works, but the SAN
controllers are smart and are acting different on randon I/O vs sequential
I/O.


Stef