-----Original Message-----
From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
owner@vger.kernel.org] On Behalf Of Keld Jørn Simonsen
Sent: Friday, May 06, 2011 4:54 PM
To: Leslie Rhorer
Cc: 'Keld Jørn Simonsen'; 'NeilBrown'; linux-raid@vger.kernel.org
Subject: Re: mdadm raid1 read performance

On Fri, May 06, 2011 at 04:20:39PM -0500, Leslie Rhorer wrote:

quoted

quoted

-----Original Message-----
From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
owner@vger.kernel.org] On Behalf Of Keld Jørn Simonsen
Sent: Thursday, May 05, 2011 6:10 AM
To: NeilBrown
Cc: Liam Kurmos; Roberto Spadim; Brad Campbell; Drew; linux-
raid@vger.kernel.org
Subject: Re: mdadm raid1 read performance

On Thu, May 05, 2011 at 09:45:38AM +1000, NeilBrown wrote:

quoted

On Thu, 5 May 2011 00:08:59 +0100 Liam Kurmos

[off-list ref]

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wrote:

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as a separate question, what should be the theoretical performance

of

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raid5?

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x(N-1)

So a 4 drive RAID5 should read at 3 time the speed of a single

drive.

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Actually, theoretically, it should be more than that for reading, more
like N minus
some overhead. In a raid5 stripe of 4 disks, when reading you do not

read

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the checksum block, and thus you should be able to have all 4 drives
occupied with reading real data. Some benchmarks back this up,
http://home.comcast.net/~jpiszcz/20080329-raid/
http://blog.jamponi.net/2008/07/raid56-and-10-benchmarks-on-

26255_10.html

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The latter reports a 3.44 times performance for raid5 reads with 4
disks, significantly over the N-1 = 3.0 mark.

For writing, you are correct with the N-1 formular.

	There have been a lot of threads here about array performance, but
one important factor rarely mentioned in these threads is network
performance.  Of course, network performance is really outside the scope

of

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this list, but I frequently see people talking about performance well in
excess of 120MBps.  That's great, but I have to wonder if their network
actually can make use of such speeds.  Of course, if the application
actually obtaining the raw data is on the machine, then network

performance

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is much less of an issue.  A database search implemented directly on the
server, for example, can use every bit of performance available to the

local

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machine.  Given that in my case the vast majority of data is squirted

across

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the LAN (e.g., these are mostly file servers), anything much in excess

of

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120MBps is irrelevant.  I mean, yeah, it’s a rather nice feeling that my
RAID arrays can deliver more than 450MBps if they are ever called upon

to do

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so, but with a 1G LAN, that's not going to happen very often.  I just

wonder

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how many people who complain of poor performance can really benefit all

that

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much from increased performance?

10 Gbit/s connections are getting commonplace these days, at least in the
environments that I operate in.

	They are certainly not unheard-of, but I'm not sure I would call
them, "commonplace".  They are definitely not in the majority.  I work for a
very large national telecommunications company, and most of the links we
sell are still less than 10M.  I'm not sure we have sold any full 10G
network links, at all, although we have certainly sold a number of 2G - 4G
links.  Of course, WAN and SAN applications are always more expensive than
LAN applications, so many companies have large intra-site links but
comparatively small inter-site links.  Our customer backbone, of course, is
much, much higher than 10G, but none of our internal LAN links at any of our
locations is more than 1G.  Most are 100M.

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