Date: Mon, 6 Nov 2006 13:02:41 -0000 From: "Steven Hartland" <killing@multiplay.co.uk> To: <freebsd-geom@FreeBSD.ORG>, "Oles Hnatkevych" <don_oles@able.com.ua> Subject: Re: geom stripe perfomance question Message-ID: <01a401c701a3$dc106ef0$b3db87d4@multiplay.co.uk> References: <200611061204.kA6C4FXt079703@lurza.secnetix.de>
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Oliver Fromme wrote:
> I wonder why people always try to use dd for benchmarking.
> It's bogus. dd is not for benchmarking. It works in a
> sequential way, i.e. it first reads 256 KB (your stripe
> size) from the first compontent, then 256 KB from the 2nd,
> and so on. While it reads from one disk, the other one is
> idle. So it is not surprising that you don't see a speed
> increase (in fact, there's a small decrease because of
> the seek time overhead when switching from on disk to
> the other). [*]
>
> The performance of a stripe should be better when you use
> applications that perform parallel I/O access.
>
> Your benchmark should be as close to your real-world app
> as possible. If your real-world app is dd (or another one
> that accesses big files sequentially without parallelism),
> then you shouldn't use striping.
Serving large files via FTP this would be just the test to
benchmark so I'd say you argument is flawed in that it makes
assumptions about the way files are accessed.
In addition to this from my tests its only geom that performs
so poorly, neither hardware RAID nore sofware RAID under
suffers from such poor performance. If your argument where
correct then they would all suffer but this is not the case.
It might be interesting from someone familiar with the geom
code to have a look at the linux RAID code to see if there
is any obvious reason why geom performs quite so poorly
under just about every test I've seen.
Steve
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