linux-kernel - Re: SSD and IO schedulers

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Message-ID: <20090204004003.26068f72@werewolf.home>
Date:	Wed, 4 Feb 2009 00:40:03 +0100
From:	"J.A. Magallón" <jamagallon@....com>
To:	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: SSD and IO schedulers

On Fri, 30 Jan 2009 22:55:47 +0100, Lorenzo Allegrucci <l.allegrucci@...il.com> wrote:

> Hi, I was wondering how IO schedulers such as as-iosched, deadline and
> cfq behave on SSD
> (that have virtually no seek time), from a theoretical point of view.
> How do they affect
> performance on these devices?
> I heard that the noop scheduler is often chosen by owners of EeePcs
> (with a SSD unit).

I'm beginning to think this is a _very_ bad idea. I have an AspireOne
A110 (512MB, 8GB SSD). This is 'not the best SSD in the world', to say
something. I had been booting with elevator=noop, because of all those
advices found along many blogs.

The fact is that the system behaved well, except when I was doing an
'urpmi --auto-update' (I use Mandriva). The bulk of rpm work rendered the
box unusable. 5-10 seconds stalls and so on.

After reading this, I killed the elevator option from grub, the system uses
the default cfq scheduler, and things are much smoother and the laptop
is perfectly usable while doing an update. I think even rpm itself works
faster.

Perhaps the reason is that, as the SSD is not so good, it behaves more
like a rotational drive ;).

> They report superior performance by using this (quite simple) scheduler.
> Are there any scientific benchmarks around?
> 

A couple questions:
- Apart from noop, which scheduler do you think is the best for SSDs ?
- I'm running 2.6.28.2. Does it have the 'SSD detector' in libata ?
  How can I verify it ?

TIA

-- 
J.A. Magallon <jamagallon()ono!com>     \               Software is like sex:
                                         \         It's better when it's free
Mandriva Linux release 2009.1 (Cooker) for x86_64
Linux 2.6.28.2-desktop-1mnb (gcc 4.3.2 (GCC) #1 Wed Jan
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