linux-kernel - Re: Fwd: [PATCH] Implement barrier support for single device DM devices

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Date:	Sat, 14 Feb 2009 10:32:13 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Mat <jackdachef@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>, agk@...hat.com,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Fwd: [PATCH] Implement barrier support for single device DM devices

On Sat, Feb 14, 2009 at 12:26:05AM +0100, Mat wrote:
> Hi Andi,
> 
> any news on this patch ?

It's in mainline as of 2.6.29-rc4.

> from what I saw it isn't included in mainline yet, if it is already
> please point to the kernel-config option where to enable it

It's always enabled for simple DM remapped devices given that its conditions
(only single underlying device etc.) are true.

> heavy flush, write operations; especially on amd64/X64:
> http://forums.gentoo.org/viewtopic-t-482731.html
> 
> I'm not sure if it's possible but I believe it could also be related
> to filesystems switching to synchronous writes (non-barrier
> write-mode) when the underlying partition is a LUKS-partition and

Note that my patch doesn't enable barriers for LUKS/dm_crypt, that would
be another patch which is not currently submtted.

> therefore
> decreasing performance (I'm not a kernel-hacker so I don't know if
> there's a possible correlation here)

You can find out by testing it without LUKS, but enabling/disabling
barriers on a file system and see how much difference it makes.
That would be better than speculating.

> 
> some factors I noticed having an impact on that are:

How did you determine that? Did you run some repeatable benchmark
that gave different numbers? 

> - the i/o scheduler hardly makes a change
> - elder boxes with PCI (non-PCIe) can be "fixed" with some latency-tweaks:
>   setpci -v -d *:* latency_timer=b0

You realize that the PCI latency has nothing to do with IO scheduler
delays? It's very unlikely that there is a correlation there.
Please double check these results.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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