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Date: Tue, 16 Feb 2016 10:10:13 +1100
From: Dave Chinner <david@fromorbit.com>
To: Premysl Kouril <premysl.kouril@gmail.com>
Cc: Theodore Ts'o <tytso@mit.edu>, Andi Kleen <andi@firstfloor.org>,
	linux-fsdevel@vger.kernel.org
Subject: Re: EXT4 vs LVM performance for VMs
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On Mon, Feb 15, 2016 at 07:56:05PM +0100, Premysl Kouril wrote:
> Hello Dave,
> 
> thanks for your suggestion. I've just recreated our tests with the XFS
> and preallocated raw files and the results seem almost same as with
> the EXT4. I again checked stuff with my Systemtap script and threads
> are waiting mostly waiting for locks in following placeS:
....
> Looking at this, does it suggest that the bottleneck is locking on the
> VFS layer? Or does my setup actually do DirectIO on the host level?
> You and Sanidhya mentioned that XFS is good at concurrent DirectIO as
> it doesn't hold lock on file, but I do see this in the trace:
> 
>  0xffffffff8176fcaf : mutex_lock+0x1f/0x2f [kernel]
>  0xffffffffc073c531 : 0xffffffffc073c531 [xfs]
>  0xffffffffc07a788a : 0xffffffffc07a788a [xfs]
>  0xffffffffc073dd4c : 0xffffffffc073dd4c [xfs]

You need to resolve these addresses to symbols so we can see where
this is actually blocking.

> So either KVM is not doing directIO or there is some lock xfs must
> hold to do the write, right?

My guess is that you didn't configure KVM to use direct Io
correctly, because if it was XFs blocking on internal locks in
direct IO it would be on a rwsem, not a mutex.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com