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Date: Fri, 28 Jun 2013 11:34:18 -0400
From: Josef Bacik <jbacik@fusionio.com>
To: Martin <m_btrfs@ml1.co.uk>
CC: <linux-btrfs@vger.kernel.org>
Subject: Re: raid1 inefficient unbalanced filesystem reads
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On Fri, Jun 28, 2013 at 02:59:45PM +0100, Martin wrote:
> On kernel 3.8.13:
> 
> Using two equal performance SATAII HDDs, formatted for btrfs raid1 for
> both data and metadata and:
> 
> The second disk appears to suffer about x8 the read activity of the
> first disk. This causes the second disk to quickly get maxed out whilst
> the first disk remains almost idle.
> 
> Total writes to the two disks is equal.
> 
> This is noticeable for example when running "emerge --sync" or running
> compiles on Gentoo.
> 
> 
> Is this a known feature/problem or worth looking/checking further?

So we balance based on pids, so if you have one process that's doing a lot of
work it will tend to be stuck on one disk, which is why you are seeing that kind
of imbalance.  Thanks,

Josef