From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joe Thornber <thornber@redhat.com>
Subject: Re: all cache blocks marked as dirty in writethrough
 mode, "Data loss may occur", constant write activity
Date: Mon, 30 Jun 2014 12:25:13 +0100
Message-ID: <20140630112513.GA3893@debian>
References: <20140628225452.GA570@schmorp.de>
 <20140630091403.GB3098@debian>
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On Mon, Jun 30, 2014 at 10:14:03AM +0100, Joe Thornber wrote:
> - There's a constant 4k background load to the metadata device.
> 
>   I'll look into this.  Sounds like the periodic commit is re writing
>   the superblock even if there's no change to the mappings.

I've added this test to the dm test suite:

https://github.com/jthornber/device-mapper-test-suite/blob/master/lib/dmtest/tests/cache/io_use_tests.rb

Using my latest development tree I can't see any spurious IO.

I also instrumented to see exactly when the superblock is written,
again nothing happens while the cache is idle.  So I'm puzzled about
what you are seeing.

- Joe