linux-kernel - Re: dm: dm-cache fails to write the cache device in writethrough mode

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Date:	Fri, 22 Mar 2013 18:34:28 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	Heinz Mauelshagen <heinzm@...hat.com>,
	Joe Thornber <ejt@...hat.com>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Paul Taysom <taysom@...omium.org>,
	Randy Dunlap <rdunlap@...radead.org>
Subject: Re: dm: dm-cache fails to write the cache device in writethrough mode

On Fri, Mar 22 2013 at  4:11pm -0400,
Darrick J. Wong <darrick.wong@...cle.com> wrote:

> The new writethrough strategy for dm-cache issues a bio to the origin device,
> remaps the bio to the cache device, and issues the bio to the cache device.
> However, the block layer modifies bi_sector and bi_size, so we need to preserve
> these or else nothing gets written to the cache (bi_size == 0).  This fixes the
> problem where someone writes a block through the cache, but a subsequent reread
> (from the cache) returns old contents.

Your writethrough blkid test results are certainly strange.  But I'm not
aware of where the block layer would modify bi_size and bi_sector;
please elaborate.

I cannot reproduce your original report.  I developed
'test_writethrough_ext4_uuids_match', apologies for the ruby code:

  def test_writethrough_ext4_uuids_match
    size = meg(10)
  
    # wipe the origin to ensure we don't accidentally have the same
    # data on it.
    with_standard_linear(:data_size => size) do |origin|
      wipe_device(origin)
    end

    uuid = "deadbeef-cafe-dead-beef-cafedeadbeef"

    # format the cache device with a specific uuid
    with_standard_cache(:format => true,
                        :io_mode => :writethrough,
                        :data_size => size) do |cache|
      fs = FS::file_system(:ext4, cache)
      fs.format(:uuid => uuid)
      FS::assert_fs_uuid(uuid, cache)
    end

    # origin should have the same uuid as the cache
    with_standard_linear(:data_size => size) do |origin|
      FS::assert_fs_uuid(uuid, origin)
    end
  end

This test was committed to the 'devel' branch of my thinp-test-suite
tree: git://github.com/snitm/thinp-test-suite.git

Also the existing 'test_writethrough' test works fine.

So for now:

Nacked-by: Mike Snitzer <snitzer@...hat.com>
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