[dm-devel] Another cache target

[Joe Thornber]

On Fri, Dec 14, 2012 at 01:51:19PM -0800, Darrick J. Wong wrote:
> Yeah, I think I've seen some odd behavior too - on one of my runs, blkid
> reported that the cache device had the same superblock as the aggregate device.
> My guess is that block 0 on the exported device got mapped to block 0 of the
> cache.  I'll see if I can make it happen again, but that brings me to another
> set of questions.

This is normal.

> First, is there a plan to have userspace tools to set up the cache, provide
> protective superblocks, etc.?

Yes, lvm2 will support it soon (hopefully).  Tools like cache_check,
cache_dump, cache_restore that manipulate the metadata device are
nearly ready.

>  As far as I can tell, the slow disk and the fast
> disk don't have headers to declare the existence of the cache, so blkid and
> friends can end up seeing things they shouldn't.  How were you planning to keep
> users from mounting the slow device before the cache comes up?

We don't label the origin device or ssd in anyway.

> Second, if the cache is in WB mode, is there a way to force it to flush the
> cache contents to disk?  Or does it do that at dmsetup create time?

  Reload the cache target with the cleaner policy.  Once it's finished
  writing everything back it'll trigger a dm event that you can catch
  with 'dmsetup wait'.  Then check the status to double check there
  are no dirty blocks.  At this point you can ditch the cache and use
  the origin directly.  See test below.


  def wait_for_all_clean(cache)
    cache.event_tracker.wait(cache) do |cache|
      status = CacheStatus.new(cache)
      status.nr_dirty == 0
    end
  end

  def test_cleaner_policy
    with_standard_cache(:format => true) do |cache|
      git_prepare(cache, :ext4)

      cache.pause do
        table = cache.active_table
        table.targets[0].args[6] = 'cleaner'
        cache.load(table)
      end

      wait_for_all_clean(cache)
    end

    # We should be able to use the origin directly now
    with_standard_linear do |origin|
      fs = FS::file_system(:ext4, origin)
      fs.with_mount('./kernel_builds', :discard => true) do
        # triggers fsck
      end
    end
  end


- Joe