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

From: Mike Snitzer
Date: Fri Mar 22 2013 - 18:34:42 EST

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

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)

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

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

Also the existing 'test_writethrough' test works fine.

So for now:

Nacked-by: Mike Snitzer <snitzer@xxxxxxxxxx>
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