[dm-devel] [PATCH 0/1] dm era: Fix digestion bug that can lead to lost writes

[Nikos Tsironis]

In case of devices with at most 64 blocks, the digestion of consecutive
eras uses the writeset of the first era as the writeset of all eras to
digest, leading to lost writes. That is, we lose the information about
what blocks were written during the affected eras.

The root cause of the bug is a failure to reinitialize the on-disk
bitset cache when the digestion code starts digesting a new writeset.

Steps to reproduce
------------------

1. Create two LVs, one for data and one for metadata

   # lvcreate -n eradata -L1G datavg
   # lvcreate -n erameta -L64M datavg

2. Fill the whole data device with zeroes

   # dd if=/dev/zero of=/dev/datavg/eradata oflag=direct bs=1M

3. Create a dm-delay device, which inserts a 500 msec delay to writes:

   # dmsetup create delaymeta --table "0 `blockdev --getsz \
     /dev/datavg/erameta` delay /dev/datavg/erameta 0 0 /dev/datavg/erameta 0 500"

4. Create a 256MiB (64 4MiB blocks) dm-era device, using the data LV for
   data and the dm-delay device for its metadata. We set the tracking
   granularity to 4MiB.

   # dmsetup create eradev --table "0 524288 era /dev/mapper/delaymeta \
     /dev/datavg/eradata 8192"

5. Run the following script:

   #!/bin/bash

   # Write to block #0 during era 1
   dd if=/dev/urandom of=/dev/mapper/eradev oflag=direct bs=4K count=1

   # Increase era to 2
   dmsetup message eradev 0 checkpoint

   # Write to block #1 during era 2
   dd if=/dev/urandom of=/dev/mapper/eradev oflag=direct bs=4K count=1 seek=1024 &

   # Increase era to 3
   dmsetup message eradev 0 checkpoint

   # Sync the device
   sync /dev/mapper/eradev

6. Remove the device, so we can examine its metadata

   # dmsetup remove eradev

7. Examine the device's metadata with `era_dump --logical /dev/mapper/delaymeta`

   <superblock uuid="" block_size="8192" nr_blocks="64" current_era="3">
       <era_array>
           <era block="0" era="2"/>
           <era block="1" era="0"/>
           <era block="2" era="0"/>
           ...
           <era block="63" era="0"/>
       </era_array>
   </superblock>

   We see that:
    a. Block #0 is marked as last written during era 2, whereas we wrote
       to it only during era 1
    b. Block #1 is not marked as written at all, whereas we wrote to it
       during era 2

8. Examining the data device, e.g., with `hexdump /dev/datavg/eradata`,
   we can see that both blocks #0 and #1 are written, as expected.

Nikos Tsironis (1):
  dm era: Reinitialize bitset cache before digesting a new writeset

 drivers/md/dm-era-target.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

-- 
2.11.0