[dm-devel] dm-cache coherence issue

[Johannes Bauer]

On 24.06.2017 15:56, Johannes Bauer wrote:

> So I seem to have a very basic misunderstanding of what the cleaner
> policy/dirty pages mean. Is there a way to force the cache to flush
> entirely? Apparently, "dmsetup wait" and/or "sync" don't do the job.

I'd like to expand on this, since I discovered something that worries me
a bit just now:

I do have a dm-cache setup for my main drive as well (3 TB HDD cached by
128 GB SSD). On top of dm-cache runs dm-crypt (LUKS). When playing
around with my root fs, I tried to discard the whole cache. Therefore, I
did -- with the LUKS container NOT open:

dmsetup suspend cache-device
dmsetup reload cache-device --table '0 5858433935 cache 253:1 253:0 8:19
512 0 cleaner 0'
dmsetup resume cache-device
dmsetup wait cache-device

Again, "dmsetup wait" hung, with no obvious I/O being done (i.e., all
disks were idle). I Ctrl-Ced out of it. The status showed no dirty pages.

Since I was suspicious, I left the device with the "cleaner" policy and
opened the LUKS container, then did an e2fsck -fn on the opened device.
No errors found.

Then I closed the LUKS container and performed "dmsetup remove" on the
cache device (as well as the two other linear mappings).

Then I re-opened LUKS on the origin device and ran e2fsck -fn there.
File system errors!

I am fully prepared to restore everything from backup. However, is this
normal behavior or an issue? How am I supposed to discard an attached
cache? How do I get the origin device back in sync with the cached device?

Best regards,
Johannes