Re: dm-cache-statistics and DMEMIT

["Bryn M. Reeves" <bmr@redhat.com>]

On Mon, Aug 18, 2025 at 12:05:08PM -0700, John Rinehart wrote:
> I'm using dm-cache in writeback mode and I'm trying to figure out how
> I can see the cache stats in "real-time" (poll every second). Like, I
> want to poll for the values in this data structure:
> https://github.com/torvalds/linux/blob/be48bcf004f9d0c9207ff21d0edb3b42f253829e/drivers/md/dm-cache-target.c#L283-L295.
> 
> But, I'm at a loss for how to do that.

That's the internal data structure that tracks the cache stats: you
could write a bpf or systemtap script to dump the contents periodically
if you really want to get at every field.
 
> There are also these DMEMIT messages that I'm really interested in
> printing to dmesg or subscribing to, or whatever, but I don't know how
> to access them (CONFIG_DM_DEBUG didn't do anything useful - which
> makes sense after reading the source a bit):
> https://github.com/torvalds/linux/blob/be48bcf004f9d0c9207ff21d0edb3b42f253829e/drivers/md/dm-cache-target.c#L3184-L3197.

This is the 'status' handler for the cache target - the DMEMIT() is what
prints the status values for the target in response to the
DM_TABLE_STATUS ioctl. The easiest way to retrieve it is with the
'dmsetup status' command.

I don't have a dm-cache device to hand right now, but here's how it
behaves for a dm-snapshot device and a thin pool. The cache target
follows the same pattern (with the fields described in the comment above
the code you linked to, on line 3134):

  # dmsetup status /dev/mapper/fedora-root--snapset_before
  0 20971520 snapshot 168344/21061632 664

  # dmsetup status /dev/mapper/fedora-pool0-tpool
  0 4194304 thin-pool 815 124/1024 1883/32768 - rw discard_passdown queue_if_no_space - 256

There's a slightly more detailed description of the status fields in the
docs: Documentation/admin-guide/device-mapper/cache.rst (other files in
that directory describe the other DM targets).

Regards,
Bryn.