Hi Robert,
here is a brief overview on these parameters:
bdev_enable_discard - flag to enable/disable trimming. Once enabled and
depending on bdev_async_discard_threads parameter discard requests
could be issued either synchronously or asynchronously. Each requests
originates when OSD releases specific disk extent, e.g. when overwriting
or removing an object (or part of it). In async mode requests are put
into a queue and are processed by one of relevant threads.
bdev_async_discard_threads - amount of threads to process discard
requests asynchronously, if set to 0 and bdev_enable_discart is set to
true - all the discard request are issued synchronously
bdev_async_discard - this is a deprecated parameter, please don't use it
I have no idea how this supposed to work with qcow2 files as the primary
targets are SSD drives but in recent Ceph releases (e.g. v19.2.3 and
upcoming v18.2.8) one can learn the amount of issued discard requests
from OSD perf counters:
ceph tell osd.0 perf dump | grep discard -B 1
"blk-kernel-device-bluestore": {
"discard_op": 0,
"discard_threads": 0
--
"blk-kernel-device-db": {
"discard_op": 0,
"discard_threads": 0
--
"blk-kernel-device-slow": {
"discard_op": 0,
"discard_threads": 0
--
"blk-kernel-device-wal": {
"discard_op": 0,
"discard_threads": 0
Hope this is helpful.
Best regards,
Igor
On 17/02/2026 12:15, Robert Sander via ceph-users wrote:
Hi,
what is the purpose and effect of these configuration options?
bdev_async_discard
bdev_async_discard_threads
bdev_enable_discard
They are not mentioned on docs.ceph.com.
They read as they would enable a regular trim job on OSDs where the
hardware supports trimming/discarding. How often would this operation
run?
On a virtual test cluster where the OSDs are backed with qcow2 files
and are presented to the VM as SSDs with discard they seem to have no
effect on the size of the qcow2 file.
With a normal filesystem an fstrim operation will reduce the size of
the qcow2 file.
Regards
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