On Mon, Aug 1, 2016 at 7:55 PM, Alex Gorbachev <ag@xxxxxxxxxxxxxxxxxxx> wrote:
> RBD illustration showing RBD ignoring discard until a certain
> threshold - why is that? This behavior is unfortunately incompatible
> with ESXi discard (UNMAP) behavior.
>
> Is there a way to lower the discard sensitivity on RBD devices?
>
>
>
> root@e1:/var/log# rbd diff spin1/testdis|awk '{ SUM += $2 } END {
> print SUM/1024 " KB" }'
> 819200 KB
>
> root@e1:/var/log# blkdiscard -o 0 -l 4096 /dev/rbd28
> root@e1:/var/log# rbd diff spin1/testdis|awk '{ SUM += $2 } END {
> print SUM/1024 " KB" }'
> 819200 KB
>
> root@e1:/var/log# blkdiscard -o 0 -l 40960 /dev/rbd28
> root@e1:/var/log# rbd diff spin1/testdis|awk '{ SUM += $2 } END {
> print SUM/1024 " KB" }'
> 819200 KB
>
> root@e1:/var/log# blkdiscard -o 0 -l 409600 /dev/rbd28
> root@e1:/var/log# rbd diff spin1/testdis|awk '{ SUM += $2 } END {
> print SUM/1024 " KB" }'
> 819200 KB
>
> root@e1:/var/log# blkdiscard -o 0 -l 4096000 /dev/rbd28
> root@e1:/var/log# rbd diff spin1/testdis|awk '{ SUM += $2 } END {
> print SUM/1024 " KB" }'
> 819200 KB
>
> root@e1:/var/log# blkdiscard -o 0 -l 40960000 /dev/rbd28
> root@e1:/var/log# rbd diff spin1/testdis|awk '{ SUM += $2 } END {
> print SUM/1024 " KB" }'
> 782336 KB
Think about it in terms of underlying RADOS objects (4M by default).
There are three cases:
discard range | command
-----------------------------------------
whole object | delete
object's tail | truncate
object's head | zero
Obviously, only delete and truncate free up space. In all of your
examples, except the last one, you are attempting to discard the head
of the (first) object.
You can free up as little as a sector, as long as it's the tail:
Offset Length Type
0 4194304 data
# blkdiscard -o $(((4 << 20) - 512)) -l 512 /dev/rbd28
Offset Length Type
0 4193792 data
Thanks,
Ilya
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