On Mon, Aug 1, 2016 at 9:07 PM, Ilya Dryomov <idryomov@xxxxxxxxx> wrote:
> 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
Just realized I've left out the most interesting bit. You can make
zero punch holes, but that's disabled by default in jewel. The option
is "filestore punch hole = true". Note that it won't be reflected in
"rbd diff" output.
Thanks,
Ilya
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