On Sat, Aug 6, 2016 at 1:10 AM, Alex Gorbachev <ag@xxxxxxxxxxxxxxxxxxx> wrote:
> Is there a way to perhaps increase the discard granularity? The way I see
> it based on the discussion so far, here is why discard/unmap is failing to
> work with VMWare:
>
> - RBD provides space in 4MB blocks, which must be discarded entirely, or at
> least hitting the tail.
>
> - SCST communicates to ESXi that discard alignment is 4MB and discard
> granularity is also 4MB
>
> - ESXI's VMFS5 is aligned on 1MB, so 4MB discards never actually free
> anything
>
> What is it were possible to make a 6MB discard granularity?
I'm confused. How can a 4M discard not free anything? It's either
going to hit an entire object or two adjacent objects, truncating the
tail of one and zeroing the head of another. Using rbd diff:
$ rbd diff test | grep -A 1 25165824
25165824 4194304 data
29360128 4194304 data
# a 4M discard at 1M into a RADOS object
$ blkdiscard -o $((25165824 + (1 << 20))) -l $((4 << 20)) /dev/rbd0
$ rbd diff test | grep -A 1 25165824
25165824 1048576 data
29360128 4194304 data
Thanks,
Ilya
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