[Vdo-devel] [BUG] Discard not working

[Louis Imershein]

The main reason that VDO doesn't pass down discards directly is that it's
complex to do this with deduplication. One challenge is, when should a
logical discard be mapped to a physical one? A second one is how do you
amortize discards for good performance as your physical address space gets
fragmented.

VDO is very good at managing allocation in flash-friendly ways and if
you've got good dedupe rates the loss of performance won't be as bad as you
think, but as usual try it.

Discards at the physical layer are definitely an issue the team is
interested in eventually addressing.

As for the comments about VDO vs ZFS compression, I'd agree for large
sequential IOs, use your native filesystem compresdion. For small 4K random
IO, use VDO compression. Both have there uses for compressible data under
different workloads.

LOUIS IMERSHEIN

PRINCIPAL PRODUCT MANAGER, DATA REDUCTION TECHNOLOGIES

Red Hat

limershe at redhat.com
M: (+1)831-588-5971



On Wed, Aug 8, 2018, 1:14 PM Michael Niewöhner <admin at mniewoehner.de> wrote:

> On Wed, 2018-08-08 at 22:09 +0200, Gionatan Danti wrote:
> > Il 08-08-2018 17:35 Michael Niewöhner ha scritto:
> > > Hi Bryan,
> > >
> > > ah, thanks... I didn't realize that.
> > > What is the reason for not passing down discard like ext*, LVM, ZFS,
> > > ... ?
> > > Using VDO on a SSD would be very bad.
> > >
> > > IMHO at least there should be an option to enable discard when needed.
> > > In my case having discard is an absolute must-have because of ZFS.
> > > Not having discard would mean ZFS never knows which blocks are
> unmapped
> > > and
> > > don't need to be synced/scrubbed/...
> > > (I don't want to use ZFS deduplication because of the memory
> > > requirements.)
> > >
> > > Best regards
> > > Michael
> >
> > Hi, before going full-on for data deduplication, I would advice you to
> > test if your data *are actually deduplicable*.
> > I evaluated both ZFS and VDO dedup and found that, for my VMs, they
> > provided minimal benefit over plain ZFS LZ4 compression at a very big
> > performance hit.
> >
> > So, if your data are deduplicable - go ahead. If not, please stop here
> > and simply use compression.
> >
> > Regards.
>
>
> Hi,
>
> since all my VMs are Linux VMs with exactly the same base, I am sure it is
> deduplicable.
> Thanks anyway.
>
> Best regards
> Michael
>
>
>
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