Thank you very much for your answer. I am copying ovirt-devel too as that
could be of interest to them.
Guillaume Pavese
Ingénieur Système et Réseau
Interactiv-Group
On Sat, Mar 2, 2019 at 7:00 AM Michael Sclafani wrote:
> Hi!
>
> 512 emulation was intended to support drivers that only do a fraction of
> their I/O in blocks smaller 4KB. It is not optimized for performance in any
> way. Under the covers, VDO is still operating on 4KB physical blocks, so
> each 512-byte read is potentially amplified to a 4KB read, and each
> 512-byte write to a 4KB read followed by a 4KB write. A workload consisting
> exclusively of 512-byte randomly-distributed writes could effectively be
> amplified by a factor of 16.
>
> We have a suite of automated tests we run in 512e mode on a nightly basis.
> That suite is a subset of our regular tests, containing only ones we expect
> would be likely to expose problems specific to the emulation.
>
> There should be no penalty to having emulation enabled on a volume that no
> longer uses it. If the I/O is 4KB-aligned and 4KB or larger, having it
> enabled won't affect it.
> It does not appear the setting can be modified by the VDO manager, but I
> cannot remember at this moment why that should be so.
>
> Hope this helps.
>
> On Fri, Mar 1, 2019 at 2:24 PM Guillaume Pavese <
> guillaume.pav...@interactiv-group.com> wrote:
>
>> Hello,
>>
>> We are planning to deploy VDO with oVirt 4.3 on centos 7.6 (on SSD
>> devices).
>> As oVirt does not support 4K devices yet, VDO volumes are created with
>> the parameter "--emulate512 enabled"
>>
>> What are the implications of this setting? Does it impact performance? If
>> so, is it IOPS or throughput that is impacted? What about reliability (is
>> that mode equally tested as standard mode)?
>>
>> As I saw on RH Bugzilla, support for 4K devices in oVirt will need to
>> wait at least for Centos 7.7
>> Once that is supported, would it be possible to transition/upgrade an
>> emulate512 vdo volume to a standard one?
>>
>> Thanks,
>>
>> Guillaume Pavese
>> Ingénieur Système et Réseau
>> Interactiv-Group
>> ___
>> Vdo-devel mailing list
>> vdo-de...@redhat.com
>> https://www.redhat.com/mailman/listinfo/vdo-devel
>>
>
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