Good idea Strahil. I guess there could be benefits, as when I mailed vdo-devel in March asking about emulate512 performance impact, the answer from RedHat's Michael Sclafani was as follows : Begin citation: 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. Best Guillaume Pavese Ingénieur Système et Réseau Interactiv-Group On Tue, Oct 1, 2019 at 6:07 PM Strahil <hunter86_bg@yahoo.com> wrote:

You can go with 512 emulation and later you can recreate the brick without that emulation (if there are benefits of doing so). After all, you gluster is either replica 2 arbiter 1 or a replica 3 volume.

Best Regards, Strahil Nikolov On Oct 1, 2019 09:26, Satheesaran Sundaramoorthi <sasundar@redhat.com> wrote:

On Tue, Oct 1, 2019 at 11:27 AM Guillaume Pavese < guillaume.pavese@interactiv-group.com> wrote:

Hi all,

Sorry for asking again :/

Is there any consensus on not using --emulate512 anymore while creating VDO volumes on Gluster? Since this parameter can not be changed once the volume is created and we are nearing production setup. I would really like to have an official advice on this.

Best,

Guillaume Pavese Ingénieur Système et Réseau Interactiv-Group

Hello Guillaume Pavese, If you are not using --emulate512 for VDO volume, then VDO volume will be created as 4K Native volume (with 4K block size).

There are couple of things that bothers here: 1. 4K Native device support requires fixes at QEMU that will be part of CentOS 7.7.2 ( not yet available ) 2. 4K Native support with VDO volumes on Gluster is not yet validated thoroughly.

Based on the above items, it would be better you have emulate512=on or delay your production setup ( if possible, till above both items are addressed ) to make use of 4K VDO volume.

@Sahina Bose <sabose@redhat.com> Do you have any other suggestions ?

-- Satheesaran S (sas)

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