[ovirt-users] Re: [Vdo-devel] impact of --emulate512 setting for VDO volumes

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.pavese(a)interactiv-group.com&gt; wrote: