[Openstack-operators] 4K block size

Sean McGinnis sean.mcginnis at gmx.com
Mon Apr 23 19:54:50 UTC 2018

On Mon, Apr 23, 2018 at 05:46:40PM +0000, Tim Bell wrote:
> Has anyone experience of working with local disks or volumes with physical/logical block sizes of 4K rather than 512?
> There seems to be KVM support for this (http://fibrevillage.com/sysadmin/216-how-to-make-qemu-kvm-accept-4k-sector-sized-disks) but I could not see how to get the appropriate flavors/volumes in an OpenStack environment?
> Is there any performance improvement from moving to 4K rather than 512 byte sectors?

I haven't seen much of a performance difference between drives with one
exception that I don't think will apply here. For backward compatiblity, there
is something that is called "512e mode". This basically takes a 4k sector size
and, using software abstraction, presents it to the host as a 512 byte sector
drive. So with this abstraction being done in between, there can be a slight
performance hit as things are translated.

As far as I know, you shouldn't need to have specific volume types and the use
of these drives should be transparent to the upper layers. At least coming from
the Cinder side. I'm not sure if there are any special considerations on the
Nova side though, so it would be great to hear from anyone that has any
experience with this and Nova.

I did find a decent write up on some of the differences with 4k vs 512. It is
focusing on SQL workloads, but I think the basics are generally applicable to
most workloads:



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