[ovirt-users] 4k alignment on FC LUN

[Yaniv Kaul]

On Wed, Jul 6, 2016 at 2:27 PM, Nir Soffer <nsoffer at redhat.com> wrote:

> On Wed, Jul 6, 2016 at 12:18 AM, David Twersky <dmtwersky at gmail.com>
> wrote:
> > We use a lot of flash arrays, and some of them require that devices be 4k
> > aligned, is it possible to create a Fibre Channel Storage Domain and
> have it
> > 4k aligned?
>

Is it required or preferred? Because while most of our data is 4K aligned,
some writes aren't - mainly metadata of the storage domain.
So you won't be usually suffering from mis-aligned writes (or reads) which
are costly, but we must be able to perform atomic 512b writes (which may
translate to a read-modify-write of course in the flash).
I'm not sure what we expose to VMs, btw - if we can expose to them 4K
drive...?


> I don't think we support this.
>

Indeed - we assume sector size to be 512b in the code:
lib/vdsm/storage/constants.py:SECTOR_SIZE = 512

Can you use a direct LUN, though?
Y.


>
> For example, we assume that writing 512 bytes blocks is atomic, if these
> writes will fail because the minimal block is 4k, block storage metadata
> operations will fail.
>
> Anyway, why not try to add such lun and create a storage domain for
> testing?
>
> Nir
> _______________________________________________
> Users mailing list
> Users at ovirt.org
> http://lists.ovirt.org/mailman/listinfo/users
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ovirt.org/pipermail/users/attachments/20160706/34c3ab46/attachment-0001.html>