On Wed, Jun 14, 2017 at 4:18 PM, FERNANDO FREDIANI <fernando.frediani@upx.com> wrote:

I normally assume that any performance gain from directlly attaching a LUN to a Virtual Machine then using it in the traditional way are so little to compensate the extra hassle to do that. I would avoid as much as I cacn use it, unless it is for some very special reason where you cannot do in any other way. The only real usage for it so far was Microsoft SQL Server Clustering requirements.

I tend to agree (from performance perspective), though I don't have numbers to back it up. It probably doesn't matter that much.

There are however other reasons to use a direct LUN - use of storage-side features, such as replication, QoS, encryption, compression, etc. that you may wish to apply (or disable) per storage.

Also, there are some strange SCSI commands that some strange applications need that require direct LUN and SCSI pass-through. Clustering (via SCSI reservations is certainly the first and foremost but not the only one).

Fernando
On 14/06/2017 03:23, Idan Shaby wrote:
Direct luns are disks that are not managed by oVirt. Ovirt communicates directly with the lun itself, without any other layer in between (like lvm in image disks).

The advantage of the direct lun is that it should have better performance since there's no overhead of another layer in the middle.

The disadvantage is that you can't take a snapshot of it (when attached to a vm, of course), can't make it a part of a template, export it, and in general - you don't manage it.

Regards,

Idan

On Mon, Jun 12, 2017 at 10:10 PM, Stefano Bovina <bovy89@gmail.com> wrote:

Thank you very much.
What about "direct lun" usage and database example?

2017-06-08 16:40 GMT+02:00 Elad Ben Aharon <ebenahar@redhat.com>:

Hi,
Answer inline

On Thu, Jun 8, 2017 at 1:07 PM, Stefano Bovina <bovy89@gmail.com> wrote:

Hi,

does a storage best practise document for oVirt exist?

Some examples:

oVirt allows to extend an existing storage domain: Is it better to keep a 1:1 relation between LUN and oVirt storage domain?

What do you mean by 1:1 relation? Between storage domain and the number of LUNs the domain reside on?

If not, is it better to avoid adding LUNs to an already existing storage domain?

No problems with storage domain extension.

Following the previous questions:

Is it better to have 1 Big oVirt storage domain or many small oVirt storage domains?

Depends on your needs, be aware to the following:

- Each domain has its own metadata which allocates ~5GB of the domain size.

- Each domain is being constatntly monitored by the system, so large number of domain can decrease the system performance.

There are also downsides with having big domains, like less flexability

There is a max num VM/disks for storage domain?

In which case is it better to use "direct attached lun" with respect to an image on an oVirt storage domain?

Example:

Simple web server: ----> image

Large database (simple example):

- root,swap etc: 30GB ----> image?

- data disk: 500GB -----> (direct or image?)

Regards,

Stefano

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