On Jun 28, 2018, at 10:55 AM, Nir Soffer <nsoffer@redhat.com> wrote:

On Thu, Jun 28, 2018 at 10:48 AM Simon Coter <simon.coter@oracle.com> wrote:

Hi Nir,

thanks for your feedback on this; inline:

On Jun 27, 2018, at 8:10 PM, Nir Soffer <nsoffer@redhat.com> wrote:

On Tue, Jun 26, 2018 at 8:45 PM Simon Coter <simon.coter@oracle.com> wrote:

I’m looking for documentation reference on oVirt max-limits, like max number of guest on single compute-node, max number of LUNs into a storage domain or others.

There is no limit to the number of LUNs you can add in a storage domain,

but I don't it is a good idea or very useful to have many LUNs in a storage

domain, since the number of volume in a storage domain is limited to about

1947.

Yep, I see. Where did you get this number for example (1947) ?

This number is internal implementation detail, so don't depend on it,

but it goes like this:

- we create 2G volume for leases

- each lease needs 1M of storage

- we reserve 100 leases for future use

- we create one lease for the SPM

- rest of the leases can be used for volumes

- every volume has one lease, create when you create the volume

So we can have 2048 leases - 101 non volume leases -> 1947 volume leases.

This limit can be removed if we support extending the leases volume, but it is                                                                                                                

not supported yet, because we don't recommend having more than 1300 volumes per

storage domains. The reason is our volumes on block storage are LVM logical

volumes, and LVM is not designed to manage 1000's of logical volumes.

So the 1947 volume limit is likely to stay in the near future.

 

The number of LUNs in a system is limited by the kernel, I think the

number of around 16000.

Why do you care about the maximum number of LUNs?

I do not strictly care about the number of LUNs; I care of possible “max” limits around the solution, to better understand how an architecture should be created/designed while using oVirt.

Generally, you want to minimize the number of storage domains. Storage domains

are not deigned for unlimited scale. You should create storage domains only if

there is a need to separate storage to different storage domains. For example

having separate storage domain for production or testing, of for different

groups of users.

However you cannot have unlimited disks in one storage domain, so one of the

factors when designing the system is how many disks you need in one storage

domain.

Every disk snapshot is a logical volume, so if you have one disk with 5

snapshot, you are using 5 logical volumes. Additionally, if you include memory

snapshot, you need another logical volume for it. For example if you have vm

with 5 disks and you want to keep memory snapshot, every snapshot will use 6

logical volumes.

So you can calculate the expected number of volume on a storage domain based on

the number of virtual machines, the number of disks per virtual machine, and if

you want to keep memory snapshot or not.

For file based storage domain none of these limits apply. Here a storage domain

is a mountpoint and we have one directory per disk, so the number of disks is

practically unlimited. Since many operations require listing all images and

snapshots in the mountpoint, having 10,000 disks will be slower then 1000

disks. 

In summary, oVirt is designed for data center, and not for the cloud, so you 

should not expect to manage unlimited number of vms, disks, or storage domains.

For 4.3 we are working on improving Cinder support. With Cinder based storage,

we have one LUN per disk, and snapshots are implemented by storage server.

Also, this LUN will be connected only to one host at a time, or two hosts during

live migration, instead of having all LUNs connected to all hosts all the time. With

this storage there will be no practical limit on oVirt side for number of disks.

Yaniv, do you have some document with general recommendations for sizing?

Nir

 

Thanks

Simon

Nir

 

Is there a reference on oVirt documentation ?

Actually the only thing I found is related to RHEV and memory (https://access.redhat.com/articles/906543)

Thanks for the help.

Simon

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