max number of snapshot per vm
Hi, What are the best pratices about vm snapshots? Is there a maximum number of snapshots per vm? Has a high number of present snapshot an impact on the vm performance? ... on how long the snapshot is completed? -- Nathanaël Blanchet Supervision réseau Pôle Infrastrutures Informatiques 227 avenue Professeur-Jean-Louis-Viala 34193 MONTPELLIER CEDEX 5 Tél. 33 (0)4 67 54 84 55 Fax 33 (0)4 67 54 84 14 blanchet@abes.fr
On Fri, Nov 23, 2018 at 1:01 PM Nathanaël Blanchet <blanchet@abes.fr> wrote:
Hi,
What are the best pratices about vm snapshots?
I think the general guideline is keep only snapshot you need, since every snapshot has a potential performance cost. qemu caches some image metadata in memory, so accessing data from an image with 20 snapshots should be efficient as image with 2 snapshots, but using more snapshots will consume more memory. Kevin, do we have performance tests comparing VMs with different amount of snapshots? On oVirt side, there is little additional overhead for every snapshot. We never measured this overhead but I don't think it will be an issue in normal use. With block storage oVirt has a soft 1300 volumes limit per storage domain, so keeping more snapshots per VM means yo can keep less VMs on the same storage domain.
Is there a maximum number of snapshots per vm?
How many snapshot do you plan to keep?
Has a high number of present snapshot an impact on the vm performance? ... on how long the snapshot is completed?
Taking a snapshot is fast - basically the time it take to create a new empty image, and then the time it take to freeze the guest file systems before the snapshot. Before 4.2 this could be slow with block storage depending on the number of snapshot in the VM: https://bugzilla.redhat.com/1395941 Taking a snapshot with memory is not fast, depending on the amount of RAM in the VM and how fast you can compress and write memory to storage. Deleting a snapshot is not fast, the operation requires copying the data in the snapshot to previous snapshot. But if the snapshot is not too old, and the VM did not write a lot of data to that snapshot, the opeartion is not very long. Reverting a VM to an older snapshot is pretty fast and require no data operations. This is basically the time to create snapshot based on the snapshot you want to revert to. You can measure these operations on your system. Clone existing VM and try to add and remove snapshots with and without memory. You can also test if having lot of snapshots cause noticeable performance issues with the planned workload. Nir
Am 25.11.2018 um 19:55 hat Nir Soffer geschrieben:
On Fri, Nov 23, 2018 at 1:01 PM Nathanaël Blanchet <blanchet@abes.fr> wrote:
What are the best pratices about vm snapshots?
I think the general guideline is keep only snapshot you need, since every snapshot has a potential performance cost.
qemu caches some image metadata in memory, so accessing data from an image with 20 snapshots should be efficient as image with 2 snapshots, but using more snapshots will consume more memory.
The memory will also only really be used when the backing file is actually accesses. So if you keep very old snapshots, but all data has already been overwritten and the old snapshot is never accessed, it doesn't need that memory. Additionally, starting with QEMU 3.1, cache-clean-interval is active by default and frees the cached table of a snapshot layer when it hasn't been accessed for 10 minutes. (You had to enable it explicitly before that, which I don't think oVirt does.)
Kevin, do we have performance tests comparing VMs with different amount of snapshots?
I'm not aware of a proper performance test where this aspect was tested. Kevin
participants (3)
-
Kevin Wolf -
Nathanaël Blanchet -
Nir Soffer