[ovirt-users] Re: cloning a VM or creating a template speed is so so slow

On Thu, Nov 11, 2021 at 4:33 AM Pascal D <pascal(a)butterflyit.com&gt; wrote: -o preallocation=metadata may work for files, we don't use it since it is not compatible with block storage (requires allocation of the entire volume upfront). We don't use the host page cache. There are several issues; - reading stale data after another host change an image on shared storage this should probably not happen with NFS. - writing to the page cache pollute the page cache with data that is unlikely to be needed, since vms also do not use the page cache (for other reasons). so you may reclaim memory that should be used by your vms during the copy. - the kernel like to buffer huge amount of data, and flush too much data at the same time. This cause delays in accessing storage during flushing. This is may break sanlock leases that must have access to storage to update the storage leases. We improved copy performance a few years ago using the -W option, allowing concurrent writes. This can speed up copy to block storage (iscsi/fc) up to 6 times[1]. When we tested this with NFS, we did not see big improvement, so we did not enable it. It also recommended to use -W for raw preallocated disk, since it may cause fragmentation. You can try to change this in vdsm/storage/sd.py: 396 def recommends_unordered_writes(self, format): 397 """ 398 Return True if unordered writes are recommended for copying an image 399 using format to this storage domain. 400 401 Unordered writes improve copy performance but are recommended only for 402 preallocated devices and raw format. 403 """ 404 return format == sc.RAW_FORMAT and not self.supportsSparseness This allows -W only on raw preallocated disks. So it will not be used for raw-sparse (NFS thin) or qcow2-sparse (snapshots on NFS), or for qcow2 on block storage. We use unordered writes for any disk in ovirt-imageio, and other tools like nbdcopy also always enable unordered writes, so maybe we should enable it in all cases. To enable unordered writes for any volume, change this to: def recommends_unordered_writes(self, format): """ Allow unordered writes only storage in any format. """ return True If you want to always enable this only for file storage (NFS, GlsuterFS, LocalFS, posix) add this method in vdsm/storage/nfsSD.py: class FileStorageDomainManifest(sd.StorageDomainManifest): ... def recommends_unordered_writes(self, format): """ Override StorageDomainManifest to allow on on qcow2 and raw sparse images. """ return True Please report how it works for you. If this give good results, file a bug to enable option. I think we can enable this based on vdsm configuration, so it will be easy to disable the option if it causes trouble with some storage domain types or image formats. With the change suggested, this command will become: /usr/bin/qemu-img convert -p -t none -T none -f qcow2 /rhev/data-center/mnt/nas1.bfit:_home_VMS/8e6bea49-9c62-4e31-a3c9-0be09c2fcdbf/images/21f438fb-0c0e-4bdc-abb3-64a7e033cff6/c256a972-4328-4833-984d-fa8e62f76be8 -O qcow2 -o compat=1.1 -W /rhev/data-center/mnt/nas1.bfit:_home_VMS/8e6bea49-9c62-4e31-a3c9-0be09c2fcdbf/images/5a90515c-066d-43fb-9313-5c7742f68146/ed6dc60d-1d6f-48b6-aa6e-0e7fb1ad96b9 You can test this in the shell without modifying vdsm to test how it affects performance. [1] https://bugzilla.redhat.com/1511891#c57 Nir