[pve-devel] qemu 7.0 : fleecing backup (aka: local temp write cache"

DERUMIER, Alexandre Alexandre.DERUMIER at groupe-cyllene.com
Sun Jul 31 17:58:26 CEST 2022


Currently, proxmox backup are using copy-before-write to have snapshot 

This works great until you have a slow  or remote storage with higher 
latencies than the vm storage.

This can slowdown a lot the vm is a lot of writes occur when backup is 
running. (or even can hang the vm if backup storage goes down during the 

qemu backup support since 1-2 years some kind of local write cache, 
called "fleecing".

When backup is running, if a block is not yet backuped, it's going to a 
temp local disk image.

Some improvements have been made in qemu 7.0, previously only qcow2 was 
allowed for the temp disk, now it's working with any disk


Could it be possible to implement same king of cache for proxmox backup ?

I think qemu implementation is a simple block filter (It's even possible 
to expose a virtual snasphot), so maybe it could be easy to implement it ?

This is really a blocker for me,I can't use pbs because I'm using nvme 
is production, and a 7200k hdd backup in a remote 200km site with 5ms 

If I could use temp nvme cache on the hypervisor, it could be wonderfull :)

I had opened a bugzilla some week ago with same kind of local cache idea 
(didn't known about qemu fleecing at this time, seem the way to go)


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