[pve-devel] KVM Live Backup performance

[Cesar Peschiera]

@Dietmar or anyone that can answer:

Note:
These questions is because I am involved in a project and the idea is to use several RAID 10 in the same PVE Host for use of a VM, and for the backup a single SATA hard drive will be used on an NFS Server

Question 1:
Then, if the backup code don't use buffers for write cache, and my Storage of NFS Server Backup is very slow compared to my storage of Virtualization, when the VM need to write to his Virtual Disk, the behavior will be sync write to both disks (to the Storage of NFS Server Backup and to Virtual Disk of the VM)?

Reasoning:
Because if the answer is positive, for my understand we have slow writes within the same VM while "KVM Live Backup" is running because will write synchronously to both two disks  (to the Storage of NFS Server Backup and to Virtual Disk of the VM).

Question 2:
If the answer of the question1 is negative. ie "KVM Live Backup" don't sync the writes to both disks, how works "KVM Live Backup" in this case?

Best regards
Cesar

----- Original Message ----- 
From: "Dietmar Maurer" <dietmar at proxmox.com>
To: "Alessandro Briosi" <ab1 at metalit.com>; "Cesar Peschiera" <brain at click.com.py>; <pve-devel at pve.proxmox.com>
Sent: Tuesday, January 28, 2014 6:05 AM
Subject: RE: [pve-devel] KVM Live Backup performance


> He is complaining that the new code enables write cache during backup.
> If there's a VM which is running a database in an HA scenario, and for some
> reasons the VM/host crashes during the backup, the database would be
> inconsistent when started on another host, cause of the write cache.

AFAIK the backup code does not change caching behavior in any way.

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