Backup in Stop mode

[Marek]

Hi - when we are doing backups, we use stop mode. It looks like this - VM is stopped than archive is made than VM is starting. When VM is online and running backup job is still running. I am struggling to find place where exactly the DIFF is placed.I know there is no snapshot involved but there must be diff stored somewhere so backup tool has full access to not changed yet state of VM.What i mean- We did some tests - while backup was running, we started to fill VHD with random files so before backup ended VHD was almost totally different - this information(difference between current state and backup`s one) must be stored somewhere - and my question is where?

 

[dcsapak]

this is built-in qemu functionality. the process is like this:
1. vm gets shutdown
2. qemu process for vm gets started but vm is not yet exectued
3. qemu creates a bitmap for the disks it is going to backup
4. the backup starts (every block written to the backup gets marked on the bitmap)
5. the vm execution starts
6. every time the vm wants to write to a block, qemu checks if it is already backed up
- if yes -> continue
- if no -> block the write, backup the block and let the vm continue

this is the same procedure for qemu vms regardless of the backup mode (i.e. snapshot,suspend,stop), the only difference is if the vm is shutdown or suspended before the backup begins

 

[Marek]

Thanks for great reply! Please tell me where are written blocks stored - the default path to it.

 

[dcsapak]

Thanks for great reply! Please tell me where are written blocks stored - the default path to it.

what exactly do you mean ? the bitmap? this is a qemu internal structure which (i guess) is held in memory

 

[Marek]

Hey let me explain - we have 2TB drive - virtual drive - we start backup, meantime (while backup) we are trying to change whole 2TBs with random data.This process is faster then backup itself.We are done with changes while backup is at 10% so in this case 90% of unchanged data should be stored somewhere.Where it is? We are afraid that we will run out of space for other VMs. Where is snapshot stored? (or something this kind) You know when we are changing disk while running backup, proxmost MUST reach for unchanged data - from beginning of this process. Process is not instant so it must store unchanged sectors somewhere as disk is already totally different.

 

[dcsapak]

Hey let me explain - we have 2TB drive - virtual drive - we start backup, meantime (while backup) we are trying to change whole 2TBs with random data.This process is faster then backup itself.We are done with changes while backup is at 10% so in this case 90% of unchanged data should be stored somewhere.Where it is? We are afraid that we will run out of space for other VMs. Where is snapshot stored? (or something this kind) You know when we are changing disk while running backup, proxmost MUST reach for unchanged data - from beginning of this process. Process is not instant so it must store unchanged sectors somewhere as disk is already totally different.

for vms, the process runs as i described, there is no snapshot stored, so if you use qemu vms i guess your writes are not fully synced to the guest disk (how do you write data to your virtual disk? what is the cache mode of the disk?)

if you use containers (and accidentally wrote vm), then yes there is a backup snapshot, where this is depends on the underlying storage