Page History

...

In the VM-initiated snapshot, an application running in the VM would ensure it flushed the latest states on the disk,
and handshake to the Local Profile Server to initiate the snapshot. This is the most straightforward approach for us, but
has its drawbacks - once the flush operation is completed, the system is allowed to continue write operations.
This means that once we rolled back to one of the snapshots, the filesystem would likely have to use its fault-tolerance
the mechanism as if a sudden power-off happened. But at least, it will be guaranteed that it has the latest data from
the application.

Let's try to take a closer look at the list of steps for executing a command to create or rollback a snapshot in this approach.
It is also worth noting that a guest agent (for example, some future eve-guest-agent) must be running on the VM in order
for the user to initiate the creation or rollback of a snapshot from the VM. Why eve-guest-agent? Because in order to implement
this approach, there is a critical need to receive feedback from EVE. Thus, it will be either a completely new guest agent tailored
for work and needs with EVE OS, or a modified one (for example, based on qemu-ga, google-ga, or others)

...

1. The user must use the functionality of the application that runs on the VM and execute a command that will flush
   the cache of this application to the “physical” disk and completely end or suspend I/O in the application
   (if creating a snapshot, you can only suspend I/O if rollback to a snapshot, then you can complete all current
   write operations). In this case, everything depends on the functionality of the N application that runs on the VM.
   As a last resort, if the application does not support such functionality for creating snapshots/backups, then
   the user can always exit the application. The user will perform this step manually (or through a script) since
   we cannot adapt to any specific applications.
2. After the successful completion of step 1, the user, through the agent application, let it be eve-guest-agent for example,
   sends a command to create a snapshot, something like: eve-guest-agent snapshot --create /dev/sdb
3. The command is sent to EVE, and all the necessary conditions for creating/rolling back a snapshot
   are calculated on EVE and if:
   1. For example, if there is not enough free space, EVE will return an error on the eve-guest-agent VM about
      not having enough space to create a snapshot on EVE. There may also be another error that appeared
      in preparation for the requested operation. And the command is interrupted.
   2. Next step
4. EVE sends a command to the VM to do a sync and freeze the available filesystems.
5. EVE checks that the filesystems on the VM have been frozen.
6. EVE is running a command to create/rollback a snapshot
   1. If something went wrong, EVE sends an FS thaw command to the VM, and eve-guest-agent returns an error to the user
   2. Next step
7. EVE thaws the FS on the guest VM and checks that it has been thawed.
8. EVE notifies the user via eve-guest-agent that the operation was successful
9. Guest VM returns to a normal state (user returns the application to its normal state)

You can read more about this in the EVE Guest Agent proposal.

Controller-Initiated snapshot

...

The steps for this are rough as follows:

1. Receive a command from the controller to create or rollback a snapshot;
2. EVE The host sends a command to qemu-guest-agent to put the application into for create snapshot/backup state;
3. Receive a command from the controller to create or rollback a snapshot;
4. qemu-guest-agent executes some command or script;
5. EVE checking Checking the state of the file system in the VM;
6. Make sure EVE checking sure the file system is working normally;
7. Flush EVE checking flush RAM to disk and freeze the file system in the VM;
8. Make sure EVE checking sure the file system in the VM is frozen;
9. Create EVE create a new snapshot or rollback to an existing snapshot;
10. After completing the command with for snapshot, EVE send the command to unfreeze the file system in the VM;
11. Make sure EVE checking sure that the file system is unfrozen and works normally;
12. The host EVE sends a command to qemu-guest-agent to put the application into a normal state.
13. qemu-guest-agent executes some command or script;
14. EVE sends Sending up-to-date information to the controller.
15. VM goes back to normal updating of their volume.

^{*Between step-5 6 and step-9 10 the guest can not do updates.}

A similar process can be implemented through the EVE Guest Agent, which also will allow you to support work with snapshots and with the initiator - VM.

Implementing a plan of snapshot functionality in EVE

...

PR: Add implementation of functionality for working with snapshots #2607

EVE Guest Agent

QEMU Guest Agent