Features/VMSnapshotEnchancement

VM Back up enhancement

This feature will enhance VM backup functionality, to make it possible taking internal/external snapshots lively, and make it works better with underlining components such as LVM.

• Name: Wenchao Xia
• Email: xiawenc@linux.vnet.ibm.com, xiaxia347os@163.com

General Summary

This feature would provide APIs that can do:

• 1 block device live snapshot as internal/external/blank delta data, export sync API for all type.
• 2 vmstate live save as internal/external data, export async API for external data, fix the size problem.
• 3 combination(internal block snapshot + internal vmstate save, internal block snapshot + external vmstate save, external block snapshot external vmstate save).
• 4 a way to screen dump in the time of snapshot complete.

Subtask Details

Now qemu support block device live external snapshot, live migration to file, static internal block snapshot + internal vmstate save, following are the blanks need to be filled:

• 1 expose block device live internal snapshot.
• 2 add and expose block device drain.
• 3 provide 1,2 together with block external snapshot in unified style.
• 4 make vmstate save lively.
• 5 add progress query.
• 6 fix the vmstate size issue.
• 7 add vmstate save to external file which have the format that qemu support.
• 8 provide vmstate save internal/external in unified style, user can specify whether cal GA FS freeze before complete, whether vm pause after complete.
• 9 add vm lively save interface in qemu(only for internal vmstate+ internal block snapshot, in which case the content is managed

by qemu).

• 10 related information retrieving enhancement as qmp/hmp interface.

General Goal

General goal from backup application persperctive:

Pic 1, principle for snapshot data for an application

Pic 2, general goal on backup server

User Cases

As show above, how deep it can recover to, resulting a choice of whether to save vmstate. Who take the action, resulting a choice of whether qemu write/read the snapshot itself. For question two, things is a bit complicated, who take the action, what type of qemu's action to be(internal/external), will resulting many cases. Following picture shows the general relationship of components, to complete a snapshot for VM:

pic3, co-operation relationship in the big picture

Typical cases: take LVM2 as an example as third party tool, vmstate save are optional, following are the typical cases:

• Case 1: external image snapshot data + external vmstate data

This is what qemu 1.3 support. Step:

 1 save vmstate to external place.
 2 blkdev-snapshot-sync each block device.
 3 Copy out data.
 4 Resume.

Todo:

 Fix the vmstate size issue(may introduce a new API), provide a interface integrate the calls.

Advantage:

 less dependence, chain and block bitmap are managed by qemu.

• Case 2: internal image snapshot data + external vmstate data(not draw in the picture)

Step:

 1 save vmstate to external place.
 2 pause VM(may call GA before).
 3 internal snapshot each block device.
 4 LVM create snapshot.
 5 resume.

Todo:

 Fix the vmstate size issue, add block internal snapshot support, provide an interface integrate the calls.

Advantage:

 Internal snapshot are a bit faster, qemu managed the block snapshot consistence.

Lack:

 live commit internal snapshots now, so fit more for desktop usage now.

• Case 3: internal image blank data (drain) + external vmstate data

Step:

 1 save vmstate to external place.
 2 pause VM(may call GA before).
 3 drain each block device.
 4 LVM create snapshot.
 5 resume.

Todo:

 Fix the vmstate size issue, add block device drain support, provide a interface integrate the calls.

Advantage:

 Fast, backing chain and block bitmap management can be offloaded from qemu to lower component, this also gives a chance to lower software/hardware to accelerate it.

• Case 4: internal image snapshot data + internal vmstate data

This is what qemu support as static method. Step:

 1 save vmstate to internal place.
 2 internal snapshot each block device.
 (3) pause the vm.
 (4) LVM create snapshot.
 (5) resume.

Todo:

 Fix the vmstate size issue, change it to commit.

Advantage:

 qemu can manage bitmap and backing chain, so it is consistent.

Lack:

 Can't delete internal snapshot lively, so fit better for desktop usage.

As a summary: focus on case 4, 1 for desktop usage on windows/Linux, focus on case 3, 1 for server usage on Linux.

API design

TBD.

Progress

TBD.