Mapped-ram — QEMU documentation

# Mapped-ram

Mapped-ram is a new stream format for the RAM section designed to supplement the existing `file:` migration and make it compatible with `multifd`. This enables parallel migration of a guest’s RAM to a file.

The core of the feature is to ensure that RAM pages are mapped directly to offsets in the resulting migration file. This enables the `multifd` threads to write exclusively to those offsets even if the guest is constantly dirtying pages (i.e. live migration). Another benefit is that the resulting file will have a bounded size, since pages which are dirtied multiple times will always go to a fixed location in the file, rather than constantly being added to a sequential stream. Having the pages at fixed offsets also allows the usage of O_DIRECT for save/restore of the migration stream as the pages are ensured to be written respecting O_DIRECT alignment restrictions.

## Usage

On both source and destination, enable the `multifd` and `mapped-ram` capabilities:

> `migrate_set_capability multifd on`

> `migrate_set_capability mapped-ram on`

Use a `file:` URL for migration:

> `migrate file:/path/to/migration/file`

Mapped-ram migration is best done non-live, i.e. by stopping the VM on the source side before migrating.

For best performance enable the `direct-io` parameter as well:

> `migrate_set_parameter direct-io on`

## Use-cases

The mapped-ram feature was designed for use cases where the migration stream will be directed to a file in the filesystem and not immediately restored on the destination VM[1]. These could be thought of as snapshots. We can further categorize them into live and non-live.

- Non-live snapshot

If the use case requires a VM to be stopped before taking a snapshot, that’s the ideal scenario for mapped-ram migration. Not having to track dirty pages, the migration will write the RAM pages to the disk as fast as it can.

Note: if a snapshot is taken of a running VM, but the VM will be stopped after the snapshot by the admin, then consider stopping it right before the snapshot to take benefit of the performance gains mentioned above.

- Live snapshot

If the use case requires that the VM keeps running during and after the snapshot operation, then mapped-ram migration can still be used, but will be less performant. Other strategies such as background-snapshot should be evaluated as well. One benefit of mapped-ram in this scenario is portability since background-snapshot depends on async dirty tracking (KVM_GET_DIRTY_LOG) which is not supported outside of Linux.

[1]

While this same effect could be obtained with the usage of snapshots or the `file:` migration alone, mapped-ram provides a performance increase for VMs with larger RAM sizes (10s to 100s of GiBs), specially if the VM has been stopped beforehand.

## RAM section format

Instead of having a sequential stream of pages that follow the RAMBlock headers, the dirty pages for a RAMBlock follow its header instead. This ensures that each RAM page has a fixed offset in the resulting migration file.

A bitmap is introduced to track which pages have been written in the migration file. Pages are written at a fixed location for every ramblock. Zero pages are ignored as they’d be zero in the destination migration as well.

    Without mapped-ram:                  With mapped-ram:

    ---------------------               --------------------------------
    | ramblock 1 header |               | ramblock 1 header            |
    ---------------------               --------------------------------
    | ramblock 2 header |               | ramblock 1 mapped-ram header |
    ---------------------               --------------------------------
    | ...               |               | padding to next 1MB boundary |
    ---------------------               | ...                          |
    | ramblock n header |               --------------------------------
    ---------------------               | ramblock 1 pages             |
    | RAM_SAVE_FLAG_EOS |               | ...                          |
    ---------------------               --------------------------------
    | stream of pages   |               | ramblock 2 header            |
    | (iter 1)          |               --------------------------------
    | ...               |               | ramblock 2 mapped-ram header |
    ---------------------               --------------------------------
    | RAM_SAVE_FLAG_EOS |               | padding to next 1MB boundary |
    ---------------------               | ...                          |
    | stream of pages   |               --------------------------------
    | (iter 2)          |               | ramblock 2 pages             |
    | ...               |               | ...                          |
    ---------------------               --------------------------------
    | ...               |               | ...                          |
    ---------------------               --------------------------------
                                        | RAM_SAVE_FLAG_EOS            |
                                        --------------------------------
                                        | ...                          |
                                        --------------------------------

where:

  - ramblock header: the generic information for a ramblock, such as idstr, used_len, etc.

  - ramblock mapped-ram header: the information added by this feature: bitmap of pages written, bitmap size and offset of pages in the migration file.

## Restrictions

Since pages are written to their relative offsets and out of order (due to the memory dirtying patterns), streaming channels such as sockets are not supported. A seekable channel such as a file is required. This can be verified in the QIOChannel by the presence of the QIO_CHANNEL_FEATURE_SEEKABLE.

The improvements brought by this feature apply only to guest physical RAM. Other types of memory such as VRAM are migrated as part of device states.