Changes

When performing [[incremental dumps]] or [[iterative migration]], a layered stack of memory images is created. In that stack, some data is duplicated (i.e. same memory page is present in multiple images). This article describes ways to deduplicate such data by punching holes in image files (using <code>fallocate()</code> syscall with <code>FALLOC_FL_PUNCH_HOLE</code> flag), effectively freeing used disk space.

When performing [[incremental dumps]] or [[iterative migration]], a layered stack of memory images is created. In that stack, some data is duplicated (i.e. same memory page is present in multiple images). This article describes ways to deduplicate such data by punching holes in image files (using <code>fallocate()</code> syscall with <code>FALLOC_FL_PUNCH_HOLE</code> flag), effectively freeing used disk space.

Two ways to deduplicate memory images are available.

Two ways to deduplicate memory images are available.

== Deduplication action ==

=== Offline ===

The <code>criu dedup</code> command opens the image directory and punches holes in the ''parent'' images where ''child'' images would replace them.

The <code>criu dedup</code> command opens the image directory and punches holes in the ''parent'' images where ''child'' images would replace them.

== On the fly deduplication ==

=== On the fly ===

 

 

 

 

Deduplication only makes sense for incremental memory dumps. For now CRIU can only track changes, create incremental checkpoints and do dedup for anonymous memory. Changes tracking, increments and deduplication for shared memory is currently (August 2016) available in CRIU development branch.

 

== Implementation notes ==

The <code>--auto-dedup</code> option can be used for <code>criu dump</code> and <code>criu page-server</code>. It causes every write to images with process' pages to punch holes in the respective parent images.

Memory images are stored into two files: ''pagemap'' and ''pages'' (see [[memory dumps]] for details). Note that the deduplication process does not change ''pagemap'' in any way, it only punches holes in ''pages'' image files.

The option is extremely useful in [[disk-less migration]] scenario.

Note that having a hole in an image file have totally different meaning that is in no way similar to the one of '''in_parent''' flag in ''pagemap'' entry (described in [[memory dumps]]).

== See also ==

== See also ==

* [[Iterative migration]]

* [[Iterative migration]]

* [[Incremental dumps]]

* [[Incremental dumps]]

[[Category: Memory]]

[[Category: Memory]]

[[Category: Images]]

[[Category: Images]]