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Such a process incurs significant I/O overhead and slows down the migration. This overhead can be avoided by doing a direct memory to memory copy, avoiding any disk I/O.+
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== Rationale ==
== Rationale ==
−For process tree migration, the biggest part of transfer data is the memory used by the processes. Therefore, optimizing this memory transfer would have the most benefit.
+For process tree migration, the biggest part of transfer data is the memory used by the processes. Therefore, optimizing this memory transfer would be beneficial.
+[[Image:Memory_migration_without_page_server.png|thumb|500px|right|Memory migration without page server]]
Without the page server, migrating the user memory pages consists of:
Without the page server, migrating the user memory pages consists of:
−* dumping the memory to files on disk;
+* dumping (writing) the memory to files on disk;
−* copying the files over network to the destination system;
+* reading the files and sending the data over the network to the destination host;
+* receiving files on the destination host, writing to files on disk;
* restoring by reading the files into memory.
* restoring by reading the files into memory.
−In other words, all the memory is written to the disk twice, and read from the disk also twice. It incurs significant I/O overhead and slows down the migration. The overhead is further multiplied when using <code>criu pre-dump</code> (such as for [[iterative migration]]), as in this case memory is dumped not once but a few times.
+A way to mitigate the disk I/O overhead is to use [[wikipedia:tmpfs|tmpfs]], a filesystem that use RAM as a storage (see [[Disk-less migration]] for details). This eliminates the disk I/O, but not the double read/write. To eliminate it, a page server mechanism was implemented.
+<br clear="both"/>
== Operation ==
== Operation ==
−[[Image:Memory_migration_with_page_server.png|thumb|500px|right|Memory migration with page server]]
+When using page server, the memory is dumped not to disk, but directly to network, thus eliminating any disk reads/writes on the source. On the destination system, a page server runs, receiving the data from network and writing it to files on disk or tmpfs.
+=== Pages deduplication ===
+When iterative memory dumping feature (<code>criu pre-dump</code>) is used, memory is sent to page server a few times. The page server can automatically deduplicate pages, by punching holes in a parent images where child is replacing the page. This functionality is turned on by <code>--auto-dedup</code> option of <code>criu page-server</code> command.
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+== Usage ==
+See https://asciinema.org/a/15847.
+== See also ==
+* [[P.Haul]]
+* [[Live migration]]
+* [[Lazy migration]]
+== External links ==
+* Using page server: https://asciinema.org/a/15847
[[Category: HOWTO]]
[[Category: HOWTO]]
[[Category: Development]]
[[Category: Development]]