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jsteemann 2017-02-21 15:49:47 +01:00
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arangod/MMFiles/README.md
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MMFilesEngine MMFilesEngine
============= =============
How operations are Stored - Overview How operations are stored - Overview
------------------------------------ ------------------------------------
All operations like insert or delete are written as markers to a write ahead All operations like insert or remove are written as markers to a write ahead
log (WAL). This WAL consists of slices of a certain size and if such a slice is log (WAL). This WAL consists of files of a certain size and if such a file is
full for is manually flushed all relevant markers are transferred full (or is manually flushed), all relevant markers are transferred
(transferMarkers()) to the journals of the respective collections. During the (transferMarkers()) to the journals of the respective collections. During the
transfer obsolete markers will be thrown away: a sequence of insert, remove, transfer any obsolete markers will be thrown away: a sequence of insert, remove,
insert on the same document will result in the last insert discarding the insert on the same document will result in the last insert discarding the
previous operations. When a journal file of size (journalSize()) is full it previous operations. When a journal file of size (journalSize()) is full, it
will be sealed and renamed. By applying this operations it will become a will be sealed and renamed. By applying these operations it will become a
datafile, that is read only. Datafiles will eventually be merged by a compactor datafile that is read-only. Datafiles will eventually be merged by a compactor
task/thread, that does the about the same work as the transferMarkers function thread that does the about the same work as the transferMarkers function,
reducing the size of the stored data. reducing the size of the stored data.
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