arangodb/Documentation/Books/Users/AdministratingArango

!CHAPTER Administrating ArangoDB 

!SECTION AppendOnly/MVCC 

Instead of overwriting existing documents, ArangoDB will create a new version of 
modified documents. This is even the case when a document gets deleted. The
two benefits are:

* Objects can be stored coherently and compactly in the main memory.
* Objects are preserved, isolated writing and reading transactions allow
  accessing these objects for parallel operations.

The system collects obsolete versions as garbage, recognizing them as
forsaken. Garbage collection is asynchronous and runs parallel to other
processes.

!SECTION Mostly Memory/Durability

Database documents are stored in memory-mapped files. Per default, these
memory-mapped files are synced regularly but not instantly. This is often a good
tradeoff between storage performance and durability. If this level of durability
is too low for an application, the server can also sync all modifications to
disk instantly. This will give full durability but will come with a performance
penalty as each data modification will trigger a sync I/O operation.

!SUBSECTION Durability Configuration

!SUBSUBSECTION Global Configuration 

There are global configuration values for durability, which can be adjusted by
specifying the following configuration options:

@startDocuBlock databaseWaitForSync


@startDocuBlock databaseForceSyncProperties


@startDocuBlock WalLogfileSyncInterval


!SUBSUBSECTION Per-collection configuration

You can also configure the durability behavior on a per-collection basis.
Use the ArangoDB shell to change these properties.


@startDocuBlock collectionProperties


!SUBSUBSECTION Per-operation configuration

Many data-modification operations and also ArangoDB's transactions allow to specify 
a *waitForSync* attribute, which when set ensures the operation data has been 
synchronized to disk when the operation returns.

!SECTION Disk-Usage Configuration

The amount of disk space used by ArangoDB is determined by a few configuration
options. 

!SUBSECTION Global Configuration 

The total amount of disk storage required by ArangoDB is determined by the size of
the write-ahead logfiles plus the sizes of the collection journals and datafiles.

There are the following options for configuring the number and sizes of the write-ahead
logfiles:

<!-- arangod/Wal/LogfileManager.h -->
@startDocuBlock WalLogfileReserveLogfiles


<!-- arangod/Wal/LogfileManager.h -->
@startDocuBlock WalLogfileHistoricLogfiles


<!-- arangod/Wal/LogfileManager.h -->
@startDocuBlock WalLogfileSize


<!-- arangod/Wal/LogfileManager.h -->
@startDocuBlock WalLogfileAllowOversizeEntries


<!-- arangod/Wal/LogfileManager.h -->
@startDocuBlock WalLogfileSuppressShapeInformation


When data gets copied from the write-ahead logfiles into the journals or datafiles
of collections, files will be created on the collection level. How big these files
are is determined by the following global configuration value:

<!-- arangod/RestServer/ArangoServer.h -->
@startDocuBlock databaseMaximalJournalSize


!SUBSECTION Per-collection configuration

The journal size can also be adjusted on a per-collection level using the collection's
*properties* method.