<commit_before>Write-ahead log
===============
The Write-ahead log is part of the MMFiles storage engine; This doesn't apply to your
ArangoDB if you are running with the [RocksDB](../../Administration/Configuration/RocksDB.md)
storage engine.

Starting with version 2.2 ArangoDB stores all data-modification operation in
its write-ahead log. The write-ahead log is sequence of append-only files containing
all the write operations that were executed on the server.

It is used to run data recovery after a server crash, and can also be used in
a replication setup when slaves need to replay the same sequence of operations as
on the master.

By default, each write-ahead logfile is 32 MB big. This size is configurable via the
option *--wal.logfile-size*.

When a write-ahead logfile is full, it is set to read-only, and following operations will
be written into the next write-ahead logfile. By default, ArangoDB will reserve some
spare logfiles in the background so switching logfiles should be fast. How many reserve
logfiles ArangoDB will try to keep available in the background can be controlled by the
configuration option *--wal.reserve-logfiles*.

Data contained in full datafiles will eventually be transferred into the journals or
datafiles of collections. Only the "surviving" documents will be copied over. When all<commit_msg>Feature/documentation storageengine (#2620)<commit_after>Write-ahead log
===============
The Write-ahead log is part of the MMFiles storage engine; This doesn't apply to your
ArangoDB if you are running with the [RocksDB](../Administration/Configuration/RocksDB.md)
storage engine.

Starting with version 2.2 ArangoDB stores all data-modification operation in
its write-ahead log. The write-ahead log is sequence of append-only files containing
all the write operations that were executed on the server.

It is used to run data recovery after a server crash, and can also be used in
a replication setup when slaves need to replay the same sequence of operations as
on the master.

By default, each write-ahead logfile is 32 MB big. This size is configurable via the
option *--wal.logfile-size*.

When a write-ahead logfile is full, it is set to read-only, and following operations will
be written into the next write-ahead logfile. By default, ArangoDB will reserve some
spare logfiles in the background so switching logfiles should be fast. How many reserve
logfiles ArangoDB will try to keep available in the background can be controlled by the
configuration option *--wal.reserve-logfiles*.

Data contained in full datafiles will eventually be transferred into the journals or
datafiles of collections. Only the "surviving" documents will be copied over. When all
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