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Doc - WAL: MMFile / RocksDB distinction (#4939)

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Documentation/Books/Manual/Architecture/WriteAheadLog.md
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@ -1,9 +1,7 @@
Write-ahead log 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.
Both storage engines use a form of write ahead logging (WAL).
Starting with version 2.2 ArangoDB stores all data-modification operation in 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 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. all the write operations that were executed on the server.
@ -12,9 +10,11 @@ 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 a replication setup when slaves need to replay the same sequence of operations as
on the master. on the master.
MMFiles WAL Details
-------------------
By default, each write-ahead logfile is 32 MiB in size. This size is configurable via the By default, each write-ahead logfile is 32 MiB in size. This size is configurable via the
option *--wal.logfile-size*. option *--wal.logfile-size*.
When a write-ahead logfile is full, it is set to read-only, and following operations will 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 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 spare logfiles in the background so switching logfiles should be fast. How many reserve
@ -38,3 +38,22 @@ them if required. How many collected logfiles will be kept before they get delet
configurable via the option *--wal.historic-logfiles*. configurable via the option *--wal.historic-logfiles*.
For all write-ahead log configuration options, please refer to the page [Write-ahead log options](../Administration/Configuration/Wal.md). For all write-ahead log configuration options, please refer to the page [Write-ahead log options](../Administration/Configuration/Wal.md).
RocksDB WAL Details
-------------------
The options mentioned above only apply for MMFiles. The WAL in the rocksdb storage engine
works slightly differently.
_Note:_ In rocksdb the WAL options are all prefixed with
`--rocksdb.*`. The `--wal.*` options do have no effect.
The individual RocksDB WAL files are per default about 64 MiB big. The size will always be proportionally
sized to the value specified via `--rocksdb.write-buffer-size`. The value specifies the amount of
data to build up in memory (backed by the unsorted WAL on disk) before converting it to a sorted on-disk file.
Larger values can increase performance, especially during bulk loads. Up to `--rocksdb.max-write-buffer-number`
write buffers may be held in memory at the same time, so you may wish to adjust this parameter to control memory usage. A larger write buffer will result in a longer recovery time the next time the database is opened.
The RocksDB WAL only contains committed transactions. This means you will never see partial transactions
in the replication log, but it also means transactions are tracked completely in-memory. In practice
this causes RocksDB transaction sizes to be limited, for more information see the [RocksDB Configuration](../Administration/Configuration/RocksDB.md)