Multi backend config needs more detail [JIRA: PMT-62]

[roooms]

http://docs.basho.com/riak/1.4.12/ops/advanced/backends/multi/#Configuring-Multiple-Backends

Customer has requested the location for configuration tuples be documented for all options. Currently we have some examples but aren't clear on them all:

Multi-Backend settings
Certain settings, such as Bitcask's merge_window, are set per-node, rather than per-backend, and as such must be set in the top level backend sections of your app.config.

[hcs42]

These are the bitcask settings according to the Bitcask documentation:

• data_root -- Can be local according to Multi-Backend docs
• open_timeout
• sync_strategy
• max_file_size
• require_hint_crc
• io_mode
• merge_window -- Global-only according to Bitcast docs
• frag_merge_trigger
• dead_bytes_merge_trigger
• frag_threshold
• dead_bytes_threshold
• small_file_threshold
• log_needs_merge -- Global-only according to Bitcast docs
• max_fold_age
• max_fold_puts
• expiry_secs -- Can be local according to the example in Multi-Backend docs
• expiry_grace_time

The other backends have different settings. The LevelDB Backend and Memory Backend documentation pages don't talk about global vs local settings.

[roooms]

As an example, testing max_file_size suggests it is per-backend.

• Specify different values in {bitcask, []} (default) and {multi_backend, []} (specific)
• Set a buckets backend property to use the specific multi-backend
• Write data to a non-configured bucket and the bucket with custom backend
• Check file sizes

vagrant@riak:/var/lib/riak$ ls -l bitcask*/0/*
-rw-r--r-- 1 riak riak   460427 Mar 12 12:06 bitcask/0/1.bitcask.data
-rw-r--r-- 1 riak riak      286 Mar 12 12:06 bitcask/0/1.bitcask.hint
-rw-r--r-- 1 riak riak   460431 Mar 12 12:06 bitcask/0/2.bitcask.data
-rw-r--r-- 1 riak riak      291 Mar 12 12:06 bitcask/0/2.bitcask.hint
-rw-r--r-- 1 riak riak   460431 Mar 12 12:06 bitcask/0/3.bitcask.data
-rw-r--r-- 1 riak riak      291 Mar 12 12:06 bitcask/0/3.bitcask.hint
-rw-r--r-- 1 riak riak   394655 Mar 12 12:06 bitcask/0/4.bitcask.data
-rw-r--r-- 1 riak riak      234 Mar 12 12:06 bitcask/0/4.bitcask.hint
-rw------- 1 riak riak       44 Mar 12 12:06 bitcask/0/bitcask.write.lock
-rw-r--r-- 1 riak riak 12562768 Mar 12 12:06 bitcask_big/0/1.bitcask.data
-rw-r--r-- 1 riak riak     7028 Mar 12 12:06 bitcask_big/0/1.bitcask.hint
-rw------- 1 riak riak       48 Mar 12 12:06 bitcask_big/0/bitcask.write.lock

[lukebakken]

Still causing confusion:

http://lists.basho.com/pipermail/riak-users_lists.basho.com/2015-November/017810.html