MongoDB 2.8.0-rc3 is released

MongoDB 2.8.0-rc3 is out and is ready for testing. This is the culmination of the 2.7.x development series.

This release candidate includes an on-disk format change for the WiredTiger storage engine files. Users with pre-existing WiredTiger data files need to either create new data files or run mongod --storageEngine wiredTiger --repair to upgrade to this new format.

Also, this release candidate replaces the raw WiredTiger config strings with MongoDB configuration options.
See http://docs.mongodb.org/v2.8/reference/configuration-options/#storage.wiredTiger for details.

As a result of the WiredTiger configuration options work, there are also changes to the following startup options:

• --wiredTigerEngineConfig is now --wiredTigerEngineConfigString
• --wiredTigerCollectionConfig is now --wiredTigerCollectionConfigString
• --wiredTigerIndexConfig is now --wiredTigerIndexConfigString

Fixed in this release candidate:
SERVER-16222 Enable prefix compression by default on indexes in WiredTiger
SERVER-16313 Whitelist storage engine options for WiredTiger
SERVER-16353 listDatabases is very slow on wiredtiger primary
SERVER-16355 Extended pauses with heavy write load under WiredTiger
SERVER-16497 Index entry too long breaks foreground index builds, even with FailIndexKeyTooLong set to false
SERVER-16429 Concurrent mapReduces with same replacement collection fail with WT
SERVER-16235 Performance issue in capped collections under WiredTiger
SERVER-16173 mongod --repair not working under WiredTiger
SERVER-6981 maxPasses assertion (on allocation failure) can make capped collection unreadable

2.8 Release Notes: http://docs.mongodb.org/manual/release-notes/2.8
Downloads: http://www.mongodb.org/downloads
Change Log: https://jira.mongodb.org/browse/SERVER/fixforversion/14591

As always, please let us know of any issues.

Can you elaborate on SERVER-16313 Whiltelist storage engine options for WiredTiger?

Sure.  We've limited the number of options exposed to the user in terms of knobs to turn when using the WiredTiger storage engine.

The full list of supported options is here:

http://docs.mongodb.org/v2.8/reference/configuration-options/#storage.wiredTiger

But to summarize, this allows users to tune the following options at start up:

* What cache size to configure (the greater of 1/2 of available memory or 1GB by default)

* Whether to compress data using snappy, zlib or run without compression (use snappy by default)

* The checkpoint interval in seconds (60 secs by default)

* Whether to compress the write ahead journal using snappy, zlib or run without compression (use snappy by default)

* Whether to use a separate directory for indexes or not (off by default)

* Whether or not to use prefix compression for indexes (on by default)

* Whether to log WT internal statistics and at what interval (off by default)

In the next RC, we're also going to extend this to the createCollection and createIndexes commands, so it will look something like this:

> // this is not supported yet!

> db.createCollection('test',  {storageEngine: {wiredTiger: {blockCompressor: "zlib"}}});

There is also a "backdoor" to enable you to use all the nitty-gritty WiredTiger options you want (as before) using the following params:

--wiredTigerEngineConfigString

--wiredTigerCollectionConfigString

--wiredTigerIndexConfigString

We may decide to expose additional knobs as necessary, but the idea is to keep this as minimal as possible.