touch Command Added the touch command to read the data and/or indexes from a collection into memory. See: SERVER-2023 859 and touch for more information. 

indexCounters No Longer Report Sampled Data indexCounters now report actual counters that reflect index use and state. In previous versions, these data were sampled. See SERVER-5784 860 and indexCounters for more information. 

Padding Specifiable on compact Command See the documentation of the compact and the SERVER-4018 861 issue for more information. 

Added Build Flag to Use System Libraries The Boost library, version 1.49, is now embedded in the MongoDB code base. 

If you want to build MongoDB binaries using system Boost libraries, you can pass scons using the --use-system-boost flag, as follows: 

scons --use-system-boost 

When building MongoDB, you can also pass scons a flag to compile MongoDB using only system libraries rather than the included versions of the libraries. For example: 

scons --use-system-all 

See the SERVER-3829 862 and SERVER-5172 863 issues for more information. 

Memory Allocator Changed to TCMalloc To improve performance, MongoDB 2.2 uses the TCMalloc memory allocator from Google Perftools. For more information about this change see the SERVER-188 864 and SERVER- 4683$^{865}$. For more information about TCMalloc, see the documentation of TCMalloc 866 itself. 

## Replication 

Improved Logging for Replica Set Lag When secondary members of a replica set fall behind in replication, mongod now provides better reporting in the log. This makes it possible to track replication in general and iden- tify what process may produce errors or halt replication. See SERVER-3575 867 for more information. 

Replica Set Members can Sync from Specific Members The new replSetSyncFrom command and new rs.syncFrom() helper in the mongo shell make it possible for you to manually configure from which mem- ber of the set a replica will poll oplog entries. Use these commands to override the default selection logic if needed. Always exercise caution with replSetSyncFrom when overriding the default behavior. 

