Ensuring good performance
On large repositories, git depends on compression to keep the history information from taking up too much space on disk or in memory.
This compression is not performed automatically. Therefore you should occasionally run git gc:
$ git gc
to recompress the archive. This can be very time-consuming, so you may prefer to run git-gc when you are not doing other work.
The git fsck command runs a number of self-consistency checks on the repository, and reports on any problems. This may take some time. The most common warning by far is about “dangling” objects:
$ git fsck
dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb
dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f
dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e
dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085
dangling tree b24c2473f1fd3d91352a624795be026d64c8841f
Dangling objects are not a problem. At worst they may take up a little extra disk space. They can sometimes provide a last-resort method for recovering lost work.
Check original at http://book.git-scm.com/4_maintaining_git.html