Cassandra at least used to do disc cleanup as a side effect of garbage collection through finalizers. (This is a mistake for the reason outlined below.)
It is important to understand that you can *never* "force* a gc in java. Even calling System.gc() is merely a hint to the VM. What you are doing is telling the VM that you are * willing* to give up some processor time right now to gc, how much it choses to actually collect or not collect is totally up to the VM. The *only* garbage collection guarantee in java is that it will make a "best effort" to collect what it can to avoid an out of memory exception at the time that it runs out of memory. You are not guaranteed when *if ever*, a given object will actually be collected. Since finalizers happen when an object is collected, and not when it becomes a candidate for collection, the same is true of the finalizer. You are not guaranteed when, if ever, it will run. On Fri, Aug 31, 2012 at 9:03 AM, Alexander Shutyaev <shuty...@gmail.com>wrote: > Hi All! > > I have a problem with using cassandra. Our application does a lot of > overwrites and deletes. If I understand correctly cassandra does not > actually delete these objects until gc_grace seconds have passed. I tried > to "force" gc by setting gc_grace to 0 on an existing column family and > running major compaction afterwards. However I did not get disk space back, > although I'm pretty much sure that my column family should occupy many > times fewer space. We have also a PostgreSQL db and we duplicate each > operation with data in both dbs. And the PosgreSQL table is much more > smaller than the corresponding cassandra's column family. Does anyone have > any suggestions on how can I analyze my problem? Or maybe I'm doing > something wrong and there is another way to force gc on an existing column > family. > > Thanks in advance, > Alexander > -- It's always darkest just before you are eaten by a grue.
