Hi Jeffrey, I think I described the problem wrong :) I don't want to do Java's memory GC. I want to do cassandra's GC - that is I want to "really" remove deleted rows from a column family and get my disc space back.
2012/8/31 Jeffrey Kesselman <jef...@gmail.com> > 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. >
