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From Jeffrey Kesselman <>
Subject Re: force gc?
Date Fri, 31 Aug 2012 16:50:40 GMT
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 <>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.

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