Even if the gc call cleaned all files, it is not really acceptable on a decent sized cluster due to the impact full gc has on performance. Especially non-needed ones.
The delay in file deletion can also at times make it hard to see how much spare disk you actually have. We easily see 100% increase in disk use which extends for long periods of time before anything gets cleaned up. This can be quite misleading and I believe on a couple of occasions we seen short term full disk scenarios during testing as a result of cleanup not happening entirely when it should... Terje On Wed, Jun 15, 2011 at 11:50 PM, Shotaro Kamio <kamios...@gmail.com> wrote: > We've encountered the situation that compacted sstable files aren't > deleted after node repair. Even when gc is triggered via jmx, it > sometimes leaves compacted files. In a case, a lot of files are left. > Some files stay more than 10 hours already. There is no guarantee that > gc will cleanup all compacted sstable files. > > We have a great interest on the following ticket. > https://issues.apache.org/jira/browse/CASSANDRA-2521 > > > Regards, > Shotaro > > > On Fri, May 27, 2011 at 11:27 AM, Jeffrey Kesselman <jef...@gmail.com> > wrote: > > Im also not sure that will guarantee all space is cleaned up. It > > really depends on what you are doing inside Cassandra. If you have > > your on garbage collect that is just in some way tied to the gc run, > > then it will run when it runs. > > > > If otoh you are associating records in your storage with specific > > objects in memory and using one of the post-mortem hooks (finalize or > > PhantomReference) to tell you to clean up that particular record then > > its quite possible they wont all get cleaned up. In general hotspot > > does not find and clean every candidate object on every GC run. It > > starts with the easiest/fastest to find and then sees what more it > > thinks it needs to do to create enough memory for anticipated near > > future needs. > > > > On Thu, May 26, 2011 at 10:16 PM, Jonathan Ellis <jbel...@gmail.com> > wrote: > >> In summary, system.gc works fine unless you've deliberately done > >> something like setting the -XX:-DisableExplicitGC flag. > >> > >> On Thu, May 26, 2011 at 5:58 PM, Konstantin Naryshkin > >> <konstant...@a-bb.net> wrote: > >>> So, in summary, there is no way to predictably and efficiently tell > Cassandra to get rid of all of the extra space it is using on disk? > >>> > >>> ----- Original Message ----- > >>> From: "Jeffrey Kesselman" <jef...@gmail.com> > >>> To: user@cassandra.apache.org > >>> Sent: Thursday, May 26, 2011 8:57:49 PM > >>> Subject: Re: Forcing Cassandra to free up some space > >>> > >>> Which JVM? Which collector? There have been and continue to be many. > >>> > >>> Hotspot itself supports a number of different collectors with > >>> different behaviors. Many of them do not collect every candidate on > >>> every gc, but merely the easiest ones to find. This is why depending > >>> on finalizers is a *bad* idea in java code. They may well never get > >>> run. (Finalizer is one of a few features the Sun Java team always > >>> regretted putting in Java to start with. It has caused quite a few > >>> application problems over the years) > >>> > >>> The really important thing is that NONE of these behaviors of the > >>> colelctors are guaranteed by specification not to change from version > >>> to version. Basing your code on non-specified behaviors is a good way > >>> to hit mysterious failures on updates. > >>> > >>> For instance, in the mid 90s, IBM had a mode of their Vm called > >>> "infinite heap." it *never* garbage collected, even if you called > >>> System.gc. Instead it just threw away address space and counted on > >>> the total memory needs for the life of the program being less then the > >>> total addressable space of the processor. > >>> > >>> It was *very* fast for certain kinds of applications. > >>> > >>> Far from being pedantic, not depending on undocumented behavior is > >>> simply good engineering. > >>> > >>> > >>> On Thu, May 26, 2011 at 4:51 PM, Jonathan Ellis <jbel...@gmail.com> > wrote: > >>>> I've read the relevant source. While you're pedantically correct re > >>>> the spec, you're wrong as to what the JVM actually does. > >>>> > >>>> On Thu, May 26, 2011 at 3:14 PM, Jeffrey Kesselman <jef...@gmail.com> > wrote: > >>>>> Some references... > >>>>> > >>>>> "An object enters an unreachable state when no more strong references > >>>>> to it exist. When an object is unreachable, it is a candidate for > >>>>> collection. Note the wording: Just because an object is a candidate > >>>>> for collection doesn't mean it will be immediately collected. The JVM > >>>>> is free to delay collection until there is an immediate need for the > >>>>> memory being consumed by the object." > >>>>> > >>>>> > http://java.sun.com/docs/books/performance/1st_edition/html/JPAppGC.fm.html#998394 > >>>>> > >>>>> and "Calling the gc method suggests that the Java Virtual Machine > >>>>> expend effort toward recycling unused objects" > >>>>> > >>>>> > http://download.oracle.com/javase/6/docs/api/java/lang/System.html#gc() > >>>>> > >>>>> It goes on to say that the VM will make a "best effort", but "best > >>>>> effort" is *deliberately* left up to the definition of the gc > >>>>> implementor. > >>>>> > >>>>> I guess you missed the many lectures I have given on this subject > over > >>>>> the years at Java One Conferences.... > >>>>> > >>>>> On Thu, May 26, 2011 at 3:53 PM, Jonathan Ellis <jbel...@gmail.com> > wrote: > >>>>>> It's a common misunderstanding that system.gc is only a suggestion; > on > >>>>>> any VM you're likely to run Cassandra on, System.gc will actually > >>>>>> invoke a full collection. > >>>>>> > >>>>>> On Thu, May 26, 2011 at 2:18 PM, Jeffrey Kesselman < > jef...@gmail.com> wrote: > >>>>>>> Actually this is no gaurantee. Its a common misunderstanding that > >>>>>>> System.gc "forces" gc. It does not. It is a suggestion only. The > vm always > >>>>>>> has the option as to when and how much it gcs > >>>>>>> > >>>>>>> On May 26, 2011 2:51 PM, "Jonathan Ellis" <jbel...@gmail.com> > wrote: > >>>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> -- > >>>>>> Jonathan Ellis > >>>>>> Project Chair, Apache Cassandra > >>>>>> co-founder of DataStax, the source for professional Cassandra > support > >>>>>> http://www.datastax.com > >>>>>> > >>>>> > >>>>> > >>>>> > >>>>> -- > >>>>> It's always darkest just before you are eaten by a grue. > >>>>> > >>>> > >>>> > >>>> > >>>> -- > >>>> Jonathan Ellis > >>>> Project Chair, Apache Cassandra > >>>> co-founder of DataStax, the source for professional Cassandra support > >>>> http://www.datastax.com > >>>> > >>> > >>> > >>> > >>> -- > >>> It's always darkest just before you are eaten by a grue. > >>> > >> > >> > >> > >> -- > >> Jonathan Ellis > >> Project Chair, Apache Cassandra > >> co-founder of DataStax, the source for professional Cassandra support > >> http://www.datastax.com > >> > > > > > > > > -- > > It's always darkest just before you are eaten by a grue. > > > > > > -- > Shotaro Kamio >
