It is mentioned here btw: http://www.datastax.com/dev/blog/improving-jbod
On Wed, Feb 24, 2016 at 8:14 AM, Marcus Eriksson <krum...@gmail.com> wrote: > If you don't use RandomPartitioner/Murmur3Partitioner you will get the old > behavior. > > On Wed, Feb 24, 2016 at 2:47 AM, Jack Krupansky <jack.krupan...@gmail.com> > wrote: > >> I just wanted to confirm whether my understanding of how JBOD allocates >> device space is correct of not... >> >> Pre-3.2: >> On each memtable flush Cassandra will select the directory (device) which >> has the most available space as a percentage of the total available space >> on all of the listed directories/devices. A random weighted value is used >> so it won't always pick the same directory/device with the most space, the >> goal being to balance writes for performance. >> >> As of 3.2: >> The ranges of tokens stored on the local node will be evenly distributed >> among the configured storage devices - even by token range, even if that >> may be uneven by actual partition sizes. The code presumes that each of the >> configured local storage devices has the same capacity. >> >> The relevant change in 3.2 appears to be: >> Make sure tokens don't exist in several data directories (CASSANDRA-6696) >> >> The code for the pre-3.2 model is still in 3.x - is there some other code >> path which will cause the pre-3.2 behavior even when runing 3.2 or later? >> >> I see this code which seems to allow for at least some cases where the >> pre-3.2 behavior would still be invoked, but I'm not sure what user-level >> cases that might be: >> >> if (!cfs.getPartitioner().splitter().isPresent() || localRanges.isEmpty()) >> return Collections.singletonList(new >> FlushRunnable(lastReplayPosition.get(), txn)); >> >> return createFlushRunnables(localRanges, txn); >> >> IOW, if the partitioner does not have a splitter present or the >> localRanges for the node cannot be determined. But... what exactly would a >> user do to cause that? >> >> There is no doc for this stuff - can a committer (or adventurous user!) >> confirm what is actually implemented, both pre and post 3.2? (I already >> pinged docs on this.) >> >> Or if anybody is actually using JBOD, what behavior they are seeing for >> device space utilization. >> >> Thanks! >> >> -- Jack Krupansky >> > >
