It would be nice to get this info into the doc or at least a blog post. -- Jack Krupansky
On Tue, Mar 1, 2016 at 4:37 PM, Tyler Hobbs <ty...@datastax.com> wrote: > > On Tue, Mar 1, 2016 at 6:13 AM, Vlad <qa23d-...@yahoo.com> wrote: > >> So commit log can't keep more than memtable size, why is difference in >> commit log and memtables sizes? > > > In order to purge a commitlog segment, *all* memtables that contain data > from that segment must be flushed to disk. > > Suppose you have two tables: > - table A has extremely high throughput > - table B has low throughput > > Every commitlog segment will have a mixture of writes for table A and > table B. The memtable for table A will fill up rapidly and will be flushed > frequently. The memtable for table B will slowly filly up, and will not be > flushed often. Since table B's memtable isn't flushed, none of the commit > log segments can purged/recycled. Once the commitlog hits its size limit, > it will force a flush of table B. > > This behavior is good, because it allows table B to be flushed in large > chunks instead of hundreds of tiny sstables. If the commitlog space were > equal to the memtable space, Cassandra would have to force a flush of table > B's memtable approximately every time table A is flushed, despite being > much smaller. > > To summarize: if you use more than one table, it makes sense to have a > larger space for commitlog segments. > > -- > Tyler Hobbs > DataStax <http://datastax.com/> >
