Everything as I thought, thank you! 2011/2/23 Matthew Dennis <mden...@datastax.com>
> Data is in Memtables from writes before they get flushed (based on first > threshold of ops/size/time exceeded; all are configurable) to SSTables on > disk. > > There is a keycache and a rowcache. The keycache caches offsets into > SSTables for the rows. the rowcache caches the entire row. There is also > the OS page cache which is heavily used. > > When a read happens, the keycache is updated with the information for the > SSTables the row was eventually found in. If there are too many entries now > in the keycache, some are ejected. Overall the keycache uses very little > memory per entry and can cut your disk IO in half so it's a pretty big win. > > If you read an entire row it goes in the row cache. Like the keycache, > this may result in older entries being ejected from the cache. If you > insert lots of really large rows in the rowcache you can OOM your JVM. The > rowcache is kept up to date with the memtables as writes come in. > > When a read comes in, C* will collect the data from the SSTables and > Memtables and merge them together but data only goes into Memtables from > writes. > > > On Tue, Feb 22, 2011 at 3:32 AM, Viktor Jevdokimov > <vjevdoki...@gmail.com>wrote: > >> Hello, >> >> Write path is perfectly documented in architecture overview. >> >> I need Reads to be clarified: >> >> How memory is used >> 1. When data is in the Memtable >> 2. When data is in the SSTable >> >> How cache is used alongside with Memtable? >> >> Are records created in the Memtable from writes only or from reads also? >> >> What I need to know is, how Cassandra uses memory and Memtables for reads? >> >> >> Thenk you, >> Viktor >> > >
