Yes, that makes sense. If you never have a warm cache then it's probably disk seek time creating that latency, in which case there isn't a whole lot you can do about it short of adding more capacity (so at least it's cached at the OS level).
iostat -x could substantiate this guess. On Thu, May 6, 2010 at 12:56 PM, Ran Tavory <ran...@gmail.com> wrote: > Jonathan, I think it's the case of large values in the columns. The > problematic CF is a key-value store, so it has only one column per row, > however the value of that column can be large. It's a java serialized object > (uncompressed) which, may be 100s of bytes, maybe even a few megs. This CF > also suffers from zero cache hits since each time a read is for a unique > key. > I ran stress.py and I see much better results (reads are < 1ms) so I assume > my cluster is healthy, so I need to fix the app. Would 1meg bytes object > explain a 30ms (sometimes even more) read latency? The boxes aren't fancy, > not sure exactly what hardware we have there but it's "commodity"... > Thanks! > > On Thu, May 6, 2010 at 5:22 PM, Jonathan Ellis <jbel...@gmail.com> wrote: >> >> columns, not CFs. >> >> put another way, how wide are the rows in the slow CF? >> >> On Wed, May 5, 2010 at 11:30 PM, Ran Tavory <ran...@gmail.com> wrote: >> > I have a few CFs but the one I'm seeing slowness in, which is the one >> > with >> > plenty of cache misses has only one column per key. >> > Latency varies b/w 10m and 60ms but I'd say average is 30ms. >> > >> > On Thu, May 6, 2010 at 4:25 AM, Jonathan Ellis <jbel...@gmail.com> >> > wrote: >> >> >> >> How many columns are in the rows you are reading from? >> >> >> >> 30ms is quite high, so I suspect you have relatively large rows, in >> >> which case decreasing the column index threshold may help. >> >> -- >> Jonathan Ellis >> Project Chair, Apache Cassandra >> co-founder of Riptano, the source for professional Cassandra support >> http://riptano.com > > -- Jonathan Ellis Project Chair, Apache Cassandra co-founder of Riptano, the source for professional Cassandra support http://riptano.com
