oh, as for the number of rows, it's 1650000. How long would you expect it to be read back?
On Thu, Nov 15, 2012 at 3:57 AM, Wei Zhu <wz1...@yahoo.com> wrote: > Good information Edward. > For my case, we have good size of RAM (76G) and the heap is 8G. So I set > the row cache to be 800M as recommended. Our column is kind of big, so the > hit ratio for row cache is around 20%, so according to datastax, might just > turn the row cache altogether. > Anyway, for restart, it took about 2 minutes to load the row cache > > INFO [main] 2012-11-14 11:43:29,810 AutoSavingCache.java (line 108) > reading saved cache /var/lib/cassandra/saved_caches/XXX-f2-RowCache > INFO [main] 2012-11-14 11:45:12,612 ColumnFamilyStore.java (line 451) > completed loading (102801 ms; 21125 keys) row cache for XXX.f2 > > Just for comparison, our key is long, the disk usage for row cache is > 253K. (it only stores key when row cache is saved to disk, so 253KB/ 8bytes > = 31625 number of keys). It's about right... > So for 15MB, there could be a lot of "narrow" rows. (if the key is Long, > could be more than 1M rows) > > Thanks. > -Wei > ------------------------------ > *From:* Edward Capriolo <edlinuxg...@gmail.com> > *To:* user@cassandra.apache.org > *Sent:* Tuesday, November 13, 2012 11:13 PM > *Subject:* Re: unable to read saved rowcache from disk > > http://wiki.apache.org/cassandra/LargeDataSetConsiderations > > A negative side-effect of a large row-cache is start-up time. The > periodic saving of the row cache information only saves the keys that > are cached; the data has to be pre-fetched on start-up. On a large > data set, this is probably going to be seek-bound and the time it > takes to warm up the row cache will be linear with respect to the row > cache size (assuming sufficiently large amounts of data that the seek > bound I/O is not subject to optimization by disks) > > Assuming a row cache 15MB and the average row is 300 bytes, that could > be 50,000 entries. 4 hours seems like a long time to read back 50K > entries. Unless the source table was very large and you can only do a > small number / reads/sec. > > On Tue, Nov 13, 2012 at 9:47 PM, Manu Zhang <owenzhang1...@gmail.com> > wrote: > > "incorrect"... what do you mean? I think it's only 15MB, which is not > big. > > > > > > On Wed, Nov 14, 2012 at 10:38 AM, Edward Capriolo <edlinuxg...@gmail.com > > > > wrote: > >> > >> Yes the row cache "could be" incorrect so on startup cassandra verify > they > >> saved row cache by re reading. It takes a long time so do not save a > big row > >> cache. > >> > >> > >> On Tuesday, November 13, 2012, Manu Zhang <owenzhang1...@gmail.com> > wrote: > >> > I have a rowcache provieded by SerializingCacheProvider. > >> > The data that has been read into it is about 500MB, as claimed by > >> > jconsole. After saving cache, it is around 15MB on disk. Hence, I > suppose > >> > the size from jconsole is before serializing. > >> > Now while restarting Cassandra, it's unable to read saved rowcache > back. > >> > By "unable", I mean around 4 hours and I have to abort it and remove > cache > >> > so as not to suspend other tasks. > >> > Since the data aren't huge, why Cassandra can't read it back? > >> > My Cassandra is 1.2.0-beta2. > > > > > > >
