Curious where did you see this?
Thanks. -Wei Sent from my Samsung smartphone on AT&T -------- Original message -------- Subject: Re: unable to read saved rowcache from disk From: Manu Zhang <owenzhang1...@gmail.com> To: user@cassandra.apache.org CC: OOM at deserializing 747321th row On Thu, Nov 15, 2012 at 9:08 AM, Manu Zhang <owenzhang1...@gmail.com> wrote: 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. > >
