To expand on what Robert said, Cassandra is a log-structured database: - writes are append operations, so both correctly configured disk volumes and SSD are fast at that
- reads could be helped by SSD if they're not in cache (ie. on disk) - but compaction is definitely helped by SSD with large data loads (compaction is the trade-off for fast writes) Thanks, James Briggs. -- Cassandra/MySQL DBA. Available in San Jose area or remote. Mailbox dimensions: 10"x12"x14" ________________________________ From: Robert Coli <rc...@eventbrite.com> To: "user@cassandra.apache.org" <user@cassandra.apache.org> Sent: Tuesday, September 16, 2014 5:42 PM Subject: Re: no change observed in read latency after switching from EBS to SSD storage On Tue, Sep 16, 2014 at 5:35 PM, Mohammed Guller <moham...@glassbeam.com> wrote: Does anyone have insight as to why we don't see any performance impact on the reads going from EBS to SSD? > What does it say when you enable tracing on this CQL query? 10 seconds is a really long time to access anything in Cassandra. There is, generally speaking, a reason why the default timeouts are lower than this. My conjecture is that the data in question was previously being served from the page cache and is now being served from SSD. You have, in switching from EBS-plus-page-cache to SSD successfully proved that SSD and RAM are both very fast. There is also a strong suggestion that whatever access pattern you are using is not bounded by disk performance. =Rob
