On 6/30/12, Cameron Byrne <cb.li...@gmail.com> wrote: > On Jun 30, 2012 12:25 AM, "joel jaeggli" <joe...@bogus.com> wrote: >> On 6/30/12 12:11 AM, Tyler Haske wrote: > Geo-redundancy is key. In fact, i would take distributed data centers over > RAID, UPS, or any other "fancy pants" © mechanisms any day.
Geo-redundancy is more expensive than any of those technologies, because it directly impacts every application and reduces performance. It means that, for example, if an application needs to guarantee something is persisted to a distributed database, such as a record that such and such user's credit card has just been charged $X or such and such user has uploaded this blob to the web service ; The round trip time of the longest latency path between any of the redundancy sites, is added to the critical path of the WRITE transaction latency during the commit stage. Because you cannot complete a transaction and ensure you have consistency or correct data, until that transaction reaches a system at the remote site managing the persistence, and is acknowledged as received intact. For example, if you have geo sites, which are a minimum of 250 miles apart; if you recall, light only travels 186.28 miles per millisecond. That means you have a 500 mile round-trip and therefore have added a bare minimum of 2.6 milliseconds of latency to every write transaction, and probably more like 15 milliseconds. If your original transaction latency was at 1 milliseconds. or 1000 transactions per second, AND you require only that the data reaches the remote site and is acknowledged (not that the transaction succeeds at the remote site, before you commit), you are now at a minimum of 2.6 milliseconds average 384 transactions per second. To actually do it safely, you require 3.6 milliseconds, limiting you to an average of 277 transactions per second. If the application is not specially designed for remote site redundancy, then this means you require a scheme such as synchronous storage-level replication to achieve clustering; which has even worse results if there is significant geographic dispersion. RAID transactional latencies are much lower. UPSes and redundant power do not increase transaction latencies at all. -- -JH
