On Thu, 22 Nov 2007, Gabor Gombas wrote: > On Tue, Nov 20, 2007 at 09:56:37AM -0800, David Lang wrote: > >> for cyrus you should have the same sort of requirements that you would have for >> a database server, including the fact that without a battery-backed disk cache >> (or solid state drive) to handle your updates, you end up being throttled by >> your disk rotation rate (you can only do a single fsync write per rotation, and >> that good only if you don't have to seek), RAID 5/6 arrays are even worse, as >> almost all systems will require a read of the entire stripe before writing a >> single block (and it's parity block) back out, and since the stripe is >> frequently larger then the OS readahead, the OS throws much of the data away >> immediatly. > > You're mixing things up. Readahead has absolutely zero influence on when > data is evicted from the cache. if the system is set to do a 1M readahead and to do that readahead it needs to read in 5M of data to verify the integrity, the system doesn't keep all 5M of data in it's cache, only the 1M that is it's readahead (or at least in some cases this is true) David Lang ---- Cyrus Home Page: http://cyrusimap.web.cmu.edu/ Cyrus Wiki/FAQ: http://cyrusimap.web.cmu.edu/twiki List Archives/Info: http://asg.web.cmu.edu/cyrus/mailing-list.html
