Also, keep in mind writes to O_DIRECT devices have to wait for the data to get on the platters rather than into the kernel cache.
--------------------------------------------------------------------------- Tom Lane wrote: > "Jim C. Nasby" <[EMAIL PROTECTED]> writes: > >> DB2 and Oracle, from memory, allow users to pass hints to the planner to > >> use/not use file system caching. > > > Might it make sense to do this for on-disk sorts, since sort_mem is > > essentially being used as a disk cache (at least for reads)? > > If sort_mem were actually being used that way, it might be ... but it > isn't, and so I doubt O_DIRECT would be an improvement. It seems more > likely to force disk I/O that otherwise might not happen at all, if the > kernel happens to have sufficient buffer space on hand. > > I'll concede though that a large sort would probably have the effect of > blowing out the kernel's disk cache. So while O_DIRECT might be a net > pessimization as far as the sort itself is concerned, it would probably > be more friendly to the rest of the system, by leaving disk buffers free > for more productive uses. It'd all depend on your workload ... > > regards, tom lane > > ---------------------------(end of broadcast)--------------------------- > TIP 7: don't forget to increase your free space map settings > -- Bruce Momjian | http://candle.pha.pa.us [EMAIL PROTECTED] | (610) 359-1001 + If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania 19073 ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org
