On Fri, Feb 20, 2009 at 4:42 PM, Nick Cummings <[email protected]> wrote: > 2) How much does hard drive cache size matter in a Linux system?=20 > Obviously, I understand that the HD cache is much faster than the disk > itself, but I would have assumed that the OS would already do caching of > data from the hard drive where possible, so it's hard to see how a > measly extra 8, 16, or 24 MB of cache could really make much of a > difference. But I don't know much about hardware, so there's probably > something important I'm missing here.
They serve quite different purposes. The kernel buffer cache exists primarily to avoid redundant reads/writes to disk, although it is also used in the kernel's elevator algorithm. However, the kernel has far less information about a drive's geometry (especially modern drives which do dynamic bad-block remapping and all kinds of other crazy magic), so this elevator algorithm basically just clusters reads/writes to similar locations in the disk as well as possible. The drive then uses its buffer space to implement its own request re-ordering, read-ahead, and so on, based on the much more accurate information it has about the drive geometry. In any kind of dynamic situation (basically anything more than linear reads or linear writes), a larger drive cache will buy you better performance. Dustin -- Storage Software Engineer http://www.zmanda.com
