"Jean-David Beyer" <jeandavid8@xxxxxxxxxxx> writes: > Gregory Stark wrote (in part): > >> The extra spindles speed up sequential i/o too so the ratio between sequential >> and random with prefetch would still be about 4.0. But the ratio between >> sequential and random without prefetch would be even higher. >> > I never figured out how extra spindles help sequential I-O because > consecutive logical blocks are not necessarily written consecutively in a > Linux or UNIX file system. They try to group a bunch (8 512-bit?) of blocks > together, but that is about it. So even if you are reading sequentially, the > head actuator may be seeking around anyway. That's somewhat true but good filesystems group a whole lot more than 8 blocks together. You can do benchmarks with dd and compare the speed of reading from a file with the speed of reading from the raw device. On typical consumer drives these days you'll get 50-60MB/s raw and I would expect not a whole lot less than that with a large ext2 file, at least if it's created all in one chunk on a not overly-full filesystem. (Those assumptions is not necessarily valid for Postgres which is another topic, but one that requires some empirical numbers before diving into.) -- Gregory Stark EnterpriseDB http://www.enterprisedb.com ---------------------------(end of broadcast)--------------------------- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
