Developing a buffer manager for a database system, I want to find out how many pages the operating system fetches from the disk on every read operation (so that, on every read() I read into the bufferpool as many pages as possible). My questions are the following: a) Does the value of /sys/block/sda/queue/read_ahead_kb (typically 128k) decide the amount of data that every read() fetches from the disk ? (this value can also be decided by hdparm -a /dev/sda, but is measured in sectors). If this is true, then I guess that each time I do a read() operation for 128 consecutive kbytes, I can be sure that at most one IO request will reach the disk. b) hdparm specifies that parameter -m specifies the sector count for the Multiple Sector mode (aka IDE Block mode), which "permits the transfer of multiple sectors per I/O interrupt, rather than the usual one sector per interrupt". Are these multiple sectors consecutive (physically) or not? Because I can't see any benefit if they are not consecutive. And if they are indeed consecutive, isn't it the same thing with read_ahead_kb ? c) Playing around a little with hdparm, I benchmarked my disk (Seagate ST340014A) using the --direct mode (which performs operations with O_DIRECT). Using read_ahead_kb = 128 i got the following (each measurement was repeated 3 times): $ sudo hdparm -t /dev/sda /dev/sda: Timing buffered disk reads: 160 MB in 3.03 seconds = 52.76 MB/sec Then i used --direct and got: $ sudo hdparm -t --direct /dev/sda /dev/sda: Timing O_DIRECT disk reads: 168 MB in 3.02 seconds = 55.69 MB/sec I disabled the read_ahead feature (hdparm -A0 /dev/sda). Then I repeated the above and got: $ sudo hdparm -t /dev/sda /dev/sda: Timing buffered disk reads: 30 MB in 3.03 seconds = 9.91 MB/sec and: $ sudo hdparm -t --direct /dev/sda /dev/sda: Timing O_DIRECT disk reads: 88 MB in 3.04 seconds = 28.98 MB/sec How do you explain this difference? Any help would be greatly appreciated... -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
