Hi, Le Sat, 8 Sep 2007 18:51:44 +0100, "Ioannis Koltsidas" <[EMAIL PROTECTED]> a écrit :
> 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 ? Not for every read() : for uncached read()s only. E.g, if you read() 4 Kb from a disk, through the cache, it will in fact read 128 kb from the disk to the cache, and then return the 4 kb to your application. When you'll request the next 4 kb, they'll already be in cache. The read-ahead functionality improves performances for sequential read() operations. For random reads, read-ahead has no effect, or can even degrade performances. > 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 ? No. The multiple sector mode allows the disk driver to request for more than one sector at once. For example, to read 4 kb, without multiple sector mode, the disk driver would have to send 8 requests to the disk controller. Using multiple sector mode, it can simply ask for 8 sectors at once (4 kb). I would say that on current disks, multiple sector mode is always available. > I disabled the read_ahead feature (hdparm -A0 /dev/sda). Then I > repeated the above and got: You are making a confusion between software read-ahead and hardware look-ahead. Software read-ahead is done by the linux kernel, and can be configured using /sys/block/sda/queue/read_ahead_kb. Hardware look-ahead is configured using hdparm's -A option. For more information about hardware look-ahead, here is an excerpt from [1] : « Read Look Ahead Feature ----------------------- The Read Look Ahead feature improves performance by reading sectors into a buffer before the sectors are actually requested by the host. Assuming the disc is formatted for a 1:1 interleave, a series of sequential reads of small blocks of data, rather than a single large block, can cause a performance loss of up to one revolution per command. The performance loss occurs when the controller is busy sending the next Read command to the drive. During this interval, the next sectors to be read can spin past the head. If this occurs, reading the logical sector after the previously read sector requires an extra revolution of the disc. The Read Look Ahead feature improves performance by reading the logical sector after the previously read sector into a buffer for possible later use. » Sincerly, Thomas [1] http://stason.org/TULARC/pc/hard-drives-hdd/seagate/ST1201A-SWIFT-178MB-3-5-HH-IDE-AT.html -- PETAZZONI Thomas - [EMAIL PROTECTED] http://{thomas,sos,kos}.enix.org - Jabber: [EMAIL PROTECTED] http://{agenda,livret}dulibre.org - http://www.toulibre.org Fingerprint : 0BE1 4CF3 CEA4 AC9D CC6E 1624 F653 CB30 98D3 F7A7
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