On Fri, 2006-03-10 at 23:57 -0800, David Lang wrote: > On Sat, 11 Mar 2006, Joost Kraaijeveld wrote: > > > On Fri, 2006-03-10 at 13:40 +0000, Richard Huxton wrote: > >> Your ATA disk is lying about disk caching being turned off. Assuming > >> each insert is in a separate transaction, then it's not going to do > >> 10,000 / 6 = 1667 transactions/sec - that's faster than it's rotational > >> speed. > > Could you explain the calculation? Why should the number of transactions > > be related to the rotational speed of the disk, without saying anything > > about the number of bytes per rotation? > > each transaction requires a sync to the disk, a sync requires a real > write (which you then wait for), so you can only do one transaction per > rotation. Not according to a conversation I had with Western Digital about the write performance of my own SATA disks. What I understand from their explanation their disk are limited by the MB/sec and not by the number of writes/second, e.g. I could write 50 MB/sec *in 1 bit/write* on my disk. This would suggest that the maximum transactions of my disk (overhead of OS and PostgreSQL ignored) would be 50MB / (transaction size in MB) per second. Or am I missing something (what would not surprise me, as I do not understand the perforance of my system at all ;-))? -- Groeten, Joost Kraaijeveld Askesis B.V. Molukkenstraat 14 6524NB Nijmegen tel: 024-3888063 / 06-51855277 fax: 024-3608416 e-mail: J.Kraaijeveld@xxxxxxxxxx web: www.askesis.nl
