-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Gregory Stark wrote: > "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. # date; dd if=/dev/sda8 of=/dev/null;date Tue Sep 11 14:27:36 EDT 2007 8385867+0 records in 8385867+0 records out 4293563904 bytes (4.3 GB) copied, 71.7648 seconds, 59.8 MB/s Tue Sep 11 14:28:48 EDT 2007 # date; dd bs=8192 if=/dev/sda8 of=/dev/null;date Tue Sep 11 14:29:15 EDT 2007 524116+1 records in 524116+1 records out 4293563904 bytes (4.3 GB) copied, 68.2595 seconds, 62.9 MB/s Tue Sep 11 14:30:23 EDT 2007 # date; dd bs=8192 if=/srv/dbms/dataA/pgsql/data/pg_xlog/000000010000002B0000002F of=/dev/null;date Tue Sep 11 14:34:25 EDT 2007 2048+0 records in 2048+0 records out 16777216 bytes (17 MB) copied, 0.272343 seconds, 61.6 MB/s Tue Sep 11 14:34:26 EDT 2007 The first two are the partition where the W.A.L. is in (and a bit more: [/srv/dbms/dataA/pgsql/data]# ls -l total 104 - -rw------- 1 postgres postgres 4 Aug 11 13:32 PG_VERSION drwx------ 5 postgres postgres 4096 Aug 11 13:32 base drwx------ 2 postgres postgres 4096 Sep 11 14:35 global drwx------ 2 postgres postgres 4096 Sep 10 18:58 pg_clog - -rw------- 1 postgres postgres 3396 Aug 11 13:32 pg_hba.conf - -rw------- 1 root root 3396 Aug 16 14:32 pg_hba.conf.dist - -rw------- 1 postgres postgres 1460 Aug 11 13:32 pg_ident.conf drwx------ 4 postgres postgres 4096 Aug 11 13:32 pg_multixact drwx------ 2 postgres postgres 4096 Sep 10 19:48 pg_subtrans drwx------ 2 postgres postgres 4096 Aug 12 16:14 pg_tblspc drwx------ 2 postgres postgres 4096 Aug 11 13:32 pg_twophase drwx------ 3 postgres postgres 4096 Sep 10 19:53 pg_xlog - -rw------- 1 postgres postgres 15527 Sep 8 00:35 postgresql.conf - -rw------- 1 postgres postgres 13659 Aug 11 13:32 postgresql.conf.dist - -rw------- 1 root root 15527 Sep 4 10:37 postgresql.conf~ - -rw------- 1 postgres postgres 56 Sep 8 08:12 postmaster.opts - -rw------- 1 postgres postgres 53 Sep 8 08:12 postmaster.pid It is tricky for me to find a big enough file to test. I tried one of the pg_xlog files, but I cannot easily copy from there because it acts a bit interactive and the time is mostly my time. If I copy it elsewhere and give it to non-root, then it is all in the cache, so it does not really read it. > > (Those assumptions is not necessarily valid for Postgres which is another > topic, but one that requires some empirical numbers before diving into.) > - -- .~. Jean-David Beyer Registered Linux User 85642. /V\ PGP-Key: 9A2FC99A Registered Machine 241939. /( )\ Shrewsbury, New Jersey http://counter.li.org ^^-^^ 14:30:04 up 33 days, 17:52, 1 user, load average: 5.50, 4.67, 4.29 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.5 (GNU/Linux) Comment: Using GnuPG with CentOS - http://enigmail.mozdev.org iD8DBQFG5uM4Ptu2XpovyZoRAhtlAKDFs5eP/CGIqB/z207j2dpwDSHOlwCfevp4 lBWn3b2GW6gesaq+l3Rbooc= =F4H6 -----END PGP SIGNATURE----- ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
