Two postgresql server with same configuration, different performance

I got two identical servers, in both is installed postgresql server version 9.0.4 with the same configuration. If I launch a .sql file that performs about 5k inserts, on the first one it takes a couple of seconds, on the second one it takes 1 minute and 30 seconds.

If I set synchronous_commit, speed dramatically reduces (as expected), and the performances of the two servers are comparable. But if I set synchronous_commit to on, on one server the insert script execution time increa开发者_如何转开发ses of less than one second, on the other one it increases too much, as I said in the first period.

Any idea about this difference in performances? Am I missing some configuration?

Update: tried a simple disk test: time sh -c "dd if=/dev/zero of=ddfile bs=8k count=200000 && sync"

fast server output:

1638400000 bytes (1.6 GB) copied, 1.73537 seconds, 944 MB/s

real    0m32.009s
user    0m0.018s
sys 0m2.298s

slow server output:

1638400000 bytes (1.6 GB) copied, 4.85727 s, 337 MB/s

real    0m35.045s
user    0m0.019s
sys 0m2.221s

Common features (both servers):

SATA, RAID1, controller: Intel Corporation 82801JI (ICH10 Family) SATA AHCI Controller, distribution: linux centOS. mount -v output:
/dev/md2 on / type ext3 (rw)
proc on /proc type proc (rw)
none on /dev/pts type devpts (rw,gid=5,mode=620)
/dev/md1 on /boot type ext3 (rw)

fast server: kernel 2.6.18-238.9.1.el5 #1 SMP

Disk /dev/sda: 750.1 GB, 750156374016 bytes
255 heads, 63 sectors/track, 91201 cylinders, total 1465149168 sectors
Units = sectors of 1 * 512 = 512 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            3906     4209029     2102562   fd  Linux raid autodetect
/dev/sda2         4209030     4739174      265072+  fd  Linux raid autodetect
/dev/sda3         4739175  1465144064   730202445   fd  Linux raid autodetect

slow server: kernel 2.6.32-71.29.1.el6.x86_64 #1 SMP

Disk /dev/sda: 750.2 GB, 750156374016 bytes
64 heads, 32 sectors/track, 715404 cylinders, total 1465149168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0006ffc4

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048     4194303     2096128   fd  Linux raid autodetect
/dev/sda2         4194304     5242879      524288   fd  Linux raid autodetect
/dev/sda3         5242880  1465147391   729952256   fd  Linux raid autodetect

Could it be useful to address the performance issue?

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I suppose your slow server with newer kernel has working barriers. This is good, as otherwise you can loose data in case of a power failure. But it is of course slower than running with write cache enabled and without barriers, aka running with scissors.

You can check if barriers are enabled using mount -v — search for barrier=1 in output. You can disable barriers for your filesystem (mount -o remount,barrier=0 /) to speed up, but then you risk data corruption.

Try to do your 5k inserts in one transaction — Postgres won't have to write to disk on every row inserted. The theoretical limit for number of transactions per second wound be comparable to disk rotational speed (7200rpm disk ≈ 7200/60 tps = 120 tps) as a disk can only write to a sector once per rotation.

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To me this sounds like in the "fast" server there is a write cache enbled for the harddisk(s), whereas in the slow server the harddisk(s) are really writing the data when PG writes it (by calling fsync)