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From: Carlos H. Reimer <carlos.reimer@opendb.com.br>
To: pgsql-performance@postgresql.org
Subject: Bad iostat numbers
Date: Thu, 30 Nov 2006 22:44:25 -0200
Message-ID: <PEEPKDFEHHEMKBBFPOOKKEHCDKAA.carlos.reimer@opendb.com.br> (raw)

Hi,

I was called to find out why one of our PostgreSQL servers has not a
satisfatory response time. The server has only two SCSI disks configurated
as a RAID1 software.

While collecting performance data I discovered very bad numbers in the I/O
subsystem and I would like to know if I´m thinking correctly.

Here is a typical iostat -x:

avg-cpu:  %user   %nice %system %iowait   %idle

          50.40    0.00    0.50    1.10   48.00



Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s    rkB/s    wkB/s
avgrq-sz avgqu-sz   await  svctm  %util

sda          0.00   7.80  0.40  6.40   41.60  113.60    20.80    56.80
22.82 570697.50   10.59 147.06 100.00

sdb          0.20   7.80  0.60  6.40   40.00  113.60    20.00    56.80
21.94 570697.50    9.83 142.86 100.00

md1          0.00   0.00  1.20 13.40   81.60  107.20    40.80    53.60
12.93     0.00    0.00   0.00   0.00

md0          0.00   0.00  0.00  0.00    0.00    0.00     0.00     0.00
0.00     0.00    0.00   0.00   0.00



Are they not saturated?



What kind of parameters should I pay attention when comparing SCSI
controllers and disks? I would like to discover how much cache is present in
the controller, how can I find this value from Linux?



Thank you in advance!



dmesg output:

...

SCSI subsystem initialized

ACPI: PCI Interrupt 0000:04:02.0[A] -> GSI 18 (level, low) -> IRQ 18

scsi0 : Adaptec AIC79XX PCI-X SCSI HBA DRIVER, Rev 3.0

        <Adaptec (Dell OEM) 39320 Ultra320 SCSI adapter>

        aic7902: Ultra320 Wide Channel A, SCSI Id=7, PCI 33 or 66Mhz, 512
SCBs



  Vendor: SEAGATE   Model: ST336607LW        Rev: DS10

  Type:   Direct-Access                      ANSI SCSI revision: 03

 target0:0:0: asynchronous

scsi0:A:0:0: Tagged Queuing enabled.  Depth 4

 target0:0:0: Beginning Domain Validation

 target0:0:0: wide asynchronous

 target0:0:0: FAST-160 WIDE SCSI 320.0 MB/s DT IU QAS RDSTRM RTI WRFLOW
PCOMP (6.25 ns, offset 63)

 target0:0:0: Ending Domain Validation

SCSI device sda: 71132959 512-byte hdwr sectors (36420 MB)

sda: Write Protect is off

sda: Mode Sense: ab 00 10 08

SCSI device sda: drive cache: write back w/ FUA

SCSI device sda: 71132959 512-byte hdwr sectors (36420 MB)

sda: Write Protect is off

sda: Mode Sense: ab 00 10 08

SCSI device sda: drive cache: write back w/ FUA

 sda: sda1 sda2 sda3

sd 0:0:0:0: Attached scsi disk sda

  Vendor: SEAGATE   Model: ST336607LW        Rev: DS10

  Type:   Direct-Access                      ANSI SCSI revision: 03

 target0:0:1: asynchronous

scsi0:A:1:0: Tagged Queuing enabled.  Depth 4

 target0:0:1: Beginning Domain Validation

 target0:0:1: wide asynchronous

 target0:0:1: FAST-160 WIDE SCSI 320.0 MB/s DT IU QAS RDSTRM RTI WRFLOW
PCOMP (6.25 ns, offset 63)

 target0:0:1: Ending Domain Validation

SCSI device sdb: 71132959 512-byte hdwr sectors (36420 MB)

sdb: Write Protect is off

sdb: Mode Sense: ab 00 10 08

SCSI device sdb: drive cache: write back w/ FUA

SCSI device sdb: 71132959 512-byte hdwr sectors (36420 MB)

sdb: Write Protect is off

sdb: Mode Sense: ab 00 10 08

SCSI device sdb: drive cache: write back w/ FUA

 sdb: sdb1 sdb2 sdb3

sd 0:0:1:0: Attached scsi disk sdb

ACPI: PCI Interrupt 0000:04:02.1[B] -> GSI 19 (level, low) -> IRQ 19

scsi1 : Adaptec AIC79XX PCI-X SCSI HBA DRIVER, Rev 3.0

        <Adaptec (Dell OEM) 39320 Ultra320 SCSI adapter>

        aic7902: Ultra320 Wide Channel B, SCSI Id=7, PCI 33 or 66Mhz, 512
SCBs

...



Reimer

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