Bad iostat numbers

"Carlos H. Reimer" <carlos.reimer@xxxxxxxxxxxxx> · Thu, 30 Nov 2006 22:44:25 -0200

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