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
