hi devin,
so there is only 1 disk in your system according to dmesg (a 500GB SATA
disk) and dd reports speeds > 1GB/s. you are either the lucky owner of a
wonder SATA disk or you are measuring linux caching ;)
when you run the tests, can you also run
watch grep Dirty /proc/meminfo
and check if it starts increasing, what the maximum is and when it
starts decreasing.
if you have default SL settings /proc/sys/vm/dirty_ratio is 40, so
that's 19GB of dirty memory allowed. with your dd you won't reach that,
but default /proc/sys/vm/dirty_expire_centisecs is 3000, so after 30
seconds, the system will start to write the dirty memory away, which is
probably when the "reduced performance" starts to hit you.
you can safely lower /proc/sys/vm/dirty_ratio to 2 (which is still
almost 1GB of dirty memory) to get a more stable performance (i think
recent kernel do this automated based on total size of memory)
and if you want to measure your disk performance, use iozone/bonnie/IOR
with sync options (or add 'sync' to you timing stats)
btw, try dstat (sort of iostat and vmstat combined, with colors ;)
stijn
> Hi Konstantin,
>
> Thank you for the reply. Here is the output of smartctl. We will see if a different drive model helps.
>
> Sincerely,
> Devin
>
> [root@lnx4103 ~]# smartctl -a /dev/sda
> smartctl version 5.38 [x86_64-redhat-linux-gnu] Copyright (C) 2002-8 Bruce Allen
> Home page is http://smartmontools.sourceforge.net/
>
> === START OF INFORMATION SECTION ===
> Device Model: WDC WD5002ABYS-02B1B0
> Serial Number: WD-WCASYDXXXXXX
> Firmware Version: 02.03B03
> User Capacity: 500,107,862,016 bytes
> Device is: Not in smartctl database [for details use: -P showall]
> ATA Version is: 8
> ATA Standard is: Exact ATA specification draft version not indicated
> Local Time is: Wed Sep 8 23:34:17 2010 EDT
> SMART support is: Available - device has SMART capability.
> SMART support is: Enabled
>
> === START OF READ SMART DATA SECTION ===
> SMART overall-health self-assessment test result: PASSED
>
> General SMART Values:
> Offline data collection status: (0x84) Offline data collection activity
> was suspended by an interrupting command from host.
> Auto Offline Data Collection: Enabled.
> Self-test execution status: ( 0) The previous self-test routine completed
> without error or no self-test has ever
> been run.
> Total time to complete Offline
> data collection: (9480) seconds.
> Offline data collection
> capabilities: (0x7b) SMART execute Offline immediate.
> Auto Offline data collection on/off support.
> Suspend Offline collection upon new
> command.
> Offline surface scan supported.
> Self-test supported.
> Conveyance Self-test supported.
> Selective Self-test supported.
> SMART capabilities: (0x0003) Saves SMART data before entering
> power-saving mode.
> Supports SMART auto save timer.
> Error logging capability: (0x01) Error logging supported.
> General Purpose Logging supported.
> Short self-test routine
> recommended polling time: ( 2) minutes.
> Extended self-test routine
> recommended polling time: ( 112) minutes.
> Conveyance self-test routine
> recommended polling time: ( 5) minutes.
> SCT capabilities: (0x303f) SCT Status supported.
> SCT Feature Control supported.
> SCT Data Table supported.
>
> SMART Attributes Data Structure revision number: 16
> Vendor Specific SMART Attributes with Thresholds:
> ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE
> 1 Raw_Read_Error_Rate 0x002f 197 197 051 Pre-fail Always - 900
> 3 Spin_Up_Time 0x0027 239 236 021 Pre-fail Always - 1050
> 4 Start_Stop_Count 0x0032 100 100 000 Old_age Always - 42
> 5 Reallocated_Sector_Ct 0x0033 200 200 140 Pre-fail Always - 0
> 7 Seek_Error_Rate 0x002e 200 200 000 Old_age Always - 0
> 9 Power_On_Hours 0x0032 100 100 000 Old_age Always - 401
> 10 Spin_Retry_Count 0x0032 100 253 000 Old_age Always - 0
> 11 Calibration_Retry_Count 0x0032 100 253 000 Old_age Always - 0
> 12 Power_Cycle_Count 0x0032 100 100 000 Old_age Always - 40
> 192 Power-Off_Retract_Count 0x0032 200 200 000 Old_age Always - 39
> 193 Load_Cycle_Count 0x0032 200 200 000 Old_age Always - 2
> 194 Temperature_Celsius 0x0022 121 112 000 Old_age Always - 26
> 196 Reallocated_Event_Count 0x0032 200 200 000 Old_age Always - 0
> 197 Current_Pending_Sector 0x0032 200 200 000 Old_age Always - 0
> 198 Offline_Uncorrectable 0x0030 100 253 000 Old_age Offline - 0
> 199 UDMA_CRC_Error_Count 0x0032 200 200 000 Old_age Always - 0
> 200 Multi_Zone_Error_Rate 0x0008 100 253 000 Old_age Offline - 0
>
> SMART Error Log Version: 1
> No Errors Logged
>
> SMART Self-test log structure revision number 1
> No self-tests have been logged. [To run self-tests, use: smartctl -t]
>
>
> SMART Selective self-test log data structure revision number 1
> SPAN MIN_LBA MAX_LBA CURRENT_TEST_STATUS
> 1 0 0 Not_testing
> 2 0 0 Not_testing
> 3 0 0 Not_testing
> 4 0 0 Not_testing
> 5 0 0 Not_testing
> Selective self-test flags (0x0):
> After scanning selected spans, do NOT read-scan remainder of disk.
> If Selective self-test is pending on power-up, resume after 0 minute delay.
>
>
> On Sep 8, 2010, at 6:46 PM, Konstantin Olchanski wrote:
>
> > On Wed, Sep 08, 2010 at 04:42:43PM -0400, Devin Bougie wrote:
> >> Hi, All. We are seeing periodic I/O delays on a new large compute node ...
> >
> >
> > Hi, you do not say what your disks are (model number as reported by smartctl),
> > but I have seen same symptoms as you describe with some WDC 2TB "advanced format" disks.
> > The problem vanished with the next shipment of these 2TB disks, which also happen
> > to have a slightly different model number.
> >
> > So I say, if you see disk delays, try different disks (different maker, different
> > model, different capacity).
> >
> >
> > --
> > Konstantin Olchanski
> > Data Acquisition Systems: The Bytes Must Flow!
> > Email: olchansk-at-triumf-dot-ca
> > Snail mail: 4004 Wesbrook Mall, TRIUMF, Vancouver, B.C., V6T 2A3, Canada
--
http://hasthelhcdestroyedtheearth.com/
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