RE: Miserable Disks
- From: "Kevin Closson" <kevinc@xxxxxxxxxxxxx>
- To: <charlottejanehammond@xxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 24 May 2006 09:48:53 -0700
>>>But even together I didn't expect things to be *this* bad.
>>>Am I looking for a non-existent fourth problem?
>>>:-) My current focus is on the 512-byte writes seen on
>>>iostat - any ideas why I could be seeing this?
No doubt you are unhappy with the performance and to your
credit, Charlotte, you are not surprised by the performance.
Shame on whoever chose this technology for you.
Some points. First, 2 second iostat resolution is very
unreliable. Generally iostat is better on a 15-30 second
interval in my experience. Second, you are complaining about 512
byte writes. I don't see those writes. What I see is
875 sectors by 35 transfers or 448000 Bytes. That is roughly
an average of 8.6KB per transfer. Please tell us your Oracle
block size is 8K? The .6 is likely for ext3 everhead such as
metadata reads or bogus readaheads that may or may not have resulted
in a bread hit. A real filesystem that supports real direct I/O
(guess where you can get one of those :-) ) would not have this
slight overhead. That aside, the overhead is at the noise level.
Also, (NOTE TO ALL FOLKS BECAUSE I'VE SEEN THIS "THROW ASYNC
AT A DISK LATENCY ISSUE" THING ON THIS LIST A LOT), async I/O
does NOT reduce disk transfer times. A DMA is a DMA, period. In fact,
the
performance data you are looking at is completely unaware
of whether or not the I/O request at the library level was
async or not. It is just a scsi request to iostat.
Async I/O only relates to the processor efficiency related
to a given I/O workload.
In summary, you are not seeing 512 Byte transfers--or at least
not average transfers as I'm sure there may well single
sector cntrl and redo log xfers mixed in to the average.
avg-cpu: %user %nice %sys %iowait %idle
0.64 0.00 0.13 8.71 90.52
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
rkB/s wkB/s avgrq-sz avgqu-sz await svctm
%util
sdc 0.00 39.18 76.29 35.05 1321.65 875.26
660.82 437.63 19.73 1.19 10.75 9.00
100.26
>>>
>>>
>>>Thanks
>>>Charlotte
>>>
>>>--- Dennis Williams <oracledba.williams@xxxxxxxxx>
>>>wrote:
>>>
>>>> Charlotte,
>>>>
>>>> Is this directly attached to the server or is it NAS?
>>>>
>>>> Dennis Williams
>>>>
>>>
>>>
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>>>
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