linux-kernel - Re: Abysmal disk performance, how to debug?

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Date:	Sat, 27 Jan 2007 14:43:04 -0500
From:	Bill Davidsen <davidsen@....com>
To:	linux-kernel@...r.kernel.org
Cc:	Sunil Naidu <akula2.shark@...il.com>,
	Ismail Dönmez 
	<ismail@...dus.org.tr>, linux-kernel@...r.kernel.org
Subject:  Re: Abysmal disk performance, how to debug?

Willy Tarreau wrote:
> On Sat, Jan 20, 2007 at 02:56:20PM -0500, Stephen Clark wrote:
>> Sunil Naidu wrote:
>>
>>> On 1/20/07, Willy Tarreau <w@....eu> wrote:
>>>
>>>
>>>> It is not expected to increase write performance, but it should help
>>>> you do something else during that time, or also give more responsiveness
>>>> to Ctrl-C. It is possible that you have fast and slow RAM, or that your
>>>> video card uses shared memory which slows down some parts of memory
>>>> which are not used anymore with those parameters.
>>>>   
>>>>
>>> I did test some SATA drives, am getting these value for 2.6.20-rc5:-
>>>
>>> [sukhoi@...hoon ~]$ time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024
>>> 1024+0 records in
>>> 1024+0 records out
>>> 1073741824 bytes (1.1 GB) copied, 21.0962 seconds, 50.9 MB/s
>>>
>>> What can you suggest here w.r.t my RAM & disk?
>>>
>>>
>>>
>>>> Willy
>>>>   
>>>>
>>> Thanks,
>>>
>>> ~Akula2
>>> -
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>>>
>>>
>>>
>> Hi,
>> whitebook vbi s96f core 2 duo t5600 2gb hitachi ATA      HTS721060G9AT00 
>> using libata
>> time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024
>> 1024+0 records in
>> 1024+0 records out
>> 1073741824 bytes (1.1 GB) copied, 10.0092 seconds, 107 MB/s
>>
>> real    0m10.196s
>> user    0m0.004s
>> sys     0m3.440s
> 
> You have too much RAM, it's possible that writes did not complete before
> the end of your measurement. Try this instead :
> 
> $ time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024 | sync
> 
I'm not sure that does what you think it does, the sync runs first, and 
data is still in the cache. Replace sync with "/bin/echo RAN" if you 
doubt me.

What you want is this:
   sync;
   time bash -c "dd if=/dev/zero of=/tmp/1GB bs=1M count=1024; sync"
which will actually time the write with the time command.


-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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