Re: Harddisks: Seek, Read, Write, Read, Write, Slow ?
From: Nathan McNulty (525676_at_betaweb.com)
Date: 07/14/04
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Date: Wed, 14 Jul 2004 13:30:12 -0700
Actually, what is happening is that the heads on the platter need to
reconfigure themselves each time you read or write. It is a hardware
limitation. By reading, the head can stay in the same location, but
once it is told to write, the head has to position itself to change the
magnetism of the sector of the platter it is on. Then once you request
to read again, the head has to move again. It is the same as seek time
when you have two files on different partitions it is going to take
longer to find those files than if they were right next to each other.
Nathan McNulty
kony wrote:
> On Wed, 14 Jul 2004 17:53:52 +0200, "Skybuck Flying"
> <nospam@hotmail.com> wrote:
>
>
>>Hi,
>>
>>Take these 3 concepts and then look at the implementation/performance:
>>
>>( 3 Concepts for reading/writing with harddisks )
>>
>>Concept 1:
>>
>>Seek, Read, Read, Read, Read, Read, Etc
>>
>>Results: FAST
>>
>>Concept 2:
>>
>>Seek, Write, Write, Write, Write, Write, Etc
>>
>>Results: FAST
>>
>>Concept 3:
>>
>>Seek, Read, Write, Read, Write, Read, Write,
>>
>>Results: SLOW ???
>>
>>The pseudo code is like:
>>
>>Seek( 0 ); // offset 0
>>
>>For I:=0 to FileBlocks-1 do // number of 4KB blocks in file.
>>begin
>> if Random(2) = 0 then
>> Read( 4 KB )
>> Else
>> Write( 4 KB );
>>end;
>>
>>The original concept is:
>>
>>Do a seek once.
>>
>>Then read or write a block of data. The head is automatically forwarded to
>>the next block. So no extra seek is needed.
>>
>>For concept 1 and concept 2 this works just fine and gives good performance.
>>
>>However concept 3 has very bad performance.
>>
>>Is this a software issue ? ( Windows XP )
>>
>>Is this a hardware issue ? ( Harddisk Read Head and Harddisk Write Head
>>can't work together like this and an extra seek is needed ? )
>>
>>Or some sort of driver issue ? ( Harddisk driver / firmware issue ? )
>>
>>Bye,
>> Skybuck.
>>
>
>
>
> I may not know the answer but feel an important question might be
> "What is very bad performance?", compared to good performance...
> numbers are our friends.
>
> Could it simply be that you're switching back and forth with data
> flow so the caching (on the drive) isn't effective?
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