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 Post subject: When does HDD do Random read/write or Sequential?
PostPosted: July 17th, 2012, 23:09 
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Joined: July 17th, 2012, 22:57
Posts: 5
Location: Singapore
Hi,

I tried to google but couldn't really find a simple answer...Just like to know when does a HDD do random read/write and when does it do sequential read/write?

If I copy a Blurray file and 10GB of music and photo files, does it do random read/write or sequential?


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 Post subject: Re: When does HDD do Random read/write or Sequential?
PostPosted: July 18th, 2012, 10:10 
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Joined: May 6th, 2008, 22:53
Posts: 2138
Location: England
I can guess (perhaps correctly, perhaps incorrectly) why you are asking, but it would help if you can please explain some context - (a) why are you asking this question, and (b) why are you giving only 2 choices of answer?

It would also help if you give your approximate level of computer experience e.g. are you a programmer? An engineering student? A normal home PC user? Something else? That will help members when considering how much detail is appropriate to write in any answer.


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 Post subject: Re: When does HDD do Random read/write or Sequential?
PostPosted: July 18th, 2012, 21:06 
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Joined: July 17th, 2012, 22:57
Posts: 5
Location: Singapore
Hi, sorry, I didn't know I had to indicate the reasons here. I'm just a normal PC user bumming into some weird behavior with my external hard disk.

I was copying some files to my PC and it seems to be very slow. . So I did an experiment using HD Tune to get the throughput and measure my own throughput of file copy time of the 40GB files and it doesn't really match up.

I can't really find what is going on with the HD tune Software and what is the difference between using the software and manual calculating my throughput. So this question pops into my mind when I read about sequential and random read/write.


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 Post subject: Re: When does HDD do Random read/write or Sequential?
PostPosted: July 19th, 2012, 11:15 
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Joined: May 6th, 2008, 22:53
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Location: England
wuming79 wrote:
I didn't know I had to indicate the reasons here.

Of course you don't have to explain the reasons - but the context of questions can be important (as in this case) to avoid wasting time giving irrelevant answers or levels of detail.

This is especially true, since the answers to your specific original questions probably won't help you, which is why I was asking for the reasons behind your questions. Here are the answers to your initial questions (excluding some of the fine details involved in things like read-ahead and NCQ re-ordering), and you'll see why I didn't expect that these answers would be what you expected:

wuming79 wrote:
when does a HDD do random read/write

When it is told to do that, by receiving commands from the host system (e.g. PC).

wuming79 wrote:
when does it do sequential read/write?

When it is told to do that, by receiving commands from the host system (e.g. PC).

In summary: The disk drive does what it is told to do, by the OS on the host system. So you see that the background/context to your questions is needed, to try to understand your real issue a little better... :)

wuming79 wrote:
I was copying some files to my PC and it seems to be very slow. . So I did an experiment using HD Tune to get the throughput and measure my own throughput of file copy time of the 40GB files and it doesn't really match up.

You haven't given exact figures, but in short, HDTune is using "synthetic benchmarks" showing the best performance that the disk can give, for its specific tests. That does not mean that you will see that same performance when performing reads/writes to a filesystem, nor in most other real-life situations (although reading large files, from an unfragmented filesystem, can be close to a sequential read, for example).

wuming79 wrote:
I can't really find what is going on with the HD tune Software and what is the difference between using the software and manual calculating my throughput. So this question pops into my mind when I read about sequential and random read/write.

In general terms, there is an expected difference between raw benchmark performance figures and real life throughput - some of the reasons for the difference include filesystem & OS overhead, sub-optimal I/O size, and filesystem fragmentation which prevents sequential I/O.

You might also have a real underlying problem, in addition to that inevitable difference between file copy throughput and performance from a benchmark, but any investigation of that would require much more details.


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 Post subject: Re: When does HDD do Random read/write or Sequential?
PostPosted: July 22nd, 2012, 23:36 
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Joined: July 17th, 2012, 22:57
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Location: Singapore
Hi, wow, the answer is so overwhelming. Thanks for the details.

So do you mean in a normal file copy, the PC can suddenly change command to read/write in random or sequential and we can't really use the normal file copy to measure throughput and thus, measuring using a standard software like HD tune will be a better gauge?


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