Regarding IO to disk

[Jawahar Tadipatri]

Hi Folks,Can anyone please let me know the average/typical time taken for
completing one IO operation (read and write) to disk?
More specifically, time taken for writing a string to a file.

Thanks a load,
Jawahar
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[Ivan Voras-7]

Jawahar Tadipatri wrote:
> Hi Folks,Can anyone please let me know the average/typical time taken for
> completing one IO operation (read and write) to disk?
> More specifically, time taken for writing a string to a file.

Your question is incomplete and cannot be answered correctly.

Declared average latency for desktop hard drives (see
http://www.seagate.com/docs/pdf/datasheet/disc/ds_barracuda_7200_12.pdf)
is about 4 ms. Real performance depends on about a dozen factors.

See here: http://www.google.com/search?q=hard+drive+performance for more
information.

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[Jawahar Tadipatri]

Thanks for the info Ivan.

On Wed, Jul 8, 2009 at 5:05 PM, Ivan Voras <ivoras@...> wrote:

> Jawahar Tadipatri wrote:
>
>> Hi Folks,Can anyone please let me know the average/typical time taken for
>> completing one IO operation (read and write) to disk?
>> More specifically, time taken for writing a string to a file.
>>
>
> Your question is incomplete and cannot be answered correctly.
>
> Declared average latency for desktop hard drives (see
> http://www.seagate.com/docs/pdf/datasheet/disc/ds_barracuda_7200_12.pdf)
> is about 4 ms. Real performance depends on about a dozen factors.
>
> See here: http://www.google.com/search?q=hard+drive+performance for more
> information.
>
> _______________________________________________
> freebsd-hackers@... mailing list
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[Wojciech Puchar-5]

>
> Declared average latency for desktop hard drives (see
> http://www.seagate.com/docs/pdf/datasheet/disc/ds_barracuda_7200_12.pdf) is

latency is average time to wait until data will be below head. it's by
average half on single rotation. for 7200 rpm drive it's 120rps so half
rotation takes 1/240 second=about 4ms.

But to perform I/O you have to

- send a request to disk (<1ms)
- move head - depends of distance, up to 20-30ms, 8-10ms average on most
drives
- wait for latency - 4ms average.

you end in 13-15ms average.

in case of multiple parallel accesses - drive accept writes immediatelly,
and can accept multiple reads if it can NCQ/TCQ, then it internally sort
request queue to minimize required head moves and rotational waiting, so
under high load you may go to 6-7ms/request which is about 150IOPS.

Of course i'm talking about random/pseudorandom reads, multiple reads of
sequential addresses will be optimized by drive.

One more - i'm talking about raw I/O, FreeBSD of course optimizes
filesystem acces internally by using lots of caching (as much as available
memory) and planning requests.

small writes to file usually succeed immediatelly (to cache).
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