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#1 (**permalink**) 04-18-2008, 11:30 AM

On 4/14/05, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> That's basically what it comes down to: SCSI lets the disk drive itself
> do the low-level I/O scheduling whereas the ATA spec prevents the drive
> from doing so (unless it cheats, ie, caches writes). *Also, in SCSI it's
> possible for the drive to rearrange reads as well as writes --- which
> AFAICS is just not possible in ATA. *(Maybe in the newest spec...)
>
> The reason this is so much more of a win than it was when ATA was
> designed is that in modern drives the kernel has very little clue about
> the physical geometry of the disk. *Variable-size tracks, bad-block
> sparing, and stuff like that make for a very hard-to-predict mapping
> from linear sector addresses to actual disk locations. *Combine that
> with the fact that the drive controller can be much smarter than it was
> twenty years ago, and you can see that the case for doing I/O scheduling
> in the kernel and not in the drive is pretty weak.
>
>

So if you all were going to choose between two hard drives where:
drive A has capacity C and spins at 15K rpms, and
drive B has capacity 2 x C and spins at 10K rpms and
all other features are the same, the price is the same and C is enough
disk space which would you choose?

I've noticed that on IDE drives, as the capacity increases the data
density increases and there is a pereceived (I've not measured it)
performance increase.

Would the increased data density of the higher capacity drive be of
greater benefit than the faster spindle speed of drive A?

--
Matthew Nuzum <matt@followers.net>
www.followers.net - Makers of “Elite Content Management System”
View samples of Elite CMS in action by visiting
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#2 (**permalink**) 04-18-2008, 11:30 AM

"Matthew Nuzum" <matt.followers@gmail.com> writes:

> drive A has capacity C and spins at 15K rpms, and
> drive B has capacity 2 x C and spins at 10K rpms and
> all other features are the same, the price is the same and C is enough
> disk space which would you choose?

In this case you always choose the 15k RPM drive, at least for Postgres.
The 15kRPM reduces the latency which improves performance when fsyncing
transaction commits.

The real question is whether you choose the single 15kRPM drive or additional
drives at 10kRPM... Additional spindles would give a much bigger bandwidth
improvement but questionable latency improvement.

> Would the increased data density of the higher capacity drive be of
> greater benefit than the faster spindle speed of drive A?

actually a 2xC capacity drive probably just has twice as many platters which
means it would perform identically to the C capacity drive. If it has denser
platters that might improve performance slightly.

--
greg

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#3 (**permalink**) 04-18-2008, 11:30 AM

"Matthew Nuzum" <matt.followers@gmail.com> writes:
> So if you all were going to choose between two hard drives where:
> drive A has capacity C and spins at 15K rpms, and
> drive B has capacity 2 x C and spins at 10K rpms and
> all other features are the same, the price is the same and C is enough
> disk space which would you choose?

> I've noticed that on IDE drives, as the capacity increases the data
> density increases and there is a pereceived (I've not measured it)
> performance increase.

> Would the increased data density of the higher capacity drive be of
> greater benefit than the faster spindle speed of drive A?

Depends how they got the 2x capacity increase. If they got it by
increased bit density --- same number of tracks, but more sectors
per track --- then drive B actually has a higher transfer rate,
because in one rotation it can transfer twice as much data as drive A.
More tracks per cylinder (ie, more platters) can also be a speed win
since you can touch more data before you have to seek to another
cylinder. Drive B will lose if the 2x capacity was all from adding
cylinders (unless its seek-time spec is way better than A's ... which
is unlikely but not impossible, considering the cylinders are probably
closer together).

Usually there's some-of-each involved, so it's hard to make any
definite statement without more facts.

regards, tom lane

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#4 (**permalink**) 04-18-2008, 11:31 AM

> The real question is whether you choose the single 15kRPM drive or
> additional
> drives at 10kRPM... Additional spindles would give a much bigger

And the bonus question.
Expensive fast drives as a RAID for everything, or for the same price
many more slower drives (even SATA) so you can put the transaction log,
tables, indexes all on separate physical drives ? Like put one very
frequently used table on its own disk ?
For the same amount of money which one would be more interesting ?

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