Re: Disk geommetry, was Re: Kernel Upgrade: Why?
I think that you are missing a very important point here. A hard disk,
unlike, for example, an audio CD-ROM, spins at a fixed angular
velocity. Thus the 'linear' speed over the disk surface is faster
towards the outside of the disk than towards the centre (as in v=wr).
Thus at the
very least, it would seem likely that the reading of a large amount of
data which was continuous would be quicker from the outside of the disk
than from the innner. (This probably represents a larger performance
increase with the large web-server than with the average single-user
system).
That said, I take your point about the seek time depending on the
distance that the heads have to move, and it is evident that multihead
drives would be expected to improve performance.
Rich
Jonathan Guthrie wrote:
>
> On Thu, 22 Apr 1999, Ookhoi wrote:
>
> > Well, there was a discussion here about a benchmark Linux vs NT, and
> > some people here said that the preformance of Linux could have been
> > affected by the fact that Linux was near the center, and NT on the outer
> > side.
>
> Probably not the reason.
>
> > And the data on the outer side passes the heads much faster than the
> > data on the inner side. But then, there is much more data on the outer
> > side, and a piece of data on the outer side will go round in the same
> > amount of time as a piece of data on the inner side..
>
> I am not aware of any disks that use a higher density recording format for
> the outer tracks than they do for the inner tracks. As far as I am aware
> (and I really haven't paid much attention to such things since ST-277's
> were state of the art) the bit density of the outer tracks is LOWER than
> the bit density of the inner tracks. That's because the outer tracks are
> physically larger, but they hold the same number of bits.
>
> Not that it matters. The whole disk spins as a single unit so even if
> there were more bits on the outer tracks, you'll still wait the same
> amount of time (on average) for the sector you want to come around. Read
> on, and I'll explain.
>
> > So, is there an advantage if whe put for example swap at the outer side
> > of a disk?
>
> NO!
>
> Look, the access times for disk are dominated by two times, the time to
> seek to the correct track and the time to wait for the data to come around
> again on the disk. The time it takes the data to come around on the disk
> is, on average, one half of the time it takes for the disk to go around
> once. That's independant of everything else and is a fairly short time,
> anyway.
>
> The time it takes to seek to the correct track depends upon where you're
> seeking from and where you're seeking to. Obviously, if the heads happen
> to be at innermost cylinder, it will take longer to seek to the outermost
> cylinder than if the heads were in the middle or toward the outside. So,
> for higher performance in a situation where you're too cheap to add enough
> RAM, you'll want the swap file near where the heads are likely to be.
>
> You can also turn that around. Seeking to the middle from either extreme
> is likely to be faster than seeking to the other extreme. (This works for
> both average and worst-case times.)
>
> Predicting where the heads are likely to be takes some doing, especially
> on systems with effective disk caches, but you can take some educated
> guesses. The middle of a disk is a better guess than either extreme, but
> isn't necessarily the best guess. If you spend a lot of time reading and
> writing (especially writing) files from a particular partition, you might
> want to put the swap file near that partition on the principle that the
> heads are likely to be near there anyway, so it should reduce the amount
> of time waiting for any given swap.
>
> An extreme example of this would be where you dedicate an entire drive to
> a (fairly small) swap partition. That's how the news servers I use do it.
> For something less extreme, I kind of like the recommendation made by OS/2
> gurus: Their advice was to put the swap file in the most used partition
> on the least used drive. You might try something like that where you put
> the swap partition in the middle of a disk that isn't used for very much.
>
> In short, my recommendation for boosting the performance of a computer
> that uses a significant amount of swap is to add RAM to the computer.
>
> HTH. HAND.
> --
> Jonathan Guthrie (jguthrie@brokersys.com)
> Brokersys +281-895-8101 http://www.brokersys.com/
> 12703 Veterans Memorial #106, Houston, TX 77014, USA
>
> --
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