[TUHS] /dev/drum

Johnny Billquist bqt at update.uu.se
Tue Apr 24 09:44:06 AEST 2018

On 2018-04-24 01:30, Grant Taylor <gtaylor at tnetconsulting.net> wrote:

> On 04/23/2018 04:15 PM, Warner Losh wrote:
>> It's weird. These days lower LBAs perform better on spinning drives.
>> We're seeing about 1.5x better performance on the first 30% of a drive
>> than on the last 30%, at least for read speeds for video streaming....
> I think manufacturers have switched things around on us.  I'm used to
> higher LBA numbers being on the outside of the disk.  But I've seen
> anecdotal indicators that the opposite is now true.

That must have been somewhere in the middle of history in that case. Old 
(proper) drives had/have track 0 at the outer edge. The disk loaded the 
heads after spin up, and that was at the outer edge, and then you just 
locked on to track 0, which should be near.
Heads had to be retracted for the disk pack to be replaced.

But this whole optimization for swap based on transfer speeds makes no 
sense to me. The dominating factor in spinning rust is seek times, and 
not transfer speed. If you place the swap at one end of the disk, it 
won't matter much that transfers will be faster, as seek times will on 
average be much longer, and that will eat up any transfer gain ten times 
over before even thinking. (Unless all your disk ever does is swapping, 
at which time the heads can stay around the swapping area all the time.)

Which is also why the file system for RSX (ODS-1) placed the index file 
(equivalent of the inode table) at the middle of the disk by default.

Not sure if Unix did that optimization, but I would hope so. (Never dug 
into that part of the code.)


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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