[Registry] Re: LinuxRegistry in Freedesktop & KDE

[Alexander Larsson]

On Mon, 2004-04-19 at 12:23, Dave Cridland [Home] wrote:
> On Sun, 2004-04-18 at 19:08, Daniel M. Lambea wrote:
> 
> >  * Ability to alter parts of the configuration file, while keeping the    
> >   rest untouched. How many disk accesses we save now? And memory
> > consumption?
> 
> Well, my guess is that a typical configuration file is less than a
> couple of blocks, so, erm, LR makes a net loss there. I could be wrong,
> but I strongly suspect that LR will take more in terms of the system's
> cache, disk space, and head movement. Laying down one single block is
> the same speed, no matter whether you're writing 4k of data or 4 bytes,
> as I understand things.

Its much worse than that. Opening a new file thats not in the cache
requires seeking and waiting on average half a rotation of the disk
head. These are things in the millisecond range, and far far slower than
reading sequential data. 

Lets do some approximations given the data here:
http://storagereview.com/articles/200401/20040126WD740GD_2.html

Lets pick a really really fast HD, such as the Maxtor Atlas 15k (73 GB
Ultra320 SCSI). The average seek time to read a sector is 5.7
miliseconds, and the transfer rate is fastest 73.7 MB/sec, and slowest
55.6 MB/sec, so lets use 64.7 as average. To open a new file you need to
seek at least once (maybe more if you need to look into a new
subdirectory). In the time it takes to seek one we can read 5.7*64.7 ==
368.8 kilobytes.

For a more normal HD, lets take the Seagate barracuda ATA V (i have some
of these):

http://storagereview.com/articles/200210/20021014ST3120023A_2.html

read seek time: 13.5 ms
average transfer rate: 34.25 ms
sequentially read per seek: 462.3 KB

These are very physical limitations (rotation speed, head seek delay),
so we can't expect these to drop exponentially like memory size or cpu
speed.

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
 Alexander Larsson                                            Red Hat, Inc 
                   alexl at redhat.com    alla at lysator.liu.se 
He's a jaded gay cyborg who knows the secret of the alien invasion. She's a 
cosmopolitan red-headed opera singer who inherited a spooky stately manor from 
her late maiden aunt. They fight crime!