Re: [gentoo-user] Re: Separate /usr

["Andrey Gerasimenko" <gak@kaluga.ru>]

On Fri, 11 May 2007 01:36:03 +0400, Dan Farrell <dan@spore.ath.cx> wrote:

> On Thu, 10 May 2007 11:34:46 +0100
> Neil Bothwick <neil@digimed.co.uk> wrote:
>
>> On Thu, 10 May 2007 12:11:34 +0200, Benno Schulenberg wrote:
>>
>> > > No idea, but I tried it when I encountered that page and portage
>> > > operations were measurably faster.
>> >
>> > That might well be just the transfer effect: you went from an old
>> > fragmented file system to a fresh unfragmented one.
>>
>> I allowed for that. I created a new filesystem for /usr/portage - I
>> had been using a directory in /usr before.
>>
>>
> Well, maybe it has to do with the efficiency of reading discontiguous
> blocks from one file as opposed to a filesystem.  Since it's a sparse
> file, there might be a lot of 'space' that, if it were on an actual
> disk, the heads would have to pass over; thus there may be a way in
> which a sparse file is more efficient than a regular filesystem.
>
> Remeber that the files in portage are, except for distfiles, quite
> small.  By my calculation, the average size for files and directories
> under $PORTDIR (excluding $DISTDIR of course) is only 62 bytes.  What
> would you bet that on a disk partition, the other 962 to 4034 bytes per
> block (I couldn't have block sizes less than 1K on reiser for my
> portage, and 4096 is the default for most FS's) are filled with
> nothing, and the heads need to pass over them to read the next block.
> On a sparse file that space is merely reserved, it needn't actually
> exist.   Hope that helps you conceptualize the difference.

I guess the idea is correct, but the details are questionable. The heads  
do not move over empty tails, the spinning disk does. A head just moves to  
the track that contains the required sectors. The head movement and disk  
spinning do not influence performance directly since there are many levels  
of caching between a physical read and an application.

It looks like it takes much less buffer space to cache lots of small files  
when they are joined into a sparse file than when they are in a real file  
system, making sparse file very efficient.

-- 
Andrei Gerasimenko
-- 
gentoo-user@gentoo.org mailing list