[Coco] RBF File System Questions

[Darren A.]

>From: Gene Heskett
>Subject: Re: [Coco] RBF File System Questions
>Date: Mon, 28 May 2007 23:53:42 -0400
>
>  <snip>
>
>One way to help the fragmentation problem that I'm fond of, is to reset 
>that
>default PD.SAS from 8, to as high as FF(hex), forcing the filesystem to
>locate and allocate as many as 255 sectors in one swell foop.  The file may
>not actually use that many, and the leftovers will be returned to the free
>space pool when the file is finished and closed.  This is far faster for
>large file operations than having it open the default, claim 8 clusters,
>write them, find its out of disk space and then have to do that again 33 
>more
>times.  And yes, that is in clusters, which may not be a 1, and could be 
>any
>power of 2 that will fit in a one byte definition.  Of course, when the 
>disk
>is nearly full, then that particular scheme of using a large value for 
>FD.SAS
>has to be scaled down.  One can use the free command to determine the ideal
>size in that event.
>
>In that regard, dmode is your friend.
>
--

I found an old (1990) document on the web that seems to say that the SAS 
value is not in clusters. Can anyone confirm which case is true?  Here is 
the relevant excerpt:

----
A "cluster" is a lot like the minimum sector allocation size except that it 
is the same for all types of files and it can't be changed on the fly.  If 
the cluster size is set to 32, every file will allocate a multiple of 32 
sectors (actually 31 sectors p lus 1 file-descriptor sector for the first 
cluster) and remain there until it needs another cluster.

As far as how cluster size and sector allocation size (SAS) work together, 
the initial file size is always 1 cluster.  On disks with 1-sector clusters, 
this is used exclusively by the file-descriptor.  After that, as soon as 
data is written past the end of a segment, more space is allocated to the 
file in chunks of SAS rounded up to the next cluster size.  (eg. if cluster 
size=$08 and SAS=$23, additional space is allocated in chunks of $28 
sectors).  When the file is closed, it is shrunk to the least used number of 
clusters.
----

Darren

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