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From: lphillips@lpami.wimsey.bc.ca (Larry Phillips)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Disk fragmentation
Message-ID: <2358@van-bc.UUCP>
Date: 9 Apr 89 19:44:31 GMT
Sender: lpami@van-bc.UUCP
Lines: 50

In <5117@cs.Buffalo.EDU>, ugkamins@sunybcs.uucp (John Kaminski) writes:
>The filing system on the Amiga uses hashing with chaining.  In case you don't
>know what this is, it is generating an otherwise "useless" number by combining
>the numbers (the character code values) in the file name in a certain
>consistent way (such as adding them up) to condense it down into a single
>number to be used as an index into the disk.  For example, suppose the
>result of adding up all the character codes in the desired pathname is
>50.  At 11 sectors per track and 2 tracks per cylinder, sector 50 would
>be on cylinder 2 (starting from 0, cyl 0 has sectors 0-21, cyl 1 has 22-44,
>and cyl 2 has 45-67, etc.), track/side 0 (cyl 2 track 0 has sectors 45-56),
>sector 5.  Right there should be a file header block which has the filename
>controlled by that exact hash.

 Well, that's close. Tha hash value does not point at a sector on disk, but to a
hash table offset within either the root block or a directory block. Contained
in that hash table entry is the pointer to either a directory block or a file
header block.

>Fragmentation can occur but is generally kept to a minimum.  As usual, the
>easiest way I know to consolidate the files is to copy them with

 Fragmentation is entirely dependent upon the number and sizes of files deleted
vs. the number and sizes of files written, and on the order in which these
operations are performed. It is possible to badly fragment a disk in a matter
of minutes, though in normal operation, it takes considerably longer. There is
no way to accurately determine how long it will take to fragment a disk without
knowing a lot more about what will be deleted and written to the partition.

>  ...  If there is a utility for copying
>the contents of a hard disk to several floppies, I am unaware of it.  The dis-
>cussion above also points out that you should try to copy the files that you
>want accessed faster first so that they will be at the head of the hash chain
>or at least as close as possible to the head.

ExpressCopy, from Expressway software is a HD backup program that will do
exactly this. It will copy any arbitrary partition, directory, or subdirectory
onto multiple floppies, creating standard Amigados floppies that can be
accessed as normally generated floppies can, and at the rate of about 1 every
1.5 minutes, to multiple drives.

-larry

--
Frisbeetarianism: The belief that when you die, your soul goes up on
                  the roof and gets stuck.
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|   //   Larry Phillips                                                |
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