Path: utzoo!utgpu!watmath!att!bellcore!rutgers!bpa!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: FFS details
Message-ID: <7686@cbmvax.UUCP>
Date: 17 Aug 89 15:34:19 GMT
References: 
Distribution: comp
Organization: Commodore Technology, West Chester, PA
Lines: 33

in article , cire@dustbin.cisco.com (Eric B. Decker) says:
> Xref: cbmvax comp.sys.amiga:41248 comp.sys.amiga.tech:7424

> 1) How does the FFS deal with data integrity?  Is there some kind of
>    checksum or other protection mechanism?

There are checksums on file, directory, and extension blocks under FFS,
but not on data blocks.  These checksums, I suppose, are mainly there for
historical reasons if anything.  The responsibility for checking data is
considered the domain of the device driver, not the filesystem.  So for
floppies, for instance, there will be checksum information on hidden 
blocks outside of the 512 byte DOS managed block.  For SCSI hard drives,
data checking is managed by the drive itself; even the device driver doesn't
see any of that.

> 2) Where can I find detailed specifications of the FFS?

Not sure about that one.  The specs on the Standard FileSystem are in 
the AmigaDOS Technical Reference Manual, which is now part of the large
AmigaDOS book you find in book and computer stores.  It's not written very
well, but most of the information on SFS is there.  FFS from a low-level
point of view is very much like SFS, only the data blocks store only data;
the checksum and parent/next links have been removed.  At a higher level,
FFS is much smarter about the block allocations is makes.  I've hacked 
around with the disk format since 1985, so I can probably answer any 
related specific questions.

> Eric B. Decker

-- 
Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
           Be careful what you wish for -- you just might get it