Re: Mac OS X VFS
Re: Mac OS X VFS
- Subject: Re: Mac OS X VFS
- From: Quinn <email@hidden>
- Date: Mon, 9 May 2005 14:57:28 +0100
First up, the Q&A that describes the current status of VFS
development on Mac OS X is DTS Q&A 1242 "Developing for VFS".
<http://developer.apple.com/qa/qa2001/qa1242.html>
It hasn't yet been updated for Tiger. I will get to that shortly.
At 13:59 -0400 30/4/05, Bijoy Thomas wrote:
I read that currently the kernel allows only leaf filesystems and
not stackable ones.
Indeed. This is still true on Tiger.
What is the difference between them?
A stacking file system is one that's implemented in terms of another.
The classic example is an encryption file system, where the
underlying data is managed by a real file system and the encryption
file system is stack on top of that to do the encryption. This is
cool because the base file system doesn't need to know anything about
encryption, and the encryption file system doesn't need to know
anything about how data is stored on disk.
The Mac OS X stacking architecture, such as it is, is derived from
4.4 BSD. To learn more about it, read "The Design and Implementation
of the 4.4 BSD Operating System".
Stacking isn't supported on Mac OS X, for a variety of reasons. Many
of these persist in Tiger.
Apple does, however, support a specific flavour of stacking, as
demonstrated by the WebDAV file system. WebDAV uses HFS as a cache
for the WebDAV data. This sort of stacking is supported and very
useful for a certain class of problems. See the WebDAV source for
details.
Share and Enjoy
--
Quinn "The Eskimo!" <http://www.apple.com/developer/>
Apple Developer Technical Support * Networking, Communications, Hardware
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