Subject: Re: VPS mailing list, BSD interest?
To: Terry Lambert <>
From: Kevin P. Neal <>
List: tech-kern
Date: 10/03/1996 01:08:55
At 02:54 PM 10/2/96 -0700, Terry Lambert wrote:
>A FS type must be able to express a preference or deny an underlying
>media type.  For instance, FFS should be permitted to prevent you
>putting it on a device which has Logical Device Attributes of
>LDA_MAY_GROW or LDA_MAY_SHRINK, since handling of these operations
>requires that the FS register a callback to allow or veto the
>operation, depending on whether or not it has handlers.

It would actually break FFS for a partition to grow underneath it?

I mean, it be pretty pointless to make a partition larger than the filesystem
if the fs can't grow. 

Hmmm. I see your point. Shrinking would be the harder case.

Would newfs_ffs (or whatever) know which devices could support the 
filesystem? How would that work?

>Similarly, JFS/NTFS/LFS/etc. which can handle changing agregation
>sizes with an LVM facility (adding or deleting PP's from the volume)
>must call back to determine allowable size change increment.  I believe
>JFS requires a 4M or an 8M increment, in all cases, for instance.

Lemmie see if I have this straight:
1) User program decides to grow/shrink a partition.
2) User program tells partition to grow/shrink
3) Virtual partition asks for the permission of the filesystem
4) Assuming they agree, they work together somehow to do whatever
  -- Filesystem must make sure there is enough space available to 
     shrink down into. Non-resizable filesystems would just veto
     the whole shebang.

Is that the gist of it? 

Ok. Doesn't this assume that whatever is sitting on top of the partition
is a filesystem? What if it isn't? What if a database or something or
other is sitting on top? Would resizing be possible? Wouldn't the mechanism
of communicating the resize be different because of the user program 
hitting the disk instead of going through a filesystem?

Third case: What do you do about resizing a swap partition? Can the
FreeBSD VM system move swapped out pages out of the way of a resize?
How would the partition tell the VM system to move? I guess this might
be through the same system as a kernel-based filesystem.

Hmmmmm. Know of any good books on the subject? (Chris Dukes the other day
asked on the ncsu.os.linux newsgroup if anybody knew of a good class
offered by our Computer Science department. If there were any responses, they
didn't appear on the newsgroup. Hmmmmm.) (No, there is no ncsu.os.bsd. I
guess you can't have a newsgroup for, like, 3 people). 
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