Shrinking Filesystems... is it important?

[dcm at plato.austin.ibm.com]

In article <1991Jun12.060212.8037 at ux1.cso.uiuc.edu> shair at ux1.cso.uiuc.edu (Bob Shair) writes:
>While we're talking about dynamically shrinking filesystems, and whether
>AIX V3 might someday provide that facility, let me ask a purely
>hypothetical question... What's it worth?

	It's worth alot.  What happens if you accidentally extend /usr
	too far, using up your entire disk, then /tmp fills up?  How do
	you take those extra blocks from /usr and use them to extend /tmp?
	Right now you backup /usr, reboot off your maintenance diskettes,
	shrink /usr's logical volume, mkfs /usr, restore, then extend /tmp.
	Big Waste of Time(tm).

	Dynamically extendible filesystems is a tremendous win, however
	it loses some of its usefulness unless you can reclaim blocks
	and use them to extend other filesystems.

>If we want dynamically shrinkable filesystems, surely the developers
>will add something like additional backwards chains... I don't know
>what (I've no connection with development).  Whatever it is, it will
>take space on disk, take additional instructions to process, update
>or skip over, and end up slowing disk performance to some extent.

	I doubt that it will affect the structure of the filesystem.  Back
	in the early days of AIXV3, I was actually working on an algorithm
	for shrink fs.  It *was* working for the easy cases, and almost
	working for harder ones.  Then they turn around and change the data
	structures...  Sigh.  I never had a chance to go back and rewrite it.

	It's not that hard.  The hardest part is error recovery (what happens
	if the system crashes in the middle of the operation?).  I don't think
	it requires any changes in the filesystem itself.

>We've gotten along in Unix for some time now without this feature.

	True.  However, we need shrink to make extend more usable.

		Craig
-- 
Craig Miller			Internet:	dcm at aixwiz.austin.ibm.com
IBM Austin			Vnet:		tkg007 at ausvmq
AIXV3 Change Team (level3)	IBM internal:	dcm at littleguy.austin.ibm.com
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