mksnap_ffs, snapshot issues, again

[Kirk McKusick]

	To: Kirk McKusick <mckusick at mckusick.com>
	cc: "[iso-8859-2] Branko F. Graènar" <bfg at noviforum.si>,
		Paul Saab <ps at yahoo-inc.com>,
		Robert Watson <rwatson at tislabs.com>,
		freebsd-current at freebsd.org
	Subject: Re: mksnap_ffs, snapshot issues, again 
	From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
	In-Reply-To: Your message of "Sat, 23 Aug 2003 01:32:38 PDT."
	Date: Sat, 23 Aug 2003 11:01:28 +0200
	X-ASK-Info: Whitelist match

	In message <200308230832.h7N8Wcol050681 at beastie.mckusick.com>,
	    Kirk McKusick writes:

	>But, to get to the problem that you are having with accessing your
	>filesystem. The problem is that although the filesystem is only
	>locked briefly, the snapshot file is locked for the entire 48 minutes.
	>Thus, if you touch the snapshot file (by for example doing a "stat"
	>on it), then the process doing the stat will hang for 48 minutes.

	Isn't there some way we can loosen this aspect up ?

	Either by having stat know about it and return approximate info or
	simply by failing ? (I pressume that making the sleep interruptible
	would break all sorts of standards)

	-- 
	Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
	phk at FreeBSD.ORG         | TCP/IP since RFC 956
	FreeBSD committer       | BSD since 4.3-tahoe    

The race to the root problem in general could be largely solved
by changing lookup (VOP_LOOKUP really) to release the lock that
it holds on the directory before blocking on the next component
in the case where it is doing a lookup without intent to create.
If we did this, then a single locked node would have lookups
pile up on itself, but could not cascade to the root. A related
change would be to do an interruptable locking request on the
node so that if one did an `ls -l foo' where foo was say a
locked snapshot, it would be possible to interrupt it.

	~Kirk