Date: Tue, 18 Nov 2014 17:19:25 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Mark Felder <feld@FreeBSD.org> Cc: svn-src-head@freebsd.org, cperciva@freebsd.org, Ian Lepore <ian@FreeBSD.org> Subject: Re: svn commit: r274653 - head/usr.sbin/freebsd-update Message-ID: <20141118151925.GX17068@kib.kiev.ua> In-Reply-To: <1416321839.350194.192432157.42AB6F37@webmail.messagingengine.com> References: <201411181338.sAIDc9U3051869@svn.freebsd.org> <20141118142449.GW17068@kib.kiev.ua> <1416321839.350194.192432157.42AB6F37@webmail.messagingengine.com>
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On Tue, Nov 18, 2014 at 08:43:59AM -0600, Mark Felder wrote: > On Tue, Nov 18, 2014, at 08:24, Konstantin Belousov wrote: > > Why do you suppose that md-backed volumes are not persistent ? > > vnode-backed devices are stable. > > > > You're right, a vnode-backed filesystem would definitely be persistent. > Clearly I've done a poor job of researching this thoroughly. I've now > read md(4) in its entirety and remember mounting disk images like > Linux's "mount -o loop" in the past. I don't see a way to reliably > detect an mfs filesystem because it uses md and masquerades as ufs in > df's output. Do you have any suggestions on how to detect this reliably? > > On the other hand, anyone could write a filesystem we aren't > blacklisting and fall into the same trap. I thought this was going to > help a few people since Oliver reported it and seemed to have a valid > use-case for a tmpfs mounted /var but still wanted to use > freebsd-update. Instead I'm beginning to think we should just throw this > away and add an entry into BUGS in the man page and call it a day. We > can't keep everyone from shooting their feet off and we probably > shouldn't waste our time trying. Up to you. You could somewhat improve this by parsing mdconfig -lv output and see if the device is swap/memory or vnode backed. In either case, if the code to detect transient /var is kept around, there should be a way to turn heuristic off and just do what user asked for. Some sort of the force flag.
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