Date: Fri, 02 Feb 2007 13:30:26 -0800 From: Nate Lawson <nate@root.org> To: mjacob@freebsd.org Cc: scsi@freebsd.org Subject: Re: cvs commit: src/sys/cam/scsi scsi_da.c Message-ID: <45C3AD72.7020007@root.org> In-Reply-To: <20070202123844.U36488@ns1.feral.com> References: <20070123173026.E692416A4CD@hub.freebsd.org> <45B65710.4060607@root.org> <20070123105009.G41619@ns1.feral.com> <45B67401.9070102@samsco.org> <20070201150111.B77236@ns1.feral.com> <45C27965.1010803@samsco.org> <45C2E7DB.30204@root.org> <20070202080329.L17850@ns1.feral.com> <45C389A6.1080606@samsco.org> <20070202123844.U36488@ns1.feral.com>
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mjacob@freebsd.org wrote: >>> >>> From a silly semantic point of view to get around this, we should >>> still support and require SYNC_CACHE on close except where devices >>> don't support it (and any device that hangs on a SYNC_CACHE doesn't >>> support it- period). >> >> The problem is that we don't know if the device will misbehave until it >> does, and then we don't know if we can reliably recover it. > > This is back to what I referred to earlier by a week or so- booting > installation (or as a fallback) with a pessimization flag that avoids > all questionable commands until the system is up enough to load (via > firmware(9) or sysctl or rc scripts) better information. That wouldn't work in this case since you would need to tell GEOM not to look at certain devices (just another quirk list). >>> On detach, devices that still need to have data commited via an >>> opcode that looks remarkably like SYNC_CACHE can and should have that >>> happen- with all the infrastructure changes that go along with >>> allowing devices to be detached (w/o complaint) with a live command. >> >> What instigates this problem is that the GEOM layer will open the >> device, read a few sectors, close it, then do that again a few more >> times, long before the user tries to mount/unmount it. It's the whole >> GEOM-taste thing where it tries to essentially auto-probe the storage. >> When we unconditionally send a SYNC_CACHE in daclose(), the >> misbehaving device is dead long before the user has a chance to do >> anything. One hack might be to track if any write command were done >> while the device was open, and only issue the SYNC_CACHE if so. Since >> the GEOM tasting will only read, it'll pass this test and avoid the >> problem. > > It's not a hack to keep track of a write commands- after all, I did > exactly this for SunOS 4.1 (or was it 4.0?) to know whether you'd > dirtied the device or not- and of course *I* would be believe it to > still be perfect, eh? :-) > > This would be an excellent and cheap idea to implement and I think I'll > do so. I bet you that this will take care of nearly all of the boot time > issues. That's fine, but you'd also have to track things like MODE SELECT or COPY or FORMAT or other commands that might actually dirty the media without being a WRITE. I don't see why GEOM can't open the device read-only to do its probe. Doesn't it use a device vnode? -- Nate
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