Date: Fri, 2 Feb 2007 12:43:01 -0800 (PST) From: mjacob@freebsd.org To: Scott Long <scottl@samsco.org> Cc: scsi@freebsd.org, mjacob@freebsd.org, Nate Lawson <nate@root.org> Subject: Re: cvs commit: src/sys/cam/scsi scsi_da.c Message-ID: <20070202123844.U36488@ns1.feral.com> In-Reply-To: <45C389A6.1080606@samsco.org> 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>
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>> >> 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. > >> 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.
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