Date: Mon, 24 Sep 2012 11:50:32 +0530 From: Jack <jacks.1785@gmail.com> To: freebsd-geom@freebsd.org Subject: Re: GEOM and CAM pass behaviour Message-ID: <CACmXQA3OJmRX8CdcwocHUR_yRMfuwouP6EOigH%2BzduJ_PNQEDg@mail.gmail.com> In-Reply-To: <CACmXQA2CiBJGZm836TUny48mnz0EYCj07E2=w9udT0ES1r8fpw@mail.gmail.com> References: <CACmXQA2CiBJGZm836TUny48mnz0EYCj07E2=w9udT0ES1r8fpw@mail.gmail.com>
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Hi again, On Sat, Sep 22, 2012 at 9:50 AM, Jack <jacks.1785@gmail.com> wrote: > Hi all, > > I would like to know 2 things regarding geom and cam pass driver > behaviour. It would be helpful if someone could educate me regarding > this. > > Consider the case where a scsi/ata tape/hard drive is attached to the > system after the FreeBSD is booted(from different drive) completely. > > > 1. Is it true that GEOM layer will always taste the (hard drive/tape > drive)device whether we access device via cam pass driver or do block > access. In other words, say if we never do a block access on the > selected device, and we only access this device via cam pass driver, > will then Geom will ever have to taste(e.g. for partition table, etc. > or for other purposes) this > device.? > > or is it something like, as soon as the device is attached, geom layer > will always taste it - it doesn't matter how we access the device. > > 2. Also, is it possible that while a process is accessing the selected > device via cam pass driver, the block access is also possible at that > same time, (by same or different process) to that same device. > I know that when a process is accessing the device via block access, > it is possible for another(or even same) process to access that same > device via cam pass driver, but I would like to know whether the other > way is true also. > > > Thanks. > -- > Jack It seems that two or more processes can access a same device via cam pass driver, and the block access is also possible at that same time, (by same or different process) to that same device. Queueing is done at HBA driver layer. But I'm not confirmed about GEOM tasting scene. Is there a way so that a da/ada/sa device is not created for a particular device, leaving only pass device, so that this device is accessible only via cam pass driver and not via geom/block layer. Actually I'm developing a userland utility, and my intent is that a particular ata/scsi device attached to the system( after Freebsd is booted completely), is accessible only via cam pass driver(ie via pass device), and not via geom/block layer(ie via da/ada/sa) driver. The device is selected by the user of utility. Any suggestions, or approaches would be helpful. Regards. -- Jack
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