Date: Wed, 07 Dec 2005 15:50:30 -0600 From: Eric Anderson <anderson@centtech.com> To: Nate Lawson <nate@root.org> Cc: hackers@freebsd.org Subject: Re: scsi-target and the buffer cache Message-ID: <43975926.1010302@centtech.com> In-Reply-To: <43960F55.3010508@root.org> References: <4395BF04.50101@centtech.com> <43960F55.3010508@root.org>
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Nate Lawson wrote: > Eric Anderson wrote: > >> I'm curious about whether a target mode device would use the buffer >> cache or not. Here's a scenario: >> >> Host A: has fibre channel host adapter, in target mode, large memory >> pool, and another fiber channel host adapter connecting to fibre >> channel block device. >> Host B: Fibre channel host adapter, connecting to Host A. 'sees' the >> target mode block device created by Host A. >> >> Will Host A use the buffer cache to cache blocks between the real >> block device, and the shared target mode device? >> What about if Host A put a filesystem on the block device, created a >> single file the size of the filesystem, and shared that filesystem >> via a target mode device to Host B? >> What I'm wanting is a box (FreeBSD?) that can be placed between a >> fibre channel block device (like a RAID array), and a fibre channel >> host using that block device, and act as a block cache for that >> device, using the FreeBSD's memory. If it had a significant amount >> of memory, this could be very useful. > > > If you use the example scsi_target usermode > (usr/share/examples/scsi_target), then the buffer cache will be used > since its reads/writes are from usermode like normal. If you don't > want that behavior, you can set O_DIRECT in the open() call of the > backing store file. > > If you chose to modify the kernel side, you'd have to make sure your > accesses were through the VOP layer and then it would be cached. > > You should check to be sure the target mode performance meets your > expectations also. > I guess I would be using the user mode tool, unless there's another way? Your comment on performance also makes me a little worried about that now - do you think I would see a large performance hit? Thanks! Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology Anything that works is better than anything that doesn't. ------------------------------------------------------------------------
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