Sharing a SCSI bus between some computers
Attila Nagy
Due to the lack of enough "networking power" I started to think about
alternatives.
I would like to ask your thoughts on the following setup:
+----------+ +----------+ +----------+
+ BOX1 +-------+ SCSI BOX +------+ BOX2 +
+----------+ +----++----+ +----------+
||
+----++----+
+ BOX3 +
+----------+
BOX[1-3] would be a PC with a SCSI adapter. SCSI BOX is an external SCSI
RAID array.
The line between the boxes is a single SCSI cable, on which the devices
access the bus (it would be a correct chain, with a terminator on each
end).
The question is:
what will happen if BOX3 mounts the filesystem from the SCSI BOX RW and
BOX[1-2] mount it RO?
In the case BOX[1-2] it is only necessary to read from the array, nothing
more.
I've seen a similar setup on Sun hardware, where the disk array was used
by two computers. RW/RO, as in this case.
Does anybody have experiences regarding this? Is it possible/necessary to
hack some kind of locking into the SCSI driver? The adapters are made by
Adaptec (aic7xxx).
Thanks,
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Dan Nelson
> Due to the lack of enough "networking power" I started to think about
> alternatives.
>
>
> The question is: what will happen if BOX3 mounts the filesystem from
> the SCSI BOX RW and BOX[1-2] mount it RO?
>
> In the case BOX[1-2] it is only necessary to read from the array,
> nothing more.
As long as BOX3 never writes data, you're okay. The problem is that
BOX1/2 don't know when BOX3 has invalidated their cached data by
writing to the disk. What you want is a shared-storage filesystem, and
there is no such thing for FreeBSD.
Considering you can get a gigabit ethernet NIC for under $50 and a
D-Link 4-port gigabit switch for $300, you might just want to plug the
RAID into BOX3, and have BOX1/2 NFS-mount it.
--
Dan Nelson
dne...@allantgroup.com
Attila Nagy
> As long as BOX3 never writes data, you're okay.
If this would be the scenario, all boxes would mount the FS RO :)
> The problem is that BOX1/2 don't know when BOX3 has invalidated their
> cached data by writing to the disk.
What problems could that cause?
I don't mind if BOX[1-2] reads a file and the file disappears in the
middle of the transfer, if those boxes can recognize it and will not read
other portions of the hard disk. In short: if they won't offer corrupt
data to the clients.
> What you want is a shared-storage filesystem, and there is no such thing
> for FreeBSD.
BTW, is it possible to do it with common SCSI storage, or it would need
some sophisticated solution, like fibre channel?
> Considering you can get a gigabit ethernet NIC for under $50 and a
> D-Link 4-port gigabit switch for $300, you might just want to plug the
> RAID into BOX3, and have BOX1/2 NFS-mount it.
Oh I know this :)
Actually, this is exactly what I have. But because the machines (box1-3)
aren't as powerful at all, I would like to get the maximum out of them.
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Attila Nagy
> I believe that SGI's xfs is capable of handling shared-storage setups...
After Dan's mail I would think that only some kind of signalling would
needed to accomplish the task of one RW and multiple RO machines using the
same SCSI storage. This could be done in the hardware and in the software.
All I could get on this topic is the problem of multiple RW machines, for
which the Linux-specific GFS[1] used the SCSI locking feature of the
individual devices and then implemented some kind of external locking,
other distributed filesystems did similar methods.
How difficult would be to implement this, either into the recently
announced GEOM or some other layers?
Thanks,
[1]: http://www.sistina.com/products_gfs.htm
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Attila Nagy e-mail: Attil...@fsn.hu
Free Software Network (FSN.HU) phone @work: +361 210 1415 (194)
cell.: +3630 306 6758