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Date: Sat, 16 Jan 1999 02:20:06 -0800 (PST)
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To: current@FreeBSD.ORG
Subject: mounting double-ended SCSI disks
From: asami@cs.berkeley.edu (Satoshi Asami)
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Hi,

In our project, we're trying to connect two PCs to both ends of a SCSI 
chain.  I've got it somewhat working, but have a couple of questions
re. mounting the filesystems.

If I mount the filesystem from one machine (A) as read-write, then the
other one (B) can't mount it read-write because the clean flag is not
set.  This is ok.  Also, I can mount the disk read-only from both A
and B.  (I even read the entire contents of a 20GB partition from both
machines at the same time -- worked without a hitch.)

However, if I try to mount it from B read-only while A is mounting it
read-write, it succeeds.  This looks dangerous, as A writing data onto
the disk could cause B's cache to go stale without B knowing it.  Is
it a good idea to allow read-only mounts of a dirty filesystem anyway?
(The filesystem could be corrupted, right?)

Another problem is if A is mounting it read-only and then B tries to
mount it read-write.  This succeeds and is dangerous for the same
reason as the last example.  Since A can't write anything to the disk,
I guess there is no way we can avoid this situation.  (The only way I
could think of avoiding a crash due to stale cache data was to have A
check the clean flag before every read, but that seems excessively
expensive.)

Satoshi

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