We need to have some disks be writeable by one host,
but readable by many. The locally-written application
that uses the disks ensures that each "owning" host
modifies only its own assigned-writeable disks.
(btw, when I say "disk" I mean physical spindle; each
spindle would be read-only to all hosts but one.)
-- for performance reasons, the disks should be "locally"
attached, i.e., not NFS. A SAN-type solution would be ok.
-- for political reasons, we can't use HACMP, PPxx, HPFS,
etc.; it has to be "ordinary" AIX.
-- we can write whatever code might be needed, as long as
it doesn't require undocumented/unsupported interfaces.
-- we might be using EMC disks/frames, or IBM SSA disks.
-- for political reasons, we'd *prefer* to use JFS rather
than using raw volumes.
-- for budget reasons, we *might* not be able to use a SAN
1. Our biggest concern is OS/hardware glitches --
things like a host doing a bus-reset or permanent reserve.
What problems / solutions might we have in this respect?
What are the physical restrictions / requirements (e.g.,
cabling -- and what exactly is "twin-tailing")?
Can "hangs" be avoided without using a SAN?
2. Would there be any special benefits of using a SAN
other than eliminating "hang" problems? "Vanilla" AIX
has no special SAN-awareness that would eliminate the need
for us to do lock management in our application code, right?
3. We're under the impression that mounting a volume as
read-only might eliminate some "hang" problems, but would
also cause that host to never refresh its cached directory
data after the mount. We can think of ways around this --
e.g., by prepping each disk with fixed-size null-filled
files before the application ever uses it, so the inodes
never change. But is this necessary? Is there any way to
force AIX to physically access the directory on each open()?
(I don't think the O_DIRECT flag would help.) btw,
repeated unmounting and re-mounting isn't an option.
Thanks in advance for your time, brain-strain,
and keystrokes, colleagues.
aka nusrat AT prodigy.net
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