Summary: Disabling rewind on close devices

From: Stuart Whitby <>
Date: Wed Jan 15 2003 - 05:40:07 EST
Thanks to those who tried on this one, but it looks like there's
no way to disable "rewind on close" devices from within the
Solaris OS.

Just in case Sun is listening, here's the reason why this would
be good...

If you have a SAN attached tape device which is shared between
different systems, the OS allows you to set a SCSI Reserve when
you open the drive.  No other system can use this drive when
the reserve is set.  This is fine as long as everything is
healthy, but if a system or application crashes, the drive
remains reserved for that system until physical intervention
takes place (ie, it gets powered off and on again or the bus
is reset).

Given that tape and jukebox hardware should be set up so only
one application has access to it, you'd normally expect that
only that application can manage the drive, and as such you
don't need to reserve it.  This is true - *if* the application
is truly the only thing which has access.  In reality, there
are a bunch of utilities which will check the hardware to
ensure it's still responding as expected.  If they so much as
look at a rewind on close device and it's not reserved, the
OS will continue to retry the rewind until it succeeds.  The
other system has no way of noting the rewind and will continue
its current operation from the new position on the tape.  If
it's a read, you'll probably get an error.  If it's a write,
you start overwriting good data, and don't find out about it
until you try to read it.

HP get around this by having a kernel setting called
"st_san_safe", which blocks access and creation of all rewind on
close device files.  If an environment is fully HP, there is no
need to reserve the drive and run the risk of all systems losing
access to it when one goes down - the application takes
care of writing data from multiple locations, and the OS takes
care of the "dangerous" device files.  Under Solaris, looks like
you've got the choice of losing occasional devices for a while,
or losing data.



 > -----Original Message-----
 > From: Stuart Whitby []
 > Sent: Wednesday, 8 January 2003 11:03
 > To:
 > Subject: Disabling rewind on close devices
 > Hello.
 > I've just come from a conversation with HP, in which they
 > recommend against using SCSI reserve on shared tape drives in
 > a SAN environment.  The reason for this is that any problem
 > with the application will lock out the device entirely.  What
 > they recommend is to use an HP kernel tunable called
 > st_san_safe to stop rewind on close devices being either
 > created or used.
 > However, the environment I'm dealing with shares these
 > devices between HP, AIX and Solaris.  Does anyone have any
 > idea if there is anything similar on Solaris (and ideally
 > AIX) which will disable rewind on close devices?
 > Cheers,
 > Stuart.
 > _______________________________________________
 > sunmanagers mailing list
 >> nmanagers
sunmanagers mailing list
Received on Wed Jan 15 06:47:31 2003

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