Netbackup will not SCSI reserve the drives.
The operating system will SCSI reserve the drives if they are not
already reserved and the open() against the device file
succeeds.
However, if reserve/release has been explicitly disabled in
the tape driver, then the driver will not attempt to reserve
devices that use that driver when an open() is issued.
Therefore two or more open()'s will succeed across distinct
hosts. However, on the same host, only one open should succeed
and the other fail with busy.
At a higher level, Netbackup does arbitration of drive access. But this
is independant of SCSI reserve/release, and only Netbackup knows
about it. SCSI reserve/release is advantageous because the tape drive
makes the "decision". In an "open" and "heterogeneous" environment clearly
the tape drive is the only component that can accurately and categorically
make that decision.
One of the paragraphs in the 3.4 manuals suggests Netbackup requires
exclusive access to devices it controls. In my experience, this is
very true. This is why I now personally recommend SCSI reserve/release
is enabled, to protect against rogue programs or people from doing
things against Netbackup controlled devices. It wont stop the programs
or people from causing your backups to potentially fail. However it
will enforce a greater level of arbitration that is made visible to not
only Netbackup but other programs, people and non Netbacup SSO hosts.
If you think about the concept of "shared tape drives" you can soon
understand that devices available on demand, need to be monitored
periodically
to determine if they are still available in the big SAN cloud.
This is what the SCAN host/avrd process does. It monitors the availability/
functionality of the drives periodically. The problem occurs when processes
outside of Netbackup access those devices, which interfers with Netbackups
view of the tapes on the SAN. Netbackup cant tell other programs, people or
hosts are accessing the devices. It can only assume there is "something
wrong".
Hence why it wants exclusive access.
Regards
Peter Marelas
-----Original Message-----
From: W. Curtis Preston [mailto:curtis AT backupcentral DOT com]
Sent: Friday, 1 June 2001 5:41 AM
To: anthony.guzzi AT storability DOT com; veritas-bu AT mailman.eng.auburn DOT
edu
Subject: Re: [Veritas-bu] Re: Does SSO Use SCSI Reserve/Release?
Good answer. Actually, both the robot and the drives are shared, but the
robot is shared more "virtually." They "share" it by asking one server to
mount a tape for them. However, when it comes time to share a drive, they
do actually communicate directly with the drive.
Perhaps what I am asking is whether or not NetBackup uses SCSI
Reserve/Release command to reserve the DRIVES, or if it just keeps track of
who's using what drive when, and doesn't let two servers use the same drive
at the same time.
At 02:50 PM 5/31/2001 -0400, anthony.guzzi AT storability DOT com wrote:
>Curtis,
>
> I'm sure you'll get a lot of responses on this, so consider my comments
>just part of the chorus. If I understand SSO correctly, only the tape
>drives are shared, not the robot. The robot is controlled by a single
>server which handles all tape mounts/unmounts. In addition, this server
>also coordinates access to the tape drives. It does this by running the
>'vmd' process (on a Unix system) in 'device allocation' (DA) mode.
>
>-- Tony Guzzi
>Solutions Engineer, AssuredRestore team
>Storability, Inc.
>118 Turnpike Road
>Southborough, MA 01772
>
>www.storabilility.com
>The Leading Provider of Automated Storage Assurance for the Enterprise
>
>
>
>
>veritas-bu-request AT mailman.eng.auburn DOT edu
>Sent by: veritas-bu-admin AT mailman.eng.auburn DOT edu
>05/31/2001 01:06 PM
>Please respond to veritas-bu
>
>
> To: veritas-bu AT mailman.eng.auburn DOT edu
> cc:
> Subject: Does SSO Use SCSI Reserve/Release?
>Message: 5
>Date: Thu, 31 May 2001 09:55:36 -0700
>To: veritas-bu AT mailman.eng.auburn DOT edu
>From: "W. Curtis Preston" <curtis AT backupcentral DOT com>
>Subject: [Veritas-bu] Does SSO Use SCSI Reserve/Release?
>
>I'm getting conflicting information from a couple of different sources,
>and
>I KNOW that someone out there knows the real answer.
>
>How does SSO (for NetBackup & BE if possible) managed contention for the
>robot? It would seem that this could be done in one of two ways:
>
>1. It uses the SCSI Reserve command when it starts to use the arm.
>
> <...... text deleted. -AJG ......>
>
>---
>W. Curtis Preston
>Principal Consultant for Storage Designs, your storage experts
>Webmaster: http://www.backupcentral.com Phone: 760 631 7991
>
>
>_______________________________________________
>Veritas-bu maillist - Veritas-bu AT mailman.eng.auburn DOT edu
>http://mailman.eng.auburn.edu/mailman/listinfo/veritas-bu
---
W. Curtis Preston
Principal Consultant for Storage Designs, your storage experts
Webmaster: http://www.backupcentral.com Phone: 760 631 7991
_______________________________________________
Veritas-bu maillist - Veritas-bu AT mailman.eng.auburn DOT edu
http://mailman.eng.auburn.edu/mailman/listinfo/veritas-bu
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