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setting of "scsi reserve/release" driver flag in shared tape environment?



 
 
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  #1  
Old December 5th 03, 04:24 PM
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Default setting of "scsi reserve/release" driver flag in shared tape environment?

Hi,

scsi reserve/release driver flag is normally set in Unix environment.
That means that
the tape driver reserves each tape device during an open I/O control
and release it with scsi-command "release unit" during I/O close
process. This procedure makes sure that only one server/application
can communicate to a drive at a time.

For sure, this flag should be reset in cluster environment due to the
fact that any operating cluster node can crash anytime during w/r
operations without releasing that drive. If that happens, no other
cluster node would be able to talk to that drive, well, Legato inquire
would work fine, but any other tape I/O should be rejected with scsi
status "busy".

I'm now wondering, if it is a good advice to reset that flag also in
shared tape environment (like Legatos DDS or Veritas Master/Media
Server concept),

any comments?

René

PS: Here is the way to reset that flag:

Solaris
SCSI reserve/release is configurable as a bit setting in the st.conf
file for each
device type in use. Be sure to use the most up to date st driver that
is available for
your version of Solaris.
ST_NO_RESERVE_RELEASE is the flag position 0x200000

AIX
To reset the reserve/release setting on an AIX operating system:
1. Through the SMIT interface, select Tapes from the Devices menu.
2. Change the value for the RESERVE/RELEASE support attribute from No
to Yes.

HP-UX
To reset the reserve/release setting on an HP-UX 11 operating system:
1. Change the st_ats_enable kernel variable to a value other than
zero.
2. You may have to restart the computer to ensure the change was
implemented.
Note - The reserve/release is a fixed setting in HP-UX 10.
Sun StorEdge Enterprise Backup Software 7.1, Release Notes • September
2003 24

Tru64
SCSI reserve/release is only available on version 5.1b and later.
Device Autodetection Attributes Not Supported On Solaris

Linux
Sorry, I've no clue to that OS, any comments appreciated
  #2  
Old December 5th 03, 04:45 PM
Rob Turk
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Default

"
wrote in message
om...
Hi,

scsi reserve/release driver flag is normally set in Unix environment.
That means that
the tape driver reserves each tape device during an open I/O control
and release it with scsi-command "release unit" during I/O close
process. This procedure makes sure that only one server/application
can communicate to a drive at a time.

For sure, this flag should be reset in cluster environment due to the
fact that any operating cluster node can crash anytime during w/r
operations without releasing that drive. If that happens, no other
cluster node would be able to talk to that drive, well, Legato inquire
would work fine, but any other tape I/O should be rejected with scsi
status "busy".

I'm now wondering, if it is a good advice to reset that flag also in
shared tape environment (like Legatos DDS or Veritas Master/Media
Server concept),

any comments?


Running without reserve/release means any machine can start talking to the
tape device. Nothing stops node A from sending a 'mt -f /dev/rmt/0 rewoffl'
while node B is running a backup. The entire purpose of reserve/release is
to prevent such mishaps.

If a cluster node crashes and another node takes over, it will be the
responsibility of that node to reset all peripherals it now controls to a
known state. If the tape drive is reserved by the crashed node, the
surviving node can send a SCSI reset to lift the reserve condition.

In a shared storage environment this may be unwanted behaviour, in that case
a node can use persistent reservation to keep a device tied to a certain
node, regardless if it's up or crashed.

Rob


  #3  
Old December 5th 03, 11:31 PM
Rene Koehnen-Wiesemes
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Posts: n/a
Default

Running without reserve/release means any machine can start talking to the
tape device. Nothing stops node A from sending a 'mt -f /dev/rmt/0

rewoffl'
while node B is running a backup. The entire purpose of reserve/release is
to prevent such mishaps.

If a cluster node crashes and another node takes over, it will be the
responsibility of that node to reset all peripherals it now controls to a
known state. If the tape drive is reserved by the crashed node, the
surviving node can send a SCSI reset to lift the reserve condition.


Of course, message code 0x0c (Bus Device Reset) from any other
initiator can release this reservation, good point, thank you Rob.

In a shared storage environment this may be unwanted behaviour, in that

case
a node can use persistent reservation to keep a device tied to a certain
node, regardless if it's up or crashed.


You're right, persistent reservation is the solution. I've just checked my
T10
specifications for that particular command. The interesting thing is that
I've
never recognized that command during scsi or fc/fcp analysis - well, in fact
I did such kind of deep protocol digging very seldom, approx. 2 per year
during
the last 2-3 years.

Anyway, my question now is if current device drivers (for tape drives) or
applications (Legatos DDS or Veritas' Master/Media Server) are familiar with
the usage of that command. Or in other words, this persistent reservation
feature seems to be a very useful command for safety reasons, that should be
in use in any tape sharing environment.

Rene


 




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