10-17-2012
Well, I am sure most SAN can be configured for mirroring, which is raid-2 or something like that. To achieve the most bang for your buck, you want the mirrors to be as far apart as possible, so something between the application host and short of the storage box needs to know where there are 2 so it can sidestep the dead side as well as splitting the query load and duplicating the churn. The farther upstream it is done, the greater the reliability, but too close can load the app server and communications, unless fiber has multicast write and anycast read. There may be several layers vying to mirror your storage, so pick wisely.
Mirroring got overshadowed a bit by raid, but has always had a query bandwidth advantage, with two devices handling query. Within the devices, there can be as much striping as in raid, so that is no different. When writing, there is no parity calculation and additional write time, just two immediate simultaneous writes. With raid in sequential striping mode, you write data to 1,2,3,4 and parity to 5, then data to 5,1,2,3 and parity to 4, and so on, so while reading is 5x spindle speed, writing is 4x. A mirrored pair trades space for bandwidth. Disk is cheap, and bandwidth is golden. Finally, it seems some raid systems seem to only get defects detected by staff when 2 adjacent devices fail, so often raid5 is either also mirrored or great downtime, data loss and partial restore pain is experienced.
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LEARN ABOUT HPUX
st_ats_enabled
st_ats_enabled(5) OBSOLETED st_ats_enabled(5)
NAME
st_ats_enabled - determine whether to reserve a tape device on open (OBSOLETED)
VALUES
Default
Allowed values
or
Recommended values
DESCRIPTION
Note: this tunable is obsoleted and has been renamed to beginning with release HP-UX 11i v3.
This tunable notifies the stape or estape driver whether it needs to reserve a tape device on open and subsequently release it on close.
When this tunable is on the stape or estape driver will attempt to reserve any tape device on open. The driver has a few tape devices
flagged as suitable devices for this functionality including DLT 8000, DLT 7000, STK 9840, HP Ultrium, and others. DDS drives are not
flagged for this functionality.
The tunable allows the safe sharing of tape devices in multinode configurations like MC/ServiceGuard's Advanced Tape Sharing. The auto-
matic reserve and release protects the tape device from being accessed from multiple nodes which would corrupt a backup. If the reserve
portion of the stape or estape driver's open routine fails, a status of is returned.
Who Is Expected to Change This Tunable?
Anyone.
Restrictions on Changing
Changes to this tunable take effect immediately and are used during the next open or close.
When Should the Tunable Be Turned On?
This tunable should be turned on if the system will be used in an MC/ServiceGuard Advanced Tape Sharing configuration or the user wants the
stape or estape driver to use automatic reserve or release on open and close for multinode manageability.
What Are the Side Effects of Turning the Tunable On?
A reserve is sent to the tape device by the stape or estape driver on open and a release is sent on close. No other initiator or HBA will
be able to access the tape device while it is opened on another initiator or HBA.
When Should the Tunable Be Turned Off?
This tunable should be turned off for any SAN tape sharing solution configuration other than ATS. Most multinode backup applications (for
example, Omniback) manage device reservation themselves and any interference by the stape or estape driver may produce problems. This tun-
able should also be turned off for any multiplatform SAN configuration to keep tape access across platforms more uniform.
What Are the Side Effects of Turning the Tunable Off?
Unauthorized access from another initiator might interfere with any current tape operation.
What Other Tunables Should Be Changed at the Same Time?
None.
WARNINGS
All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its meaning changed in future releases of
HP-UX.
Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation,
some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tun-
able values, consult the documentation for the kernel software being installed. For information about optional kernel software that was
factory installed on your system, see at
AUTHOR
was developed by HP.
SEE ALSO
st_ats_enable(5), scsi_tape(7), scsimgr_estape(7).
Tunable Kernel Parameters st_ats_enabled(5)