AIX Ver. 4 SysAdmin IV:Storage Management (Unit 3) – Working with SSA (2 of 2)

SSA Subsystem Configuration Example (Diagrams)
See Text or Video

Bypass Card Modes (Diagrams)
See Text or Video

Bypass Mode LEDs
Operating Mode Status Link Status Mode Light Link Status
———————– ———- —————– —————- —————–
Automatic Inline On Off On
Automatic Bypass Off Amber Off
Forced Inline Inline Off Green Off
Forced Bypass Bypass On Amber On
Forced Open Open Off Off Off
Jumpered Forced Inline Off Green Off

Fundamentals of SSA Cabling
All adapters for RX/6000 offer two loops: A and B
– Do not exceed 48 disks per loop
– Never mix A and B connectors of the same adpater

Pulled-out disks break the loop
– Replacement or dummy disk has to be inserted soon
– Dummy disks do not amplify the signal
– No more than three dummy disks are allowed in sequence

Adapters in powered-down systems break the loop
– Use bypass cards whenever possible
– Connect both loop outlets of an adpater to the same bypass card

Cabling, Disk Positioning and Performance (1 of 2)
SSA devices on the same loop:
– Forward data received on one link to the other
– Share the bandwidth via a packetized protocol

Handling of simultaneous communication requests:
Devices closer to the host adpater are allowed to send first.

– Avoid connecting adapters directly to each other.
– Place most active disks farther away from the using adapter:
– Increases the fairness of I/O bandwidth allocation
– “Far away” means the middle of the loop in HACMP environments

SSA Cabling Guidelines
SSA cabling should be the best compromise of:
– Space requirements
– Scalability
– Availability goals
– Performance demands
– Budget

For complex environments optimization targets should be:
– Discussed before the hardware is purchased (or installed)
– Confirmed by application and system administrators.
– Written down

Using diag to Verify SSA Loops
SSA Service Aids

SSA Link Verification etc…

Some AIX Commands for SSA
ssaxlate – translates between SSA logical disks (hdisks) and SSA physical disks
– ssaxlate -l LogicalDiskName
– ssaxlate -l PhysicalDiskName

ssaadap – lists the adapters to which an SSA logical disk or physical disk is connected
– ssaadap -l LogicalDiskName
– ssaadap -l PhysicalDiskName

ssaidentify – sets or clears identify mode (blinking light) for an SSA physical disk
– ssaidentify -l PhysicalDiskName (y or n)

ssadisk – displays the names of SSA disks connected to SSA adapter
– ssadisk -a AdapterName -P -L (P=physical, L=logical)

ssa_diag -l pdisk#

ssa_speed -l pdisk# (or -a adpater) – Speed of the device

ssa_progress pdisk# (formatting progress)

Note: There are a lot of other SSA commands.

Disk Reservation
SCSI -type disk reservation
– Occurs between a single adapter and a disk
– Has to be released by the initiating adapter or broken
– Is supported with SSA in the same manner

– Also blocks “legal” accesses (cfgmgr) from other adapters
– Gets into conflict with SSA adpater redundancy
– Disks are still reserved for the failing adapter after switching
– SSA router has to maintain the reservation state per adapter internally. May not break reservations other than those issued bythe failed adpater.

SSA Note Numbers for HACMP
– Unique number > 0 assigned to a host
– Set via chdev -l ssar -a node_number=N
– Prerequisite of
–Node number-based reservation
–Host and adpater display in the link verification panel
–SSA target mode

SSA node number based reservation
– Occurs between nodes, not between single adapters
– Is released automatically by the death of a node
– Allows reading the PVID of reserved disks

Updating SSA Microcodes
– Use “all-in-one” update packages whenever possible:
New microcode often requires updated AIX drivers
– Adapter microcode
-Is downloaded to the adpater by running cfgmgr
-Has to be updated in all nodes
– Disk drive microcode
-Has to be downloaded to the device explicitly from one node only. (diag or ssadload)
-May not be downloaded concurrently or if the loop is broken
– Check microcode levels of new or replaced drives

Update packages can be obtained from

Serial Storage Expert (StorX)
Windows-based tool for:
-Configuration analysis and visualization
-Monitoring and error notification

-Live Viewer
– GUI runs on Windows 95/98/NT
– Data is provided by RPC agent in AIX:
Fileset devices.ssa.network_agent

Licensing is by number of disks managed.

SSA Spare Tool (1 of 2)
Consequences of a disk failure in an SSA configuration:
– RAID Environments:
-Hot spare drives are maintained by the adapter’s firmware and driver.
-All required panels for disk replacement can be found under smit devices SSA
-No impact on the operating system

– JBOD environments (LVM mirroring):
-No support for hot spare drives in the LVM
-Disk replacement involves LVM commands: rmdev, cfgmgr
-Complexity increases in a multihost configuration.

Solution: SSA Spare Tool
-Uses the errnotify ODM to react upon disk failures
Provides a pool of spare disks for immediate replacement

CheckPoint Questions:
1. How many disks are possible within an SSA loop? 48
Adapter Card? 96
2. Can you “hot plug” an SSA cable, disk, bypass card? Yes
3. In which state is an SSA drive if:
– The activity LED is blinking slowly? broken link
– The activity LED is flickering? busy
– The check LED is blinking? identify mode (amber light)
4. What is the purpose of bypass cards? automatic open/close of loop so another adapter can service the disk if an adapter goes down.
5. Can you use adapter redundancy with RAID in HACMP? No
6. What has to be avoided when building an SSA cabling?
– Name two or three issues? Don’t link A to B, Disk limit of 48, no more than 3 dummy disks in a series within a loop.
7. Which initial definition is necessary if you want to use SSA target mode?
chdev command to set up the node number.

Lab 2: Working with SSA
# diag
Task Selection
SSA Service Aids
Link Verification

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