AIX Ver. 4 SysAdmin IV:Storage Management (Unit 3) – Working with SSA (1 of 2)

SSA – Serial Storage Architecture

Unit Objectives
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– Describe the components of a Serial Storage Architecture subsystem and its operational characteristics
– List the subsystem and adapter types suitable for use on your systems
– Choose and implement a cabling that fits your needs of performance and/or availability
– Use AIX commands and external tools to monitor your SSA disks.

SSA Development
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History:
– Started by IBM in 1995
– SSA is an open standard (submitted to ANSI)
Motivation:
– SCSI came to its limits:
– Cable length
– Exploitation of available bus bandwidth
– 15 devices only
Need for a technology with
– Built-in options for disaster recovery
– Sustained throughput with different types of workload
– Support for a large number of individual disks
– Better reliability

Advantages of SSA
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Dual paths to devices
– Simplified cabling – cheaper, smaller cables and connectors, no separate terminators.
– Faster interconnect technology
– Not an arbitrated system
– Full-duplex, frame multiplexed serial links
– 40 MB/s total per pot, resulting in 80 MB/s total per node, and 160 MB/s total per adapter
– Spatial reuse allows concurrent activity in different portions of the same loop.
– Hot-pluggable cables and disks

SSA Operation
———————-
Four links per loop
– Two read and two rite operations simultaneously in each direction
– 20/40 MB/sec on each link

Loop Availability
————————
Loop reconfigures itself dynamically
logical – hdisk#
physical – pdisk#

ssaidentify logicaldevicename (turns on check amber light)

SSA Adapter Redundancy (1 of 2)
—————————————————
Mechanism:
– A loop may contain one or two adapters of the same host.
– If an adapter fails, the SSA Router (ssar) will
– Log an error
– Use the other adapter to acess the disks

Benefit: Best way to prevent an adapter-caused quorum loss.

Limitations:
– May be 25% slower than non-redundant cablings
– Limits for adapters and disks per loop remain the same:
– Two initiators maximum means either no redudndancy or no HACMP
– Redundancy should not be implemented if space demands grow and further adapters cannot be added.

SSA Adapter Redundancy (2 of 2)
—————————————————
A pdisk’s primary and secondary adapter can be
– Queried
– Changed manually

Example:
lsattr -El pdisk0
chdev -l pdisk0 -a primary_adapter=adapter_b

SSA Storage Subsystem Controller Card
————————————————————-
– Monitors itself. If it detects a problem, the controller card can generate an SSA error message through the disk drive modules
– Monitors the ambient air temperature is to high, the controller card can:
– Increase the speed of the fans
– Generate an SSA error message through the disk drive modules
– Shut down the 7133, if necessary

Monitors the status of the power supply assemblies. If a power supply assembly fails the controller card can generate an SSA error message through the disk drive modules

Monitors and controls the bypass cards
Monitors and controls the operator panel.

Serial Storage Architecture
—————————————
High-speed serial interface
SCSI compatible high-level command set
New attachment technology
– Full-duplex and frame-multiplexed disk I/O
– 48 disks per loop, 96 disks per adapter
– Alternate path capabilities allow hot-plugging of disks and cables
– Hardware addresses assigned to disks and adapters in the factory
– 25m copper cable between two devices
– 2.4km or 10km via optical extenders
– Four simultaneous I/Os per loop at 20 or 40 MBs each

SSA Communication Paths
—————————————–
A single break in a loop
– Causes the adapter and disk firmware to choose an alternate path.
– Will not make a device unreachable
– May reduce bandwidth by half in worst case.

– Two or more breaks may isolate devices

SSA Device Names and IDs
—————————————–
SSA introduced a new device class: pdisk
pdisks are for adminitration only (no I/O)
Relationship hdisk <--> pdisk
– JBOD: 1:1
– RAID: 1:n
-ssaxlate -l hdisk M or ssaxlate -l pdisk N
pdisk numbers are ordered by the disk’s serial numbers:

Renaming SSA Disks
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This procedure works for inactive drives only.
– Identify the drive you want to rename
– Make note of the drive’s characteristics
– Delete the old definitions
– Add the disk with new names, but same characteristics

ssaxlate -l hdisk2
lsdev -Cl pdisk0 -F ‘name type connwhere’
rmdev -dl pdisk0
mkdev -l pdisk20 -c pdisk -s ssar -t 4500mbC -p ssar -w ……………
mkdev -l hdisk20 -c disk -s ssar -t hdisk -p ssar -w …………………

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