Reserving Disk Drives; Fast-Write Cache - IBM Advanced SerialRAID Adapters SA33-3285-02 User Manual

Reserving Disk Drives

The Advanced SerialRAID Adapter, the Micro Channel SSA Multi-Initiator/RAID EL
Adapter, and the PCI SSA Multi-Initiator/RAID EL Adapter implement reservation by
using commands that are sent directly from adapter to adapter. They do not use the
SCSI reservation command. The advantages of this method are:
v System software can read the Physical Volume ID (PVID) from a reserved disk drive.
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v It is possible to use the ssa_rescheck command to determine which adapter is
holding a reservation to a disk drive.
v The diagnostics can detect particular failure conditions on reserved disk drives that
they cannot detect with the other reservation method.
v Fencing can be used on a reserved disk drive.
v Node_number locking is supported. With node_number locking, the disk drive is not
locked to an adapter, but rather to a using system. To enable a disk drive to be
locked to a using system, each using system in an SSA network must have a unique
node number. The node number is stored as the node_number attribute of ssar. It
can be queried with the lsattr command and set by using the chdev command. The
ssavfynn command (described in "ssavfynn Command" on page 371) can be used to
verify that no duplicate node numbers exist.
v If a reservation is challenged (that is, a node that does not hold the reservation
attempts to access a reserved SSA logical disk), the adapter verifies that a valid path
still exists to the node that is holding the reservation. If no path exists, the
reservation is removed, and the new node is allowed access to the disk drive.
Therefore, if an adapter is used to reserve a disk drive, and is then disconnected or
powered off, that disk drive becomes effectively unreserved.

Fast-Write Cache

SSA adapters that are on SSA loops of up to two SSA adapters can use the Fast-Write
cache feature. For a two-way configuration, the two adapters can be in the same, or in
different, using systems. Fast-write functions can be enabled for single disk drives or for
RAID arrays. Performance improvements are related to the type of disk drive (single
disk or RAID array type) and the workload. If two-way fast-write functions are used in a
two-host system configuration, the performance improvements are greater when
concurrent access is not permitted to disk drives. When concurrent access is permitted,
performance improvements are less.
Chapter 2. Introducing SSA Loops
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