max_coalesce
The maximum number of bytes that the SSAdisk device driver attempts to
transfer to or from an SSAlogical disk in one operation.
The default value is appropriate for most environments. For applications that
perform very long sequential write operations, performance improves when
data is written in blocks of 64 KB multiplied by (n-1), where n is the number of
disks in the array.For example, if the array contains six member disks, the
data would be written in blocks of 64 KB x 5. (These operations are known as
full-stride writes.) Touse full-stride writes, increase the value of this attribute to
64 KB x (n-1), or to some multiple of this number.
write_queue_mod
Alters the way in which write commands are queued to SSAlogical disks. The
default value is 0 for all SSAlogical disks that do not use the fast-write cache;
with this setting the SSAdisk device driver maintains a single seek-ordered
queue of queue_depth operations on the disk. Read operations and write
operations are queued together in this mode.
If write_queue_mod is set to a non-zero value, the SSAdisk device driver
maintains two separate seek-ordered queues: one for read operations, and
one for write operations. In this mode, the device driver issues up to
queue_depth read commands and up to write_queue_mod write commands to
the logical disk.
This facility is provided because, in some environments, it might be beneficial
to hold back write commands in the device driver so that they can be
coalesced into larger operations that can be handled as full-stride writes by the
RAID software in the adapter.
This facility is not likely to be useful, unless a large percentage of the workload
to a RAID-5 device consists of sequential write operations.
Device-Dependent Subroutines
The open,read,write, and close subroutines start typical physical volume operations.
open, read, write and close Subroutines
The open subroutine is mainly for use by the diagnostic commands and utilities.
Correct authority is required for execution. If an attempt is made to run the open
subroutine without the correct authority,the subroutine returns a value of -1, and sets
the errno global variable to a value of EPERM.
The ext parameter that is passed to the openx subroutine selects the operation for the
target device. The /usr/include/sys/ssadisk.h file defines possible values for the ext
parameter.
The ext parameter can contain any combination of the following flag values logically
ORed together:
SSADISK_PRIMARY
Opens the device by using the primary adapter as the path to the device.As a
result of hardware errors, the device driver might automatically switch to the
272 User’s Guide and Maintenance Information