Specifications

2
RAID overview
❖ Increased interleave size, from 2KB to
512KB
Stripe interleave defines how many logical blocks are
used from one drive in the stripe before the next
drive’s blocks are used. An interleave size of 64KB
(default size) indicates that 64KB of data will come
from one drive and the next 64KB will come from the
next drive. Therefore, performance optimization
attained with striping will only occur when the read or
write command is requesting more than 64KB of data.
The size of the stripe interleave should be matched to
the average size of the read and write commands.
Windows OS RAID cannot vary the interleave from 64
KB, which may not be optimum for your application.
ATTO striping provides interleave sizes from 2KB to
512KB: default is 64KB. The optimum interleave size
is best determined by trial and error. An interleave size
should be chosen and then tested via a benchmark
program or by actual tests of the system.
Generally speaking, the recommended interleave is
equivalent to the average transfer size divided by the
number of drives in the stripe set:
NOTE ATTO ExpressPCI Ultra 320 and
Ultra3/UltraWIDE drivers created before version 1.02 and
1.68, respectively, only supported interleave sizes up to
64KB. If you create a stripe group with an interleave size
greater than 64KB with an unsupported version, the stripe
group will be inaccessible and the driver will report that
members are missing from the stripe group. Do not repair
the stripe group with the RAIDUTIL.EXE program or all
data will be lost. The stripe group can only be accessed
with a supported driver.
❖ An ATTO stripe group is recognized under
BIOS and DOS environments, enabling the
boot drive to be a stripe group.
ATTO drivers support striping consistently for all
operating systems by combining drives rather than
partitions.You may boot the system off a striped set of
drives and all operating systems will be able to access
the striped drives.
ATTO drivers support striping at the miniport driver
level in Windows by detecting composite drives
during the initial power-on bus scan. During this scan,
composite drives are reported to the system only on the
last target ID of the stripe group. The miniport driver
intercepts any SCSI command sent to the composite
drive reported on the highest ID and either emulates
the command, sends it to whatever drives in the stripe
group are required to satisfy the request, or rejects it as
an illegal command. Any command sent to one of the
other drives in the stripe set is rejected with a SCSI
Selection Timeout error.
SCSI Inquiry and Read Capacity commands are
emulated. Inquiry data is fabricated to indicate a
manufacturer ID of
ATTO and a device ID of whatever
stripe group name was assigned when the group was
created. Read/write/verify requests are sent to the
drives required to satisfy the request based on the disk
address range involved. Commands such as
Test Unit
Ready
and Prevent/Allow media removal are sent to all
drives in the group. When an error occurs on any or all
drives in a stripe set, the first error which is detected is
returned with the command.
set RAIDin drives #
size transfer average
interleave optimal =
120 GB
Disk Drive
120 GB
Disk Drive
120 GB
Disk Drive
360 GB
Virtual
Stripe Group
ExpressRAID
software presents
one virtual drive
while routing data
to three physical
drives
Example of a 3-drive stripe group created by ExpressRAID