Avago Technologies 3ware SAS 9750-4i - Manual (Page 66)

Chapter 2. CLI Syntax Reference
58 3ware SATA+SAS RAID Controller Card CLI Guide, Version 10.0
group=3|4|5|6|7|8|9|10|11|12/13|14|15|16 indicates the number of disks per
group for a RAID 50 type. (This attribute can only be used when
type=raid50.) Group=13-16 is only applicable to 9690SA.
Recall that a RAID 50 is a multi-tier array. At the bottom-most layer, N
number of disks per group are used to form the RAID 5 layer. These RAID 5
arrays are then integrated into a RAID 0. This attribute allows you to specify
the number of disks in the RAID 5 level. Valid values are 3 through 16.
However, no more than 4 RAID 5 subunits are allowed in a RAID 50 unit.
Note that a sufficient number of disks are required for a given pattern or disk
group. For example, given 6 disks, specifying 3 will create two RAID 5
arrays. With 12 disks, specifying 3 will create four RAID 5 arrays under the
RAID 0 level. With only 6 disks a grouping of 6 is not allowed, as you would
basically be creating a RAID 5.
The default RAID 50 grouping varies, based on number of disks. For 6 and 9
disks, default grouping is 3. For 8 disks, the default grouping is 4. For 10
disks, the default grouping is 5, and for 12 disks, the disks can be grouped into
groups of 3, 4, or 6 drives (the group of 4 drives is set by default as it provides
the best of net capacity and performance). For 15 disks, the disks can be
grouped into 5 drives (3 drive groups would make 5 subunits, you can have a
maximum of 4 subunits). For 16 disks, the disks can be grouped into groups
of 4 or 8 drives.
Note that the indicated group number that is supported depends on the
number of ports on the controller. group=16 is the maximum and it is
available on the 9690SA.
noscan attribute instructs CLI not to notify the operating system of the
creation of the new unit. By default CLI will inform the operating system.
One application of this feature is to prevent the operating system from
creating block special devices such as /dev/sdb and /dev/sdc as some
implementations might create naming fragmentation and a moving target.
nocache or nowrcache attribute instructs CLI to disable the write cache on
the newly created unit. Enabling write cache increases write performance at
the cost of potential data loss in case of sudden power loss (unless a BBU or
UPS is installed). By default the write cache is enabled. To avoid the
possibility of data loss in the event of a sudden power loss, it is recommended
9750,
9690SA,
and
9650SE 16 N/A 16 16 N/A 16 N/A
64 64 64 64 64
256 256 256 256 256
a. Models 9500S, 9550SX, and 9590SE
Table 7: Supported Stripe Sizes (KB)
Model R0 R1 R5 R6 R10 Spare R50 Single