HP StorageWorks Scalable File Share System Installation and Upgrade Guide Version 2.2

Configuring the storage5–22
• Capacity
Each SFS20 array has twelve SATA disks of varying capacities. Depending on the size of disk used,
the type of redundancy affects the capacity as follows:
• In configurations where 160GB disks are used, all disks can be used in the case of both ADG
and RAID5 redundancy. In such configurations, the largest possible LUN when ADG
redundancy is used is smaller by one disk than the largest possible LUN when RAID5
redundancy is used.
• In configurations where 250GB disks are used, ADG redundancy uses twelve disks. One of the
disks is configured as a spare. Any one data chunk is striped over nine disks with parity being
written to both the tenth and eleventh disk.
When RAID5 redundancy is configured, eleven disks in total are used, with one or two of these
disks being used as spare disk(s). Any one data chunk is striped over nine disks, with parity
being written to the tenth disk.
ADG redundancy requires one disk more than the number required by RAID5 redundancy to
create a LUN of equivalent capacity. In each of these configurations, the largest supported LUN
size is 2TB.
• Using 500GB disks in a single disk group configuration, up to 4TB of storage can be realized
on an SFS20 array with ADG redundancy. Twelve disks are required to provide 4TB of storage
from a single SFS20 array. (Only ADG redundancy is supported for 500GB disks.)
The maximum supported LUN size is 2TB, so that where 500GB disks are used, two OST LUNs are
typically created on an SFS20 array. Only one OST LUN is typically created when 160GB or 250GB
disks are used.
• Performance
In the case of 160GB disks and 250GB disk, ADG LUNs typically provide performance of
approximately 10% (or more) less than RAID5 LUNs of equivalent size for large streaming write
operations, and can are roughly equivalent for large streaming read operations. Refer to
Appendix B of the HP StorageWorks Scalable File Share System User Guide for performance data on
Lustre configurations using ADG and RAID5 LUNs comprised of 250GB disks.
HP strongly recommends that MDS LUNs are mirrored; where mirrored MDS LUNs are used, the component
LUNs can be configured with either ADG or RAID5 redundancy (both of the component LUNs must be
configured with the same type of redundancy). In any one file system, all OST LUNs must be configured with
the same redundancy type; this does not have to be the same as the redundancy type configured on the
MDS LUNs, although it would be unusual for it to be different.
In addition, once the LUNs on an array have been used in a file system, you cannot reconfigure the array
to have a different redundancy type.
NOTE: 500GB disks are offered primarily as an option for systems where the principal concern is to
provide maximum capacity (rather than optimized bandwidth or availability).
5.3.4 Configuring SFS20 arrays attached to the administration and MDS servers
When configuring SFS20 arrays attached to the administration and MDS server, you must take account of
the following constraints:
• The administration server and the MDS server can each have a maximum of two SFS20 arrays (that
is, four SFS20 arrays for the server pair).
• The maximum supported LUN size on an SFS20 array is 2TB.
• The maximum number of disks that an SFS20 array can contain is twelve.