HP XP7 Owner Guide (H6F56-96006)

configured as either 3390-x or OPEN-x logical devices. All LDEVs in the array group must be the
same format (3390-x or OPEN-x). For open systems, each LDEV is mapped to a SCSI address, so
that it has a TID and logical unit number (LUN).
Figure 14 Sample RAID5 3D + 1P layout (data plus parity stripe)
RAID6. A RAID6 array group consists of eight data drives (6D+2P). The data is written across the
eight drives in a stripe that has six data chunks and two parity chunks. Each chunk contains either
eight logical tracks (mainframe) or 768 logical blocks (open).
In the case of RAID6, data can be assured when up to two drives in an array group fail. Therefore,
RAID6 is the most reliable of the RAID levels.
Sequential data striping
The XP7’s enhanced RAID5 implementation attempts to keep write data in cache until parity can
be generated without referencing old parity or data. This capability to write entire data stripes,
which is usually achieved only in sequential processing environments, minimizes the write penalty
incurred by standard RAID5 implementations. The device data and parity tracks are mapped to
specific physical drive locations within each array group. Therefore, each track of an LDEV occupies
the same relative physical location within each array group in the disk array.
In a RAID6 (dual parity) configuration, two parity drives are used to prevent loss of data in the
unlikely event of a second failure during a rebuild of a previous failure.
LDEV striping across array groups
In addition to the conventional concatenation of RAID1 array groups (4D+4D), the XP7 supports
LDEV striping across multiple RAID5 array groups for improved logical unit performance in open
system environments. The advantages of LDEV striping are:
• Improved performance, especially of an individual logical unit, due to an increase in the
number of data drives that constitute an array group.
• Better workload distribution: in the case where the workload of one array group is higher
than another array group, you can distribute the workload by combining the array groups,
thereby reducing the total workload concentrated on each specific array group.
The supported LDEV striping configurations are:
• LDEV striping across two RAID 5 (7D+1P) array groups. The maximum number of LDEVs in
this configuration is 1000. See Figure 15 (page 26)).
• LDEV striping across four RAID 5 (7D+1P) array groups. The maximum number of LDEVs in
this configuration is 2000. See Figure 16 (page 26).
RAID implementation overview 25