HP P2000 G3 MSA Technical White Paper page 12

Technical white paper | HP P2000 G3 MSA
Supporting large storage capacities requires advanced planning because it requires using large virtual disks with several
volumes each or many virtual disks. To increase capacity and drive usage (but not performance), you can create virtual disks
larger than 2 TB and divide them into multiple volumes with a capacity of 2 TB or less.
The largest supported Vdisk is the number of drives allowed in a RAID set multiplied by the largest drive size.
RAID 0, 3, 5, 6, 10 can support up to 16 drives.
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RAID 50 can support up to 32 drives.
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Tip 1
The best practice for creating virtual disks is to add them evenly across both controllers. With at least one virtual disk
assigned to each controller, both controllers are active. This active-active controller configuration allows maximum use of a
dual-controller configuration's resources.
Tip 2
Another best practice is to stripe virtual disks across shelf enclosures to enable data integrity in the event of an enclosure
failure. A virtual disk created with RAID 1, 10, 3, 5, 50, or 6 can sustain an enclosure failure without loss of data depending
on the number of shelf enclosures attached. The design should take into account whether spares are being used and
whether the use of a spare can break the original design. A plan for evaluation and possible reconfiguration after a failure
and recovery should be addressed. Non-fault tolerant Vdisks do not need to be dealt with in this context because a shelf
enclosure failure with any part of a non-fault tolerant Vdisk can cause the Vdisk to fail.
Chunk size
When you create a virtual disk, you can use the default chunk size or one that better suits your application. The chunk (also
referred to as stripe unit) size is the amount of contiguous data that is written to a virtual disk member before moving to the
next member of the virtual disk. This size is fixed throughout the life of the virtual disk and cannot be changed. A stripe is a
set of stripe units that are written to the same logical locations on each drive in the virtual disk. The size of the stripe is
determined by the number of drives in the virtual disk. The stripe size can be increased by adding one or more drives to the
virtual disk.
Available chunk sizes include:
16 KB
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32 KB
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64 KB (default)
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If the host is writing data in 16 KB transfers, for example, then that size would be a good choice for random transfers
because one host read would generate the read of exactly one drive in the volume. That means if the requests are random-
like, then the requests would be spread evenly over all of the drives, which is good for performance.
If you have 16-KB accesses from the host and a 64 KB block size, then some of the host's accesses would hit the same drive;
each stripe unit contains four possible 16-KB groups of data that the host might want to read.
Alternatively, if the host accesses were 128 KB in size, then each host read would have to access two drives in the virtual
disk. For random patterns, that ties up twice as many drives.
Note
On RAID 50 drives, the chunk size is displayed as: <requested Chunk size> * (Num drives in sub Vdisk—1)
For example: A requested chunk size of 32 KB with 4 drives in a sub array. The chunk size is reported as 96 KB.
Using the formula: 32 K byte* (4-1) = 96 KB.
Tip
The best practice for setting the chuck size is to match the transfer block size of the application.
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