HP StorageWorks Enterprise Virtual Array Cluster FIO Starter Kit Guía de usuario descargar pdf (Pagina 121)

Setup volumes might be spread across different arrays for additional redundancy, but remember that

all writes are mirrored, and therefore the slowest performing volume will determine when the write is

acknowledged. HP does not recommend placing all three volumes on a single array; if that is all that

is available, create only two setup volumes and use different pools if the array has that option.

Building basic storage pools

Storage pools can be optimized for performance or for capacity; however, there is only one way to

configure storage pools to enable the maximum 2 PB per domain, and another way to deliver maximum

performance, but not both at the same time. This section defines the best practices for building

capacity-optimized storage pools.

Experience with configurations in the field has indicated that pools should be built using at least 8–16

back-end LUs. Adding more back-end LUs to a pool is allowed, and in some cases may be desirable,

as long as other scalability limits are not exceeded. Less than 16 LUs should also work, but is

discouraged because it limits the capabilities of the system to distribute the I/O across the many paths

to the storage.

A simple concatenated pool should have all of its volumes presented from a single back-end array.

The volumes should have the same RAID type, and similar performance and capacity characteristics.

The pool is constructed of at least as many volumes as there are paths from the DPMs to the array

(16 for an 8-host port array). The benefits and trade-offs of this approach are as follows:

• With a concatenated pool, the pool can be expanded by adding one or more additional volumes

of arbitrary size without changing the basic performance characteristics of the virtual disks carved

out of that pool. Best practice would be to add volumes of the same size, or roughly the same

size, as the original volumes.

• By having all the back-end volumes on a single array, the availability of the virtual disks carved

from the pool is dependent only on the availability of that single array.

• By having all the back-end volumes on a single array, it is relatively straightforward to map per-

formance information from the array to the pool.

• By having all the back-end volumes on a single array it is simpler to debug issues.

• By having the same RAID type and disk drives for all the volumes of the pool, the performance

characteristics of the pool are derived from the performance characteristics of the disk drives and

RAID type. If these are mixed, it is not possible to predict what RAID type and disk drive will be

used by any front-end virtual disk, and the behavior could be very unpredictable—even between

different LBAs within a single front-end virtual disk.

• Occasionally an I/O will span two back-end volumes. This is called a split I/O. Split I/Os are

handled on the DPM soft path. I/Os handled by the soft path do not enjoy the lowest latency and

highest throughput achieved by the DPM fast path. An occasional split I/O will have an impercept-

ible impact. With concatenated pools, there are very few split I/Os because there are only as

many opportunities for split I/Os as there are adjacent back-end volumes.

• A single path is used by any one DPM to access each back-end volume. If that path fails, the DPM

will select one of the alternate paths that are available. If a pool were constructed of a single back-

end volume, then a single path will be used from each DPM to the pool. If there are ten virtual

disks using the capacity of that pool, all I/O to those ten virtual disks will be concentrated on that

single path. If there are no other pools on that array, then the resources associated with the addi-

tional ports and controllers are unused. Performance on the single path can suffer with long

latencies and even “queue full” responses.

• By having at least as many back-end volumes in the pool as paths from DPMs to the array there

is the opportunity that all those paths might be used in parallel. The odds of using multiple paths

grows as the number of back-end volumes in the pool increases. For an 8-port EVA like the

EVA8400 that is zoned to a single quad on each of two DPMs, 16 different paths are created

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