HP Modular Smart Array 500 Starter Kit Uživatelská příručka stáhnout pdf (Strana 81)

HP StorageWorks 2300 Modular Smart Array CLI reference guide 81

Cache read-ahead size

You can optimize a volume for sequential reads or streaming data by changing the amount of data read in

advance after two back-to-back reads are made. Read ahead is triggered by two back-to-back accesses to

consecutive logical block address (LBA) ranges. Read ahead can be forward (that is, increasing LBAs) or

reverse (that is, decreasing LBAs). Increasing the read-ahead size can greatly improve performance for

multiple sequential read streams. However, increasing read-ahead size will likely decrease random read

performance.

The default read-ahead size, which sets one chunk for the first access in a sequential read and one stripe

for all subsequent accesses, works well for most users in most applications. The controllers treat volumes

and mirrored vdisks (RAID 1) internally as if they have a stripe size of 64 Kbyte, even though they are not

striped.

Cache redundancy mode

In the storage system’s default operating mode, Active-Active, data for volumes configured to use

write-back cache is automatically mirrored between the two controllers. Cache redundancy has a slight

impact on performance but provides fault tolerance. You can disable cache redundancy, which permits

independent cache operation for each controller; this is called independent cache performance mode

(ICPM).

The advantage of ICPM is that the two controllers can achieve very high write bandwidth and still use

write-back caching. User data is still safely stored in nonvolatile RAM, with backup power provided by

super-capacitors should a power failure occur. This feature is useful for high-performance applications that

do not require a fault-tolerant environment for operation; that is, where speed is more important than the

possibility of data loss due to a drive fault prior to a write completion.

The disadvantage of ICPM is that if a controller fails, the other controller will not be able to fail over (that

is, take over I/O processing for the failed controller). If a controller experienced a complete hardware

failure, and needed to be replaced, then user data in its write-back cache is lost.

CAUTION: Data might be compromised if a RAID controller failure occurs after it has accepted write

data, but before that data has reached the disk drives. Do not use ICPM in an environment that requires

fault tolerance.

NOTE: Independent cache performance mode disables partner firmware upgrade. Controllers must be

upgraded manually.

The operating mode applies per storage system, not per volume.

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HP Modular Smart Array 500 Starter Kit Uživatelská příručka Strana 81