Adaptec 1542B - AHA Storage Controller Fast SCSI 10 MBps Informações Técnicas descarregar pdf (Página 41)

Chapter 2. Netfinity solutions 31

With the advent of Ultra2 SCSI speeds, cable lengths required to support

single-ended devices have become too small for any practical use (approximately

75 cm). To compensate for this, a new connection standard has been developed

that uses the signaling aspects of the older SCSI differential standard without its

associated costs.

With the differential SCSI technology, where the same signal is transmitted along

two cables that have different polarity from the signal, up to 12-meter cable

lengths can be achieved. This method will improve the clarity of the signal as it

reduces the line noise caused by the signal fluctuation. See 2.4, “Netfinity storage

enclosures” on page 39 for implementation discussions.

To implement an LVDS SCSI solution, the adapter, cabling, disks, and disk

enclosures must all support LVDS.

Note: The ServeRAID II and ServeRAID adapters do not support LVDS SCSI

devices.

2.2.4 ServeRAID adapter cache

Each of the ServeRAID adapters has cache memory installed. Each uses 60 ns

EDO memory):

• ServeRAID-3HB: 32 MB (battery backup standard)

• ServeRAID-3H: 32 MB (battery backup optional)

• ServeRAID-3L: 4 MB (no battery backup option available)

The battery backup cache feature, which is standard on the ServeRAID-3HB and

optional on the ServeRAID-3H, protects data in the adapter’s write-back cache

from being lost in the event of a power outage in the server or a problem that

occurs in the adapter itself. Once the adapter is replaced the cache option can be

reinstalled allowing the data to be restored to the server. In addition, this option

contains high-speed cache memory to optimize RAID performance.

These options are write caches and only operate when the write policy is set to

write-back (WB) mode. They offer no additional benefit when the logical drives

are set to write-through (WT) mode.

Write-back cache works because the operating system is signaled before the

actual write to the drive has occurred. This method of caching can offer greater

performance and data throughput, but there is normally an exposure to data loss

in the event of a power failure. Using one of these options means that this

exposure is reduced and at the same time the user can enjoy increased

performance using write-back cache.

In the event of a power failure, the battery will maintain the data in the cache for

approximately 10 days. During that time, you should remove the cache option

from the failed adapter or failed server and install it in a functioning system. After

10 days, the data will be lost. During normal powered operation, the battery will

be maintained in a continuously charged state. The battery has a life expectancy

of about two years.

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Adaptec 1542B - AHA Storage Controller Fast SCSI 10 MBps Informações Técnicas Página 41