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P222 cache module


dikkiedirk
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Is a cache module on the P222 controller necessary to be able to rebuild a failing disk? What exactly is the main purpose of the cache module?

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store data that is in cache until power has been returned to the machine so it can be written to disk.

 

Extract from the Smart Array Technology Manual - http://h10032.www1.hp.com/ctg/Manual/c00257513.pdf

 

Role of cache in the storage system

 

The process of transferring data to and from disk storage includes storing the data temporarily in cache memory located on the RAID (redundant array of independent disks) array controller that is managing the data transfer. Cache is comprised of high-speed silicon memory DRAM chips. The access time for writing data to or reading data from DRAM (dynamic random access memory) memory is roughly 106, or a million times faster than the typical access time for writing directly to or reading directly from a set of disk drives. In a posted-write operation, as soon as the host computer writes data to the cache, the write operation is completed; and the host is freed up immediately to perform another operation. The host does not have to wait for the write data to be transferred to disk. Therefore, use of cache memory on HP RAID array controllers significantly speeds up write operations and increases overall system performance. Posted-write operations also provide other benefits. If the host computer reads from data still residing in write cache, the RAID controller will quickly deliver this read data from its cached contents. If the host writes new data to the same location, the controller will over-write the cached contents, thus eliminating an extra disk write operation. Write cache can improve the performance of any RAID level by combining two or more adjacent write requests into a single request for a disk drive. Such an operation reduces disk rotational latency and processing overhead. Write cache can significantly improve RAID5 performance by accumulating enough write requests to perform a full-stripe write to the disk drives. This eliminates the need to read-modify-write parity data for each write access, since all members of the stripe needed to update the parity are available in the write cache.

Edited by eddie563
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Apologies, you are right, i looked in 2 documents but only linked one,

 

Data recovery (rebuild)

 

When you replace a hard drive in an array, the controller uses the fault-tolerance information on the remaining drives in the array to reconstruct the data from the original drive and write it to the replacement drive. This process is called automatic data recovery or rebuild. If fault tolerance is compromised, this data cannot be reconstructed and, likely, is lost permanently. Rebuilding an array requires that you either be in offline ACU or booted into the operating system with the driver installed. The operating system must be running for the rebuild to occur. If another drive in the array fails while fault tolerance is unavailable during rebuild, a fatal system error can occur, and all data on the array is then lost. In exceptional cases, however, failure of another drive does not always cause a fatal system error. These exceptions include the following:

• Failure after activation of a spare drive

• Failure of a drive that is not mirrored to any other failed drives (in a RAID 1+0 configuration)

 

it would only go quicker if you had a cache, without one, it will take longer. I have rebuilt with a P410 without a cache on so i don't see what the difference in the P222 would be in that scenario.

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Thanks again, I have another error on the cache module I am afraid. I replaced the capacitor packed but after it finished charging the cache wasn't enabled. So there might be a problem with the cache module I am afraid. SSA says it is permanently disabled. On booting the server I see a selftest error on the cache module. I hoped this was related to the capacitor pack but alas. Don't know how to troubleshoot further now. 

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only other way i'd say is to replace it if you can, if not, remove it if this is not a production box and can suffer a bit of data loss that would have been being written to the disk if the box goes down.

I just had a look in the quickspecs for the P222, Check out page 6 flowing to 7, the section Software Suite.

 

im not sure if you have tried the ADU for more in-depth diagnostics.

 

 

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Yeah, it is not doing anything now so I might take it out now. I was afraid afraid the disk rebuild feature was compromised so I had to replace the cache module immediately. Now I can wait for a good deal on ebay and get me one then. Does it matter what size? Is 512 MB sufficient?

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depends on the amount of data you will be pushing thought the card. but i looked at the differences previously and it was very minimal from what i read to be honest.

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