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Full Discussion: A1000 Battery Question
Top Forums UNIX for Dummies Questions & Answers A1000 Battery Question Post 53150 by Perderabo on Wednesday 7th of July 2004 01:25:58 AM
Old 07-07-2004
Cache is a holding area. When the computer writes to a disk, the RAID can acknowledge the write as soon as the data has arrived in cache. This makes disk writes seem to be much faster. But you probably want that data to actually get written to the disk. (A cache will delay for awhile in case the data is written again. That way multiple disk writes actually become a single write.) But if you lose power, the actual disks stop spinning. Now you need to preserve the data until power is restored. That's where the battery comes in.

So far, we've talked about write-behind. But cache is also used for read-ahead. The RAID is paying attention to which data the computer wants and tries to guess which read may occur in the future. If a disk goes idle, it will issue the reads to have the data in the cache ready for a read to occur.

This is big performance boost, and without the cache performance drops.

Even if I didn't care about disk i/o performance. I would replace the battery. The code in firmware of the RAID may have bugs. And they probably don't test degraded mode as thoroughly as it's normal mode. Degraded hardware always makes me nervous.
 

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CACHEINFO(5)							AFS File Reference						      CACHEINFO(5)

NAME
       cacheinfo - Defines configuration parameters for the Cache Manager

DESCRIPTION
       The cacheinfo file defines configuration parameters for the Cache Manager, which reads the file as it initializes.

       The file contains a single line of ASCII text and must reside in the /etc/openafs directory. Use a text editor to create it during initial
       configuration of the client machine; the required format is as follows:

	  <mount>:<cache>:<size>

       where

       <mount>
	   Names the local disk directory at which the Cache Manager mounts the AFS namespace. It must exist before the afsd program runs. The
	   conventional value is /afs. Using any other value prevents traversal of pathnames that begin with /afs (such as pathnames to files in
	   foreign cells that do use the conventional name).  The -mountdir argument to the afsd command overrides this value.

       <cache>
	   Names the local disk directory to use as a cache. It must exist before the afsd program runs. The standard value is /usr/vice/cache,
	   but it is acceptable to substitute a directory on a partition with more available space. Although the Cache Manager ignores this field
	   when configuring a memory cache, a value must always appear in it. The -cachedir argument to the afsd command overrides this value.

       <size>
	   Specifies the cache size as a number of 1-kilobyte blocks. Larger caches generally yield better performance, but a disk cache must not
	   exceed 90% of the space available on the cache partition (85% for AIX systems), and a memory cache must use no more than 25% of
	   available machine memory.

	   The -blocks argument to the afsd command overrides this value. To reset cache size without rebooting on a machine that uses disk
	   caching, use the fs setcachesize command. To display the current size of a disk or memory cache between reboots, use the fs
	   getcacheparms command.

EXAMPLES
       The following example cacheinfo file mounts the AFS namespace at /afs, establishes a disk cache in the /usr/vice/cache directory, and
       defines cache size as 50,000 1-kilobyte blocks.

	  /afs:/usr/vice/cache:50000

SEE ALSO
       afsd(8), fs_getcacheparms(1), fs_setcachesize(1)

COPYRIGHT
       IBM Corporation 2000. <http://www.ibm.com/> All Rights Reserved.

       This documentation is covered by the IBM Public License Version 1.0.  It was converted from HTML to POD by software written by Chas
       Williams and Russ Allbery, based on work by Alf Wachsmann and Elizabeth Cassell.

OpenAFS 							    2012-03-26							      CACHEINFO(5)
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