RAID Controller Features

    With RAID technology, data is striped across an array (a group) of hard disk drives.  Striping is the process of storing data across all the disk drives that are grouped in an array.  This data distribution scheme complements the way the operating system requests data. 

The following features further enhance the performance of your PS/2 Server 95A. 

Overlapped Input/Output Operation 
Interleave Depth 
Queue Depth 
Cache 
   Cache Size 
   Cache Size Factors 
   Cache Size and Diminishing Returns 
Algorithms 
Hot-Spare Drive and Replacement 
Data Protection (Takes you to RAID Levels)



Overlapped Input/Output Operation 
   Because the IBM RAID Controller provides multiple data paths to and from arrayed drives, your server can respond to requests from several users simultaneously.  With its overlapped input/output operation, if one user requests data that resides on the first drive of the array and a second user requests data that resides on the second drive, the controller can simultaneously deliver both pieces of information. 

Interleave Depth 
   The granularity at which data from one file is stored on one drive of the array before subsequent data is stored on the next drive of the array is called the interleave depth.  For the IBM RAID Controller in your server, the interleave depth is set at 16 sectors to maximize system performance. 
   The collection, in logical order, of these 16-sector blocks, from the first drive of the array to the last drive of the array, is called a stripe. 

Queue Depth 
   Commands are queued in the controller with a queue depth of 61.  To obtain better performance, the commands in the queue will be reordered and coalesced on a hard disk drive basis.  That is, the controller organizes the commands according to which drive will be responding, and then orders and combines two or more commands, when possible, before sending them off to the drives. 



Cache 
   The IBM RAID Controller has 4MB of cache memory, which can be configured to operate in a write-through or write-back mode on a logical drive basis. (Refer to Changing the Write Policy for more information about write-through and write-back modes.) Cache memory has parity to detect memory errors and retry algorithms to recover from errors that appear sporadically. 

Cache Size 
      How important is the amount of cache ram on the PassPlay RAID adapter--4 MB, 16 MB, 64 MB?  Under what circumstances will a cache increase pay off? (The system in question is running NetWare 4.1, but I'm interested in general info on this subject.) 
    I notice that the more recent Cheetah RAID adapter has only 4 MB with no upgrade possible.  It seems counterintuitive, but I seem to remember reading somewhere that large amounts of controller cache aren't really that useful with modern drives and operating systems. 

From Peter 
    Having a large cache is only half the truth. Bigger cache means more damage if the controller chokes and cannot write data back to the drives. Large caches on Raid controllers make sense only if they are battery-backed  (Ed. I have seen battery-backed 72 pin SIMMs) and if there is a mechanism that allowes to remove the cache (with the data), replace the adapter, plug back the cache and restart the system to that point where the operation was stopped and write the cache data down to the drives and maintain the integrity of the data / array.  
     The older Raid-controllers (Server-95 Raid "Passplay", Fast/Wide Streaming Raid /A "Cheetah" and Fast/Wide Raid PCI "DAC960") don't have battery backed cache. Even 4MB of cache memory contains a large number of "data-stripes" (usually 8K blocks). 
     These data-stripes will be lost if the machine powers down for any reason, or the controller fails, or the operating system hangs. Recalculate how many sectors fit in 4MB - and the higher the number of missing sectors the lower the chance that the Raid-Utility will be able to restore the missing data. 

Cache Size Depends On 
 a) Overall drive data-throughput (buffering x accesses while drives are in *mechanically* causes delay / dead zone / recalibration) 
b) Data-stripe size (8K normally - 64K under WinNT might be better) 
c) Operating system (WinNT and OS/2 are very "swap active") 
d) Structure of the RAID itself (Raid-5 uses the cache much more than Raid-1 ... because the mirroring is imminently fast with buffering the data). 
 e) Nature of the data blocks. Consequently high internal redundancy of the data will cause higher "hit rates" within the cache than permanent data-streaming with new data, which void the cached data and only "pass through". 

     Like on all caches there is a limit where enlarging the cache any further makes no sense. And I think this limit is at around 4MB on a 5 drives Raid-5 system running under OS/2 or WinNT. The content-redundancy of the data is mostly not given - so the cache is mostly used to buffer the Raid data-overhead between the drives (during reading / writing / synchronizing the Raid structure) - on the transfer between drive-subsystem and processor the cache does not play a major role. 
     A larger cache here costs only money and bears the above mentioned risks to render the entire array useless if something crashes.  

Cache Size and Diminishing Returns 
 From "Mark" 
       Generally speaking, increasing the amount of cache will always improve performance. The performance gain will be more for sequential access type applications than for random access type applications. 
   Typically increasing the cache from 2 to 4MB will see a bigger % gain than 4 to 16 MB and that will see a bigger % gain than 16 to 32 MB and so on. 



Algorithms 
   The IBM RAID Controller provides adaptive RAID algorithms for improved high performance. 

Hot-Spare Drive and Replacement 
   The hot-spare drive is a hard disk drive that is installed in your server and is defined for automatic use in the event of a drive failure.  The hot-spare drive must be of equal capacity or larger than the drives in the array.  You can define as many hot-spare drives as you want. 
   If a drive fails, the system automatically switches to the hot-spare drive, and a rebuild operation recreating the data in the defunct drive automatically occurs in the hot-spare drive.  The system automatically defines the replacement drive as a hot spare. 

Note:  A hot-spare drive is effective only in an array in which no logical drives are defined as RAID level 0. 

   No data loss occurs in arrays with logical drives assigned only RAID level 5 or 1 or a combination of these two levels. Data is lost in an array with any logical drive assigned RAID level 0. 
  You must have at least four hard disk drives if you want a hot-spare drive and RAID level 5.  To maintain capacity, the size of the additional drive can be larger but must be no smaller than the size of the drives that came with your server.  All the drives in an array are configured to the capacity of the smallest. 
 

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