PERFORMANCE QUESTION ON LVM DISK MIRRORING.

ITEM: RTA000027499

QUESTION:                                                                       
Customer would like more information on the impact of disk mirroring            
relative to performance.  We have researched the General Concepts and           
the System Performance and Tuning Guide but would like more specific            
information on individual parameters etc.  Is there a redbook or a              
manual on LVM that would cover this?                                            
                                                                                
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A: We do not have specific documentation or a redbook that covers               
disk mirroring(as it relates to performance).  The documented information       
is in Infoexplorer or the ASKQ database.  I can list some suggestions           
to increase performance when using disk mirroring:                              
                                                                                
  1)  Limit the number of disks per SCSI adapter.                               
  2)  Mirror between different SCSI adapters.                                  
  3)  The performance will be affected by the location of the logical           
      volume with the disk.  A contiguous, unfragmented partition in a          
      logical volume will increase performance.                                 
  4)  You can turn off mirror write consistency cache for logical volume        
      mirroring, but by doing so, consistency of data is not guaranteed         
      in case of a crash.  In this case, you would have to recopy all           
      logical volumes to make sure they are consistent.  This does              
      provide a 20% + improvement in performance, however.                      
  5)  Make sure you are using the parallel scheduling policy.                   
  6)  If any of the logical volume exists on more than one disk, stagger        
      the partitions over the disks.  This is automatically accomplished        
      when the logical volume is created with a "maximum" INTER-POLICY.         
                                                                                
  Infoexplorer has more information on item 4) above in the "Scheduler          
Layer" stanza of the "Understanding the Logical Volume Device Driver"          
article.  Furthermore, a search using the words "mirror" and "logical"          
will bring up many subroutines and articles on this subject.                    
                                                                                
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This item was created from library item Q611280      BVZKR                      
                                                                                
Additional search words:                                                        
BVZKR DASD DEC92 DISC DISKETTE IX LVM MEASURE MIRRORING OP                      
PERFORMANCE QUESTION RISCPERF RISCSYSTEM SOFTWARE SYS TUNE                      
                                                                                
                                                                                
                                                                                
                                                                               



WWQA: ITEM: RTA000027499 ITEM: RTA000027499

Dated: 03/1996 Category: RISCPERF

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