ITEM: RTA000038987

We just installed Informix On-Line on 3.2.4.  We have two volume                
groups.  One volume group contains the operating system, the other              
contain user programs and data.  On two drives within the users                 
volume group, I have defined logical volumes for Informix.  Informix            
is writing to these logical volumes directly, so they do not contain            
AIX JFS's.  When we installed Informix, we specified an offset of               
8KB to avoid writing over any control information.  Is this a large             
enough offset?  We will NOT be using the mirroring function of AIX              
on these logical volumes.  On another logical volume we did NOT                 
specify an offset.  My concern was that the logical volume control              
block would get overwritten because of this.  To check this I                   
issued dd if=/dev/hd20 ibs=1 count=65384 of=/tmp/hd20.out.  When I              
did an od -a /tmp/hd20.out, I saw what appeared to be valid control             
information for the LVM.  Is this a valid check?  Do I have to issue            
getlvcb -AT hd20 to determine if the information is corrupted?                 
For some other drives in the system, I did NOT add them to any                  
volume group.  These drives are accessed directly by Informix.  An              
lspv shows them to have valid PVID's.  We also used an 8KB offset               
for these drives.  I am pretty sure that since these drives are not             
part of any volume group, an 8K offset is large enough.                         
Do I need to reconfigure my raw logical volumes in the user volume              
group with a larger offset?  What offset do I use?  The programmers             
reference seems to mention that the first 128 sectors                           
(512 Bytes/sector) should be reserved for the LVM.  But I think this            
may be overkill since we are not doing mirroring.                               
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      Under the "mklv" command, InfoExplorer documents that the first few       
   hundred bytes within the logical volume contain the logical volume           
   control block (LVCB).  Logical volume data begins on the second              
   512-byte block.  I tested this and verified that writing after the           
   512 bytes did not affect the LVCB.  Therefore an 8K offset, although         
   very generous, will not overwrite the LVCB.  The dd command and              
   "getlvcb -AT LVNAME" command will both read the LVCB.  With the dd           
   command however, you will need to be familiar with the unformatted           
   output.  I favor using getlvcb, as its output makes it easier to             
   notice if LVCB corruption exists.                                            
      InfoExplorer states (in the article "Understanding Physical Volumes       
   and the Logical Volume Device Driver") a physical volume reserves the        
   first 128 sectors to store various types of DASD configuration and           
   operation information.  The /usr/include/sys/hd_psn.h file describes        
   the information stored on the reserved sectors.  In summary, the             
   sector layout is:                                                            
   o  The boot record (sector 0) contains information that allows the           
      ROS to boot the system.  It also contains the PVID.                       
   o  The MWC record (sector 2 and 3) used by LVM identifies which              
      logical partitions may be inconsistent if the system is not               
      shutdown properly.                                                        
   o  The LVM record (sector 7) consists of one sector and contains             
      information used by the LVM when the physical volume is a member          
      of the volume group.                                                      
   o  The bad-block directory (sector 8-29) records the blocks on the          
      physical volume that have been diagnosed as unusable by LVDD (part        
      of LVM).                                                                  
   o  Sectors 64, 70, 71 consist of LVM backup information.                     
   o  The area following the first 128 sectors contains the VGDA/VGSA           
      for the LVM.                                                              
   o  End of the physical volume is reserved as a relocation pool for           
      bad blocks that are are software-relocated by LVM.                        
   It seems as though everything after sector 0 is related to LVM.              
   Sector 0 contains the important PVID information that should not be          
   overwritten.  Therefore, the offset can be set to 512-bytes (starting        
   at sector 1), if LVM is not being used and the disk is being used as        
   a raw device.                                                                
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This item was created from library item Q652960      7QGDZ                      
Additional search words:                                                        
OFFSET OZIBM OZNEW RAW RISC VOLUMES 7QGDZ                                       

WWQA: ITEM: RTA000038987 ITEM: RTA000038987
Dated: 11/1996 Category: RISCOHW
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