ITEM: BA7083L

Which RAID provides better performance for databases.

Question:

Is RAID 1 or RAID 5 better for performance?
BP will probably use Sybase or Oracle.  His questions 
should be taken generally for RAID performance.
Response:

Raid 1 or Raid 0 provide better performance.  Databases run 
faster with mirroring which is Raid 1.  Raid 5 has a write
penalty that affects performance.

Next:   He wants to know if having one large volume group for all 
disk is better than having a vg per disk for RAID 5?
 Response:

RAID 5 is slower than RAID 1 which is slower than RAID 0

Oracle generally requests that certain things be placed on different
disks - this would be hdisks for the 7135 provided you are not using
sub-LUNs.  If you use sub-LUNs, then different hdisks are on the same
physical disks.

The 7135, especially due to the write penalty but also due to
limitations of the dac, is slow, you might want to consider SSA disks.

Assuming usage of RAID-5 only, we discussed the following:

Oracle recommends putting data tables and indexes on seperate disks
(and they actually have recommendations for up to 5 different drives,
there are more things than the tables and indexes: programs, etc.).

With the 7137 you get a choice of a single LUN or multiple LUNs.
With a single LUN you need multiple 7137s to assure that different
LVs are on different physical disks.  If you choose multi-LUN, you
will get N-1 LUNs (where N is the number of disks in the 7137).
Each of these LUNs is not spread out among all the disks.  For
the case of an 8 drive 7137, the LUNs are spread as follows:

  LUN0 is 87.5% on disk1 and 12.5% on disk2
  LUN1 is 75.0% on disk2 and 25.0% on disk3
  LUN2 is 62.5% on disk3 and 37.5% on disk4
  LUN3 is 50.0% on disk4 and 50.0% on disk5  
  LUN4 is 62.5% on disk6 and 37.5% on disk5
  LUN5 is 75.0% on disk7 and 25.0% on disk6
  LUN6 is 87.5% on disk8 and 12.5% on disk7

This means that when we use multi-LUN on the 7137, the hdisks
kind of are on separate disks.  Thus they could follow the Oracle
recommendations.  This also means that for a single LUN, the LVM
intra policies (center, middle ,edge, etc.) still apply, but
for multi-LUN, they do not apply.

On the 7135, sub-LUNs are spread evenly among the physical disks.
Thus LVM intra-policy does not apply when sub-LUNs are used.  So to
follow Oracle's recommendations would require separate LUNs (e.g., 
separate banks of disks for each LUN), without using sub-LUNs.  LVM
intra-policies apply in the case where sub-LUNs are not used.  



Support Line: Which RAID provides better performance for databases. ITEM: BA7083L

Dated: December 1995 Category: N/A

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