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Quest Pipelines

March's Tip of the Month

How Many RAID5 Drives Do I Need?
Compliments of Mike Ault, DBA Pipeline SYSOP ( and Cary Millsap
For more online references from Mike Ault, check out ROBO Books International.

Don't calculate how many RAID 5 drives you need by counting bytes of storage capacity. You must buy enough drives to provide the number of I/O operations per second (IOps) that your application will require. You know the problem: every small write call requires 4 actual physical I/O operations. So in your model of how many IOps your application will require, use the formula T = R + 4*SW + G/(G-1)*LW, where the variables are:

  • T is total number of I/O calls per second at peak
  • R is number of reads per second at peak
  • SW is number of small writes per second at peak
  • LW is number of large writes per second at peak
  • G is the number of disks in one RAID 5 array
A "large" write is any write that engages all G disks in the array for one write call. A "small" write is any write that engages fewer than G disks to execute the call.

There are many cases I've seen in which you have to buy *more* RAID 5 disks to compensate for the IOps penalty than if you had just bought a RAID1 configuration to begin with. It's not true for everyone, but I've seen it happen with some regularity.

April's Tip of the Month

Dynamic Memory Allocation in Oracle9i
Compliments of Robert Freeman, DBA Pipeline SYSOP

Oracle9i allows you to dynamically allocate or deallocate memory from the database buffer cache or the keep or recycle buffer pools, providing there is enough memory available. Use the alter system command to resize the buffer caches up or down, as required. Here is an example of such an operation:

Alter system set db_cache_size=100m scope=both;
Alter system set db_keep_cache_size=100m scope=both;
Alter system set db_recycle_cache_size=10m scope=both;
In the first example we are changing the default cache size to 100 megabytes dynamically, so that assuming no error is generated, this change will take effect dynamically. In the second example we are setting the keep buffer pool to 100m, and in the final example the recycle buffer pool is set to 10m. You can, of course, size the caches up or down, as required. Note that dynamic sizing is not available if your database buffer cache or your keep or recycle pools are allocated using the old Oracle8i parameters db_block_buffers, buffer_pool_keep, or buffer_pool_recycle.

As always there is a catch. The catch to dynamic memory management is that to add memory you must have prepared the system for such an event by setting the parameter sga_max_size. This parameter defines the maximum overall size of the SGA, and no component of the SGA can be dynamically expanded such that it exceeds the size defined by sga_max_size. Also note that on a number of systems, sga_max_size will cause the system to allocate memory up to sga_max_size at instance startup, essentially making that memory unavailable to other processes or databases. The only exception to this appears to be Solaris 2.8.



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