Oracle Concepts - Tuning Considerations Oracle Tips by Burleson Consulting

Disk Striping, Shadowing, RAID, and Other Topics

Unless you’ve been living in seclusion from the computer mainstream, you will have heard of the above topics. Let’s take a brief look at them and how they will affect Oracle tuning.

Figure 1: Example Of Improper Striping

Disk Striping 

Disk striping is the process by which multiple smaller disks are made to look like one large disk. This allows extremely large databases, or even extremely large single-table tablespaces, to occupy one logical device. This makes managing the resource easier since backups only have to address one logical volume instead of several. This also provides the advantage of spreading IO across several disks. If you will need several gigabytes of disk storage for your application, striping may be the way to go. One disadvantage to striping: If one of the disks in the set crashes, you lose them all unless you have a high reliability array with hot swap capability.

Figure 2: Example of Proper Striping

Disk Shadowing or Mirroring 

If you will have mission-critical applications that you absolutely cannot allow to go down, consider disk shadowing or mirroring. As its name implies, disk shadowing or mirroring is the process whereby each disk has a shadow or mirror disk that data is written to simultaneously. This redundant storage allows the shadow disk or set of disks to pick up the load in case of a disk crash on the primary disk or disks; thus the users never see a crashed disk. Once the disk is brought back on-line, the shadow or mirror process brings it back in sync by a process appropriately called “resilvering.” This also allows for backup, since the shadow or mirror set can be broken (e.g., the shadow separated from the primary), a backup taken, and then the set resynchronized. I have heard of two, three and even higher mirror sets, generally I see no reason for more than a three-way mirror as this allows for the set of three to be broken into a single and a double set for backup purposes.

The main disadvantage to disk shadowing is the cost: For a two hundred-gigabyte disk farm, you need to purchase four hundred or more gigabytes of disk storage.

RAID—Redundant Arrays of Inexpensive Disks 

The main strength of RAID technology is its dependability. In a RAID 5 array, the data is stored as are check sums and other information about the contents of each disk in the array. If one disk is lost, the others can use this stored information to recreate the lost data. This makes RAID very attractive. RAID has the same advantages as shadowing and striping at a lower cost. It has been suggested that if the manufacturers would use slightly more expensive disks (RASMED—redundant array of slightly more expensive disks) performance gains could be realized. A RAID system appears as one very large, reliable disk to the CPU. There are several levels of RAID to date:

* RAID-0—Known as disk striping

* RAID-1—Known as disk shadowing

* RAID-0/1—Combination of RAID-0 and RAID-1

* RAID-2—Data is distributed in extremely small increments across all disks and adds one or more disks that contain a Hamming code for redundancy. RAID-2 is not considered commercially viable due to the added disk requirements (10–20% must be added to allow for the Hamming disks).

* RAID-3—This also distributes data in small increments but adds only one parity disk. This results in good performance for large transfers, but small transfers show poor performance.

* RAID-4—In order to overcome the small transfer performance penalties in RAID-3, RAID-4 uses large data chunks distributed over several disks and a single parity disk. This results in a bottleneck at the parity disk. Due to this performance problem RAID-4 is not considered commercially viable.

* RAID-5—This solves the bottleneck by distributing the parity data across the disk array. The major problem is it requires several write operations to update parity data. The performance hit is only moderate and the other benefits outweigh this minor problem.

* RAID-6—This adds a second redundancy disk that contains error-correction codes. Read performance is good due to load balancing, but write performance suffers due to RAID-6 requiring more writes than RAID-5 for data update.

For the money, I would suggest RAID0/1, that is, striped and mirrored. It provides nearly all of the dependability of RAID5 and gives much better write performance. You will usually take at least a 20% write performance hit using RAID5. For read-only applications RAID5 is a good choice, but in high transaction/high performance environments the write penalties may be too high.

New Technologies

Oracle is a broad topic; topics related to Oracle and Oracle data storage are even broader. This section will touch on several new technologies such as Optical Disk, RAM disk, and tape systems that should be utilized with Oracle systems whenever possible. Proper use of Optical technology can result in significant savings when large volumes of static data are in use in the database (read only). RAM drives can speed access to index and small table data by several-fold. High-speed tapes can make backup and recovery go quickly and easily. Let’s examine these areas in more detail.

Optical Disk Systems 

WORM (write once, read many) or MWMR (multiple write, multiple read) optical disks can be used to great advantage in an Oracle system. Their main use will be in storage of export and archive log files. Their relative immunity to crashes and long shelf life provide an ideal solution to the storage of the immense amount of data that proper use of archive logging and exports produce. As access speeds improve, these devices will be worth considering for these applications in respect to Oracle. Another area where they have shown great benefits is in read-only tablespaces. Now in Oracle8I with transportable tablespaces it becomes possible to create an entire catalog system on one Oracle server, place the tablespaces on CD-ROMs or PDCD-ROMs and literally ship copies to all of your sites where they will up and operating the day they get there.

Tape Systems 

Nine track, 4 mm, 8 mm, and the infamous TK series from DEC can be used to provide a medium for archive logs and exports. One problem with this is the need at most installations for operator monitoring of the tape devices to switch cartridges and reels. With the event of stacker-loader drives for the cartridge tapes, this limitation has all but been eliminated in all but the smallest shops. New DAT tape technology with fast streaming tape makes for even faster backup and recovery times.

RAM Drives (Random Access Memory) 

While RAM drives have been around for several years, they have not seen the popularity their speed and reliability should be able to claim. One of the problems has been their small capacity in comparison to other storage mediums. Several manufacturers offer solid state drives of steadily increasing capacities. For index storage these devices are excellent. Their major strength is their innate speed. They also have onboard battery backup sufficient to back up their contents to their built-in hard drives. This backup is an automatic procedure invisible to the user, as is the reload of data upon power restoration. The major drawback to RAM drives is their high cost. The rapid reductions in memory chip costs with the equally rapid increase in amount of storage per chip may soon render this drawback nonexistent.

New disk arrays such as those developed by EMC Technology provide a hybrid between disk and RAM technology with their multi-gigabyte high reliability arrays and multi-gigabyte RAM caches.

This is an excerpt from the eBook "Oracle DBA made Simple".

For more details on Oracle database administration, see the "Easy Oracle Jumpstart" by Robert Freeman and Steve Karam.  It’s only $19.95 when you buy it directly from the publisher here.

Need an Oracle Health Check? Do you have bad performance after an upgrade?   Need to certify that your database follows best practices? BC Oracle performance gurus can quickly certify every aspect of your Oracle database and provide a complete verification that your database is fully optimized.