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Lessons Learned: Configuring RAID

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  TABLE OF CONTENTS
   What are the different RAID configurations?
   Will using RAID impact my workload?
   Which RAID configuration is best for a database?
   Can database logs be put on RAID devices?
   Does RAID support multiplexing or mirroring?

This week's teachers:

 

 

 

Brian Peasland
Paul Baumgartel
 
Karen Morton
(former expert)


 


  What are the different RAID configurations?
[ Return to Table of Contents ]

RAID 0: This is where datafiles are striped across mutliple disk volumes. But this is not true "RAID" since it offers no redundancy.

RAID 1: This is where data on one disk volume is completely mirrored on another disk volume. If you lose a disk volume, then no problem. The system just gets the data from the other volume.

RAID 0+1 (Sometimes called RAID 10): This combines the best of the two above. Your datafiles are striped across multiple volumes and those volumes are all mirrored. You get very nice disk throughput and very good redudancy.

RAID 3: This stripes data across multiple volumes. One volume is devoted to "parity bits." This bits are used to reconstruct data should you lose a data volume.

RAID 5: This is similar to RAID 3, where parity bits are computed and used to reconstruct lost data. The difference is that the parity bits are not stored on a separate disk, they are striped across all disk, interspersed with the data. RAID 5's biggest advantage is that it uses the least amount of disk space for recovering lost data. So you don't have to buy as much disk. Unfortunately, RAID 5 has the worst write performance of all of these RAID levels.

Excerpted from Brian Peasland's "The best RAID configuration for Oracle9i."

 


 

  Will using RAID impact my workload?
[ Return to Table of Contents ]

It depends on how RAID is configured. If it's RAID 5, for example, your workload is likely to increase, as Oracle performance is often poor with this setup. A properly-configured RAID (ideally RAID 10, or mirrored and striped volumes) storage subsystem, however, should not materially increase a DBA's workload once implemented. Most of the work involved in using RAID lies in the planning, ideally done in conjunction with your system administrator.

Excerpted from Paul Baumgartel's "How RAID affects your DBA workload."

 


 

  Which RAID configuration is best for a database?
[ Return to Table of Contents ]

RAID 5 is very good for read operations though since the file is striped across multiple disk volumes. If your database is READ MOSTLY, then you may consider RAID 5, but that is rarely the case. RAID 0+1 does not have the write penalty like RAID 5 does. And RAID 0+1 also stripes the data so the read performance is as good as RAID 5. The only downside to RAID 0+1 is that you waste more physical disk space to set up the redundancy. But if I had my vote, it would go to RAID 0+1 every time over RAID 5.

Excerpted from Brian Peasland's "Best RAID level for database."

 


  Can logs be put on RAID devices?
[ Return to Table of Contents ]

Rollback segments and redo logs are accessed sequentially (usually for writes) and therefore are not suitable candidates for being placed on a RAID-5 device. Also, datafiles belonging to temporary tablespaces are not suitable for placement on a RAID-5 device.

Another reason redo logs should not be placed on RAID-5 devices is related to the type of caching (if any) being done by the RAID system. Given the critical nature of the contents of the redo logs, catastrophic loss of data could ensue if the contents of the cache were not written to disk, e.g. because of a power failure, when Oracle was notified they had been written. This is particularly true of write-back caching, where the write is regarded as having been written to disk when it has only been written to the cache. Write-through caching, where the write is only regarded as having completed when it has reached the disk, is much safer, but still not recommended for redo logs for the reason mentioned earlier.

Excerpted from Karen Morton's "Why logs shouldn't be put on RAID devices."

 


  Does RAID support multiplexing or mirroring?
[ Return to Table of Contents ]

RAID does not support multiplexing. RAID 1, however, does support mirroring. Mirroring and multiplexing are two different animals. In mirroring, you have two copies of the same datafile, stored on two different disks. In multiplexing, you have more than one copy of the same information, but they are stored in different datafiles. Control files and online redo logs should be multiplexed. They can be mirrored, but they should always be multiplexed.

Excerpted from Brian Peasland's "RAID and multiplexing."

Go to the Lessons Learned library for additional lessons and quizzes.

 


 


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