RAID (Redundant Arrays of Inexpensive Disks) in Oracle

RAID-0:
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-   RAID-0 offers pure disk striping.
-   It does not provide either data redundancy or parity protection.  
-   In fact, RAID-0 is the only RAID level focusing solely on performance. 
-   Some vendors, such as EMC, do not consider level 0 as true RAID and do not offer solutions based on it.  
-   Pure RAID-0 significantly lowers MTBF, since it is highly prone to downtime. 
-   If any disk in the array (across which Oracle files are striped) fails, the database goes down. 


RAID-1:
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-   With RAID-1, all data is written onto two independent disks (a "disk pair") for complete data protection and redundancy. 
-   RAID-1 is also referred to as disk mirroring or disk shadowing.  
-   Data is written simultaneously to both disks to ensure that writes are almost    as fast as to a single disk. 
-   During reads, the disk that is the least busy is utilized. 
-   RAID-1 is the most secure and reliable of all levels due to full 100-percent redundancy.  
-   However, the main disadvantage from a performance perspective is that every 
    write has to be duplicated. 
-   Nevertheless, read performance is enhanced, as the read can come from either 
    disk.  RAID-1 demands a significant monetary investment to duplicate each disk; 
-   however, it provides a very high Mean time between failures (MTBF). Combining RAID level 0 and 1 (RAID-0+1) allows data to be striped across an array, in addition to mirroring each disk in the array.

RAID-0 & RAID-1:
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-   If RAID/0 is then combined with RAID/1 (mirroring) this then provides the resilience, but at a cost of having to double the number of disk drives in the configuration. 
-   There is another benefit in some RAID/1 software implementations in that the requested data is always returned from the least busy device.This can account for a further increase in performance of over 85% compared to the striped, no n-mirrored configuration.
-   Write performance on the other hand has to go to both pieces of the software mirror. If this second mirror piece is on a second controller (as would normally be recommended for controller resilience), this degradation can be as low as 4 percent.


RAID-3:
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-   In a RAID 3 configuration, a single drive is dedicated to storing error correction or parity data. Information is striped across the remaining drives.
-   RAID/3 dramatically reduces the level of concurrency that the disk subsystem can support (I/O's per second) to a comparable software mirrored solution. The worst case for a system using RAID/3, would be an OLTP environment, where the number of rapid transactions is numerous and response time is critical.
-   So to put it simply, if the environment is mainly read only (Eg Decision Support) RAID/3 provides disk redundancy with read performance slightly improved, but at the cost of write performance. Unfortunately, even decision support databases still do a significant amount of disk writing since complex joins, unique searches etc still do temporary work, thus involving disk writing.

RAID-5:
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-   Instead of total disk mirroring, RAID-5 computes and writes parity for every write operation. The parity disks avoid the cost of full duplication of the disk drives of RAID-1. 
-   If a disk fails, parity is used to reconstruct data without system loss. Both data and parity are spread across all the disks in the array, thus reducing disk bottleneck problems. 
-   Read performance is improved, but every write has to incur the additional overhead of reading old parity, computing new parity, writing new parity, and then writing the actual data, with the last two operations happening while two disk drives are simultaneously locked. This overhead is notorious as the RAID-5 write penalty. This write penalty can make writes significantly slower. 
-   Also, if a disk fails in a RAID-5 configuration, the I/O penalty incurred during the disk rebuild is extremely high. Read-intensive applications (DSS,     data warehousing) can use RAID-5 without major real-time performance degradation (the write penalty would still be incurred during batch load operations in DSS applications). 
-   In terms of storage, however, parity constitutes a mere 20-percent overhead,     compared to the 100-percent overhead in RAID-1 and 0+1. 
-   Initially, when RAID-5 technology was introduced, it was labeled as the cost-effective panacea for combining high availability and performance. Gradually, users realized the truth, and until about a couple of years ago, RAID-5 was being regarded as the villain in most OLTP shops. Many sites contemplated getting rid of RAID-5 and started looking at alternative solutions.
-   RAID 0+1 gained prominence as the best OLTP solution for people who could afford it. Over the last two years, RAID-5 is making a comeback either as hardware-based RAID-5 or as enhanced RAID-7 or RAID-S implementations. However, RAID-5 evokes bad memories for too many OLTP database architects. 

RAID-S:
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-   RAID S is EMC's implementation of RAID-5. However, it differs from pure RAID-5 in two main aspects: 
(1) It stripes the parity, but it does not stripe the data.
(2) It incorporates an asynchronous hardware environment with a write cache. This cache is primarily a mechanism to defer writes, so that the overhead of calculating and writing parity information can be done by the system, while it   is relatively less busy (and less likely to exasperate the user!). Many users    of RAID-S imagine that since RAID-S is supposedly an enhanced version of RAID-5, data striping is automatic. They often wonder how they are experiencing I/O bottlenecks, in spite of all that striping. It is vital to remember that in RAID-S, striping of data is not automatic and has to be done    manually via third-party disk-management software. 

RAID-7:
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RAID-7 also implements a cache, controlled by a sophisticated built-in real-time operating system. Here, however, data is striped and parity is not. Instead, parity is held on one or more dedicated drives. RAID-7 is a patented architecture of Storage Computer Corporation.  


2. Pro's and Cons of Implementing RAID technology
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There are benefits and disadvantages to using RAID, and those depend on the RAID level under consideration and the specific system in question. 

In general, RAID level 1 is most useful for systems where complete redundancy of data is a must and disk space is not an issue.  For large datafiles or systems with less disk space, this RAID level may not be feasible.  Writes under this level of RAID are no faster and no slower than 'usual'.  For all other levels of RAID, writes will tend to be slower and reads will be faster than under 'normal' file systems.  Writes will be slower the more frequently ECC's are calculated and the more complex those ECC's are. Depending on the ratio of reads to writes in your system, I/O speed may have a net increase or a net decrease.  RAID can improve performance by distributing I/O, however, since the RAID controller spreads data over several physical drives and therefore no single drive is overburdened. 
 
The striping of data across physical drives has several consequences besides balancing I/O.  One additional advantage is that logical files may be created which are larger that the maximum size usually supported by an operating system.  There are disadvantages, as well, however.  Striping means that it is no longer possible to locate a single datafile on a specific physical drive. This may cause the loss of some application tuning capabilities.  Also, in Oracle's case, it can cause database recovery to be more time-consuming.  If a single physical disk in a RAID array needs recovery, all the disks which are part of that logical RAID device must be involved in the recovery.  One additional note is that the storage of ECC's may require up to 20% more disk space than would storage of data alone, so there is some disk overhead involved with usage of RAID. 
 

3. RAID and Oracle 
----------------------------
 
The usage of RAID is transparent to Oracle.  All the features specific to RAID configuration are handled by the operating system and go on behind- the-scenes as far as Oracle is concerned.  Different Oracle file-types are suited differently for RAID devices.  Datafiles and archive logs can be placed on RAID devices, since they are accessed randomly. The database is sensitive to read/write performance of the Redo Logs and should be on a RAID 1 , RAID 0+1 or no RAID at all since they are accessed sequentially and performance is enhanced in their case by having the disk drive head near the last write location.  Keep in mind that RAID 0+1 does add overhead due to the 2 physical I/O's. Mirroring of redo log files, however,is strongly recommended by Oracle. In terms of administration, RAID is far simple than using Oracle techniques for data placement and striping.



Recommendations:                                                               

In general, RAID usually impacts write operations more than read operation.This is specially true where parity need to be calculated (RAID 3, RAID 5, etc). Online or archived redo log files can be put on RAID 1 devices.  You should not use RAID 5. 'TEMP'  tablespace data files should also go on  RAID1 instead of RAID5 as well. The reason for this is that streamed  write performance of distributed parity (RAID5) isn't as good as that of simple mirroring (RAID1).                          
 
Swap space can be used on RAID devices without affecting Oracle.   
         
=================================================================================
RAID    Type of RAID                Control                   Database               Redo Log                Archive Log
                                                 File                          File                       File                           File
=================================================================================
0           Striping                         Avoid*                      OK*                    Avoid*                       Avoid*     
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
1          Shadowing                      OK                         OK                 Recommended        Recommended
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
0+1       Striping +                       OK                   Recommended           OK                           Avoid     
           Shadowing                                                      (1)                                                         
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
3         Striping with                     OK                        Avoid                   Avoid                         Avoid     
          Static Parity                                                     (2)                                                                    
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
5        Striping with                       OK                      Avoid                     Avoid                        Avoid     
        Rotating Parity                                                   (2)
-----------------------------------------------------------------------------------------------------------------------------------------------------------------

*   RAID 0 does not provide any protection against failures. It requires a strong backup strategy.
(1) RAID 0+1 is recommended for database files because this avoids hot spots and gives the best possible performance during a disk failure. The disadvantage of RAID 0+1 is that it is a costly configuration.
(2) When heavy write operation involves this datafile