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There are many factors to consider when optimizing your RAID configuration, such as reliability, performance, cost, access times, application type and operations under degraded mode. These factors influence how your RAID configuration is fully optimized. Therefore, the advantages and disadvantages of various RAID levels need to be considered and compared with the IO workloads to determine the best configuration for your solution.
This paper specifically addresses SMB and SME RAID implementations with current technology and practical application, taking into account manufacturers features and capabilities targeting these two markets.
Quick Raid Level Overview
| RAID Level
| Description
| Data Protection
| Use
| Notes
|
| RAID 0 |
Stripes data across multiple disks and presents a single volume |
None |
High IO without data protection |
Best IO performance - lose a drive loose lose all the data |
| RAID 1 |
Simple mirroring of disks, presents as a single volume |
1 Disk |
Small capacities with protection |
Best suited for OS replication or very specific file integrity |
| RAID 10 |
Mirrors drives and adds mirror set together, presents as a single volume |
2 Disk |
High IO with protection |
Used to be considered best performance bet for database applications |
| RAID 0+1 |
Two RAID 0 sets are mirrored, presents a single volume |
2 Disk |
High IO with protection |
Used to be considered best performance bet for database applications |
| RAID 3 |
Places data bits across multiple drives and adds parity bits to dedicated drive(s), presents as single volume |
1 Disk |
High transfer rates |
Never practical because of bad blocks reallocation and spindle synchronization requirement |
| RAID 5 |
Stripes data across multiple disk and embeds parity information across all drives |
1 Disk |
High IO with protection without high cost per GB penalty for redundancy incurred with RAID 10 or 0+1 |
Best bet for high IO requirements, today the write penalty is greatly reduced over earlier RAID 5 |
| RAID 6 |
Stripes data across multiple disk and embeds parity information across all drives |
2 Disk |
High IO with protection without high cost per GB penalty for redundancy, plus extra drive failure protection over RAID 5 |
Best bet for high IO requirements and extra drive failure protection, slightly higher write penalty over RAID 5 |
RAID 50
RAID 60 |
Grouping of two or more RAID 5/6 sets |
1 Disk
2 disk |
High IO with protection and larger volume sizes |
Best bet for large volumes with high performance requirements |
Only Three Raid Set Tuning Objectives
There are only three RAID tuning objectives, tune for IO, transfer rates or a balance.
IO performance is dependent on five things
"Disk Access Time" which is average rotational speed plus average seek time
Correct RAID level selection
RAID stripe size
Cache tuning
Number of drives in a RAID set
Transfer rates are dependent on five things
Rotational speed and bit density of the disk
Correct RAID level selection
RAID stripe size
Cache tuning
Number of drives in a RAID set
Balanced Tuning
Balanced performance is really a compromise of tuning for IO or Transfer speed
How Many Drives Should be Used in a Single RAID Set?
Today's RAID storage solutions for the SMB and SME market can support between four to 256 drives as part of the total subsystem. This does not mean optimal performance will occur at low or high drive count extremes. Prudent use of RAID sets with drive counts between two and thirty-two will yield best performance for IO or transfer requirements. Do not mistake this with a limitation of total capacity, large usable capacities can still be accomplished by presenting two or more RAID sets using RAID50/60 or allowing the operating system or storage appliance to concatenate multiple RAID sets. The type of drive, SATA-II, SAS and SSD will also determine how many drives should be part of a RAID Set.
Recommended Drive Minimums/Maximums per RAID Set for Performance
|
| RAID 0
| RAID 10
| RAID 5
| RAID 6
|
| SSD |
2/16 |
4/16 |
3/16 |
4/16 |
| SAS 15k RPM |
2/16 |
4/16 |
5/16 |
5/16 |
| SAS 7200 RPM |
2/16 |
8/16 |
8/16 |
8/16 |
| SATA 7200 RPM |
2/16 |
8/16 |
8/16 |
8/16 |
Most hardware based RAID arrays targeting the SMB and SME markets support more than sixteen drives. Many array manufacturers offer enclosures that support 8, 16, 24, 36 and 48 drives but most have a limit of 32 drives in a RAID set. In situations where you feel compelled to create RAID5 sets containing more than sixteen drives, and this is very common when filling a 24 bay enclosures, you may want to consider using RAID6 as a hedge against more than one drive failing in a RAID set.
Your particular requirements may mandate presenting a LUN that requires using more than 16 or 32 drives meet a single LUN capacity requirement. In these situations, use the RAID50 or RAID60 feature. The RAID50/60 allows multiple RAID5/6 set to be concatenated into a very large LUN and while maintaining performance and data protection.
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