RAID

Description

In-depth explanation of RAID in all its shapes and sizes

Contents

Introduction

Redundant array of independent disks (RAID) provides capacity and reliability, and increases performance by combining an array of physical hard disks into a single logical disk. RAID is a common feature shared by most servers and almost all storage platforms as no single disk can meet the demands of capacity and performance of a busy server.

RAID is not a backup

RAID protects your data against failure of one or more disks, and increases your uptime. However, it but it does not qualify as a backup. Please consider the following when before using RAID on your server:

  • RAID is not "offsite", it is part of your server
  • RAID cannot restore old or deleted data
  • RAID requires monitoring as a failed disk may go unnoticed
Note

Responsibility to monitor your RAID arrays is on you, and not with LeaseWeb. However, if a drive fails, you can create a support ticket to have it replaced.

RAID levels

RAID has different modes of operation, called "levels". The RAID levels differ in how the data stored on the logical disk is distributed over the physical disks in the array, which determines the capacity, performance and reliability of the array.

The most common RAID levels are:

LevelMethodMin. disks requiredCapacityBenefitsDrawbacksNotes
RAID 0Striping2N
  • High performance
  • Simple
  • No redundancy
Pure RAID 0 is not recommended, it often used in combination with RAID 1
RAID 1Mirroring21/2 N
  • Good reliability
  • Good performance
  • Simple
  • Only available for two disks
Common software RAID option
RAID 5Striping with distributed parity3N - 1
  • Good reliability
  • Good performance
  • Scalable
  • Slower rebuild after disk failure
 
RAID 6Striping with double distributed parity4N - 2
  • Excellent reliability
  • Good performance
  • Scalable
  • Slower rebuild after disk failure
Not available on all hardware RAID controllers
RAID 10Mirroring and striping41/2 N
  • Good reliability
  • High performance
  • Poor scaling
Combination of RAID 1 and RAID 0
Rebuilding

If a physical disk that is part of a RAID array fails, the array is said to be "degraded". When the failed disk is replaced, the data that was originally present on the failed disk should be regenerated and rewritten to the newly placed disk. This process is called "rebuilding".

For RAID 1 and RAID 10, rebuilding is simply a copy. For RAID 6 and RAID 5, the data has to be recalculated based on the remaining data and parity data.

Because the rebuilding process generates an additional I/O load on the remaining disks, performance of the array will be reduced until the rebuild is complete.

Calculating for RAID array characteristics

Calculating the capacity, reliability, and performance of an array can be tricky. A full explanation is beyond the scope of this article, but many excellent RAID calculation tools can be found online.

As an example, please refer to this tool from Wolfram|Alpha:
RAID calculator on Wolfram|Alpha

Implementations

Hardware RAID

RAID is most commonly implemented on dedicated hardware, known as the "RAID controller". This is especially true for both RAID 5 and RAID 6 that require complicated parity calculations. The RAID controller also provides a well-maintained set of management features.

To improve performance, especially for writes to the array, most hardware RAID controllers also include an amount of DRAM cache memory. But, in the event of a power failure, the information in the cache at that moment will be lost. To avoid this situation, a dedicated Battery Back-up Unit (BBU) can be attached to the controller to keep the contents of the cache memory available in the event of a power failure.

 

AdvantagesDisadvantages
  • Dedicated controller processor and memory offloads the main CPU
  • Lower chance of data loss in the event of a power failure
  • Well defined and implemented feature set
  • Supports larger numbers of disks
  • Supports hot-swapping failed disks
  • Most controllers support SAS disks
  • Vendor-specific implementation
  • Not available on all servers

For LeaseWeb, the IBM X3630 RAID controller is not equipped with a BBU by default. Please contact our Sales department if you want to add a BBU.

All other servers available at LeaseWeb that feature a hardware RAID controller are equipped with a BBU by default.

Software RAID

Software RAID uses software to emulate a logical disk from the physical disks present in the server. All modern operating systems provide support for software RAID. Compared to hardware RAID, it is almost never a preferable choice, but it is universally available.

Software RAID is most commonly used for simple RAID 1 redundancy on servers without a hardware RAID controller.

 

AdvantagesDisadvantages
  • Supports any kind of storage
  • Standardized set of tools
  • Low cost
  • Vendor agnostic
  • Flexible, can be mixed with non-RAID partitions
  • OS specific
  • Hot-swapping support is limited by OS and server model
  • OS may still be crashed by a failed disk
  • OS reboot may not work with a failed disk
  • No cache memory protection in the event of a power failure

RAID at LeaseWeb

LeaseWeb offers RAID on all servers containing 2 or more disks - a hardware RAID if the server model features a hardware RAID controller, or else a software RAID. Server reinstallations with software or hardware RAID are offered at no additional cost. If you wish to reinstall your server, and the desired RAID option is not available through our Customer Portal, you can request it via a support ticket.

Hard disk and RAID troubleshooting

Further reading

Wikipedia: RAID

Ars Technica: The skinny on RAID - May 2000, by Matt "Panders" Anderson & Ken "Caesar" Fisher

EECS Department, University of California, Berkeley: A Case for Redundant Arrays of Inexpensive Disks (RAID) - December 1987, by David A. Patterson, Garth Gibson, Randy H. Katz

 


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