CompTIA A+ 902 - RAID

Types of RAID?

A number of standard schemes have evolved. These are called levels. Originally, there were five RAID levels, but many variations have evolved, notably several nested levels and many non-standard levels (mostly proprietary). RAID levels and their associated data formats are standardized by the Storage Networking Industry Association (SNIA) in the Common RAID Disk Drive Format (DDF) standard: - RAID 0 - striping - RAID 1 - mirroring - RAID 5 - striping with parity - RAID 6 - striping with double parity - RAID 10 - combining mirroring and striping

What is RAID?

RAID (array of independent disks) is a data storage virtualization technology that combines multiple physical disk drive components into a single logical unit for the purposes of data redundancy, performance improvement, or both.

Software RAID

Software RAID implementations are provided by many modern operating systems. Software RAID can be implemented as: A layer that abstracts multiple devices, thereby providing a single virtual device (e.g. Linux kernel's md and OpenBSD's softraid) A more generic logical volume manager (provided with most server-class operating systems, e.g. Veritas or LVM) A component of the file system (e.g. ZFS, GPFS or Btrfs) A layer that sits above any file system and provides parity protection to user data (e.g. RAID-F)

How does it work?

Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the required level of redundancy and performance. The different schemas, or data distribution layouts, are named by the word RAID followed by a number, for example RAID 0 or RAID 1. Each schema, or RAID level, provides a different balance among the key goals: reliability, availability, performance, and capacity. RAID levels greater than RAID 0 provide protection against unrecoverable sector read errors, as well as against failures of whole physical drives.

What is parity?

Many RAID levels employ an error protection scheme called "parity", a widely used method in information technology to provide fault tolerance in a given set of data. Most use simple XOR, but RAID 6 uses two separate parities based respectively on addition and multiplication in a particular Galois field or Reed-Solomon error correction. RAID can also provide data security with solid-state drives (SSDs) without the expense of an all-SSD system. For example, a fast SSD can be mirrored with a mechanical drive. For this configuration to provide a significant speed advantage an appropriate controller is needed that uses the fast SSD for all read operations. Adaptec calls this "hybrid RAID".

Operating systems RAID

Many operating systems provide RAID implementations, including the following: - Apple's OS X and OS X Server support RAID 0, RAID 1, and RAID 1+0. - FreeBSD supports RAID 0, RAID 1, RAID 3, and RAID 5, and all nestings via GEOM modules and ccd. - Linux's md supports RAID 0, RAID 1, RAID 4, RAID 5, RAID 6, and all nestings. Certain reshaping/resizing/expanding operations are also supported. - Microsoft's server operating systems support RAID 0, RAID 1, and RAID 5. Some of the Microsoft desktop operating systems support RAID. For example, Windows XP Professional supports RAID level 0, in addition to spanning multiple drives, but only if using dynamic disks and volumes. Windows XP can be modified to support RAID 0, 1, and 5. Windows 8 and Windows Server 2012 introduces a RAID-like feature known as Storage Spaces, which also allows users to specify mirroring, parity, or no redundancy on a folder-by-folder basis.

How does RAID 0 work?

RAID 0 consists of striping, without mirroring or parity. The capacity of a RAID 0 volume is the sum of the capacities of the disks in the set, the same as with a spanned volume. RAID 0 divides a set of data evenly among multiple hard drives, usually two drives. The main goal and benefit of using RAID 0 is improved speed/performance. With this level, there is no redundancy of data, that is, each individual batch of data is stored on only one disk. This makes RAID 0 systems vulnerable to data loss, since the failure of one disk on the array affects the entire system. The data is "striped" across both hard drives so none of the data on the failed disks can be recovered without a repairing a failed disk. This leads to the unfortunate fact that the more hard drives you have in a RAID 0 configuration, the less you should rely on it. The more drives you have, the greater the chance that one of them will fail and the data on that drive will be lost, affecting the rest of the data as well.

How does RAID 1 work?

RAID 1 consists of data mirroring, without parity or striping. RAID 1, stores an exact copy of your data on two or more drives. This makes your data much more secure; if one drive in the system fails, your data can simply be retrieved from any other drive in the system. This is known as data redundancy. With your files copied and stored on every drive in a RAID 1 system, you'd have to have all the drives fail simultaneously to lose all of your data. Another significant difference between RAID 1 and RAID 0 is that with RAID 1, the reliability of the system increases with each drive used in the configuration. The more drives you use, the more copies there are of your data and the less likely that all drives will fail at the same time, causing the loss of your data. The downside of the mirroring RAID 1 is that in certain situations if one drive fails or you accidentally delete a portion of your data the other hard drive will mirror the damaged one.

How does RAID 10 work?

RAID 1+0: creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses so long as no mirror loses all its drives

How does RAID 5 work?

RAID 5 offers the advantages of data storage redundancy along with a high level of performance. RAID 5 offers the advantages of data storage redundancy along with a high level of performance. It works differently from RAID 0 and 1, using striping (distributing small segments of data across a number of hard drives) and parity (a technique that allows data to be reconstructed from any drive in the system if it is lost). The combination of striping and parity adds a high degree of fault tolerance to your system, meaning that it can still function effectively even in the face of multiple drive failures. Your data is copied and spread out among all the hard drives in the configuration, so the chance of experiencing a complete system failure eliminating all your files is much lower.

A user is installing Windows 7 on a PC with a RAID card. The user has verified all of the cables are connected correctly, but the installation media does not detect any hard drives. Which of the following should the user do to help detect the hard drives?

Select load driver. Often when installing Windows onto an external RAID array, a driver is required for Windows to detect the hard drives.

What are the most commonly used RAID levels?

The most common RAID configurations are levels 0, 1 and 5. There are also various combinations of these levels such as RAID 10 or RAID 50. Each level carries certain advantages and disadvantages depending on how you plan to use it.

You are installing Windows Server 2008 R2 onto a new company file server that uses a RAID card. You have verified that all cabling is correctly seated, but Windows is not recognizing any storage mediums for installation. Which of the following will most likely correct the issue?

Use the correct RAID driver Many RAID cards require a vendor specific driver to function with Windows. In order to install Windows, you first need to provide it with the proper RAID driver.