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Have questions re terminology?

 

What is a .......?

 

NAS, SAN, RAID   

Logical Drives    Logical Volumes    NAS (Network Attached Storage)    SAN (Storage Area    RAID (Redundant Array of Independent)   

Mirroring, Snapshot, Replication

 

Logical Drives and Volumes
 

Redundant Arrays of Independent Disks, or RAID, offers the following advantages:

 

Availability, Capacity, and Performance.

 

Choosing the right RAID level and drive failure management can increase Availability, subsequently increasing Performance and Capacity.

 

What is a Logical Drive?
 

The advantages mentioned above are achieved by creating “logical drives.”

A logical drive is simply an array of independent physical drives. The logical drive appears to the host the same as a local hard disk drive does.

 

 

The following section describes the different methods by which logical drives are created, such as spanning, mirroring and data parity. These methods are referred to as “RAID levels.”


 

What is a logical volume?

The concept of a logical volume is very similar to a logical drive. A logical volume is composed of one or several logical drives, the member logical drives can be the same RAID level or different RAID levels.

The logical volume can be divided into a maximum of 8 partitions. During operation, the host sees a non-partitioned logical volume or a partition of a partitioned logical volume as one single physical drive.


NAS (Network Attached Storage)

The advantages of NAS over conventional server-attached storage can be summed up in three words: economy, speed, and ease. The NAS device sits on the network and is optimized for a single purpose: to pump data to users efficiently without the overhead and complexity of general-purpose servers.  NAS devices can produce improved file access performance at a substantially lower cost than general-purpose network servers. When factoring in the additional cost savings generated with plug-and-play installation that literally takes just minutes and ongoing reduced management costs, NAS is a true bargain for network storage needs. 

NAS is often contrasted with SANs, but NAS is actually under the "storage network" umbrella. The major difference is that the SAN is channel attached, and the NAS is network attached.  It should also be noted that adding or removing a NAS system is like adding or removing any network node.


What is a SAN

A Storage Area Network (SAN) is an independent network for storage subsystems, free from the rest of the computer network. In effect, a SAN removes the storage from the servers; thus liberating the storage devices from the ownership of the servers. In such a setup where no server has ownership of storage subsystems, any server can gain access to any storage device. In other words, any user can gain access to any storage device of the SAN, regardless of the physical location of the storage or the user.

In addition to offering any-to-any connections, a SAN creates a scaleable environment. Since storage and servers are independent from each other, storage devices or servers can be added or removed from their respective networks without affecting each other. Storage devices can be added to the SAN without any worry about a server's configurations. Isolating the potential disruptions of the servers from those of the storage reduces potential for interruptions.

SAN (Storage Area Networking) is designed around and encapsulated SCSI protocol. The most popular physical connections are based on high-speed Optical and Copper Fiber interconnects and can be shared via a hub or switched, much like the more common networking protocols. In this type of system data is transferred over a storage loop to the various peripheral devices on the SAN. This is essentially a private network whose bandwidth is 100MB/sec and that can support up to 128 devices. There is also a switching technology available that will allow over 15 million devices to be addressed and configured within a single switched fabric network, but the full specifications for this standard have not yet been formalized. The storage medium for SAN is based on SCSI disk and tape drives or on the newer Fiber Channel interface drives now entering the market.

The creation of an independent SAN further enhances the workflow of information among storage devices and other systems on the network. Additionally, moving storage-related functions and storage-to-storage data traffic to the SAN relieves the front end of the network, the Local Area Network (LAN), of time consuming burdens such as restore and backup.


The Various Types of RAID

RAID stands for Redundant Array of Independent (sometimes Inexpensive) Disks. Using a RAID storage subsystem has the following advantages:

Provides disk spanning by weaving all connected drives into one single volume.
Increases disk access speed by breaking data into several blocks when reading/writing to several drives in parallel. With RAID, storage speed increases as more drives are added.
Provides fault-tolerance by mirroring or parity operation.
 

A few terms that you will need to understand include:

Data Striping. Data is split across multiple drives in a RAID array to form a single logical storage unit. Each drive's storage space is partitioned into stripes, ranging in size from one sector (512 bytes) to multiple megabytes. The stripes then are interleaved so that the logical storage unit is made up of alternating stripes from each drive.

Mirroring. Used in RAID levels 1 and 1+0 for data recovery. Data is duplicated through mirroring across two disks. If one drive fails, the data remains available on the other disk. It's sort of like low-end clustering.

Parity. Information Used in RAID levels 3, 4 and 5 for data recovery. In the event of a drive failure, parity information can be combined with the other remaining data to regenerate the missing information.

What are the RAID levels?
 

RAID Level
Describe
Minimum Drives
Data Availability
Performance Sequential
Performance Random
NRAID Non-RAID 1   Drive Drive
RAID 0 Disk Striping N ==NRAID R: Highest

W: Highest

R: High

W: Highest

RAID 1 (0+1) Mirroring Plus Striping (if N>1) N+1 >>NRAID

==RAID 5

R: High

W: Medium

R: Medium

W: Low

RAID 3 Striping with Parity on Dedicated Disk N+1 >>NRAID

==RAID 5

R: High

W: Medium

R: Medium

W: Low

RAID 5 Striping with Interspersed Parity N+1 >>NRAID

==RAID 5

R: High

W: Medium

R: High

W: Low

 

NRAID

Disk Spanning

Minimum Disks Required=1

Capacity=N

Redundancy=No

NRAID stands for Non-RAID. The capacity of all the drives is combined to become one logical drive (no block striping). In other words, the capacity of the logical drive is the total capacity of the physical drives. NRAID does not provide data redundancy.

 

JBOD

Single Drive Control

Minimum Disks Required=1

Capacity=1

Redundancy=No

JBOD stands for Just a Bunch of Drives. The controller treats each drive as a stand-alone disk, therefore each drive is an independent logical drive. JBOD does not provide data redundancy.

 

RAID 0

Disk Striping

Minimum Disks Required=2

Capacity=N

Redundancy=No

RAID 0 provides the highest performance but no redundancy. Data in the logical drive is striped (distributed) across several physical drives.

 

RAID 1

Disk Mirroring

Minimum Disks Required=2

Capacity=N/2

Redundancy=Yes

RAID 1 mirrors the data stored in one hard drive to another. RAID 1 can only be performed with two hard drives. If there are more than two hard drives, RAID (0+1) will be performed automatically.
 

RAID (0+1)

Disk Striping with Mirroring
Minimum Disk Required=4

Capacity=N/2

Redundancy=Yes

RAID (0+1) combines RAID 0 and RAID 1 - Mirroring and Striping. RAID (0+1) allows multiple drive failure because of the full redundancy of the hard drives. If there are more than two hard drives assigned to perform RAID 1, RAID (0+1) will be performed automatically.

IMPORTANT: “RAID (0+1)” will not appear in the list of RAID levels supported by the controller. If you wish to perform RAID 1, the controller will determine whether to perform RAID 1 or RAID (0+1). This will depend on the number of drives that has been selected for the logical drive.
 

RAID 3

Disk Striping with Dedicated Parity Disk
Minimum Disk Required=3

Capacity=N-1

Redundancy=Yes

RAID 3 performs Block Striping with Dedicated Parity. One drive member is dedicated to storing the parity data. When a drive member fails, the controller can recover/ regenerate the lost data of the failed drive from the dedicated parity drive.
 

RAID 5

Striping with Interspersed Parity
Minimum Disk Required=3

Capacity=N-1

Redundancy=Yes

RAID 5 is similar to RAID 3 but the parity data is not stored in one dedicated hard drive. Parity information is interspersed across the drive array. In the event of a failure, the controller can recover/regenerate the lost data of the failed drive from the other surviving drives.

 

Mirroring is duplication of data by creating two I/Os from a single I/O. Disk mirroring is created on host systems through OS or volume management software. Disk mirroring is a local option that depends on the platform and the local connectivity characteristics. Certainly works with DAS and SAN and most NAS products support it.

Snapshots comes in three basic flavors: File system based, subsystem based and volume manager/virtualization based. All three are considerably different. Snapshots are an extremely important function for business continuity, but there are a lot of details to work through. You need a strategy for snapshots as well as a decent understanding of how you will establish operations to work with them. They will change your daily operations and they require constant, ongoing administration. Platform specific operations for flushing cache (file system buffers) matter a whole lot.

Replication is the transport of data objects (files -- tables) over a TCP/IP network. The transfer is made from system to system not between storage devices or subsystems.


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