Monday, May 21, 2012

Storage Strategies -> SAN / NAS | RAID

Before we dive in, let's see some basic terms. This is by no means a complete list of key definitions, but it should make things clearer, or at least be enough to make this article comprehensible.

Direct Attached Storage (DAS): The server stores data on disks that are in the same box. Redundant Array of Independent Disks (RAID) is used heavily in this approach.

Storage Area Network (SAN): A collection of computers and devices are connected over a high-speed network and are dedicated to the task of storing and protecting data. Instead of storing data locally, each server sends data across the network to a shared pool of storage.

Disk Array: A large array of disks in one box, it is often used as part of a SAN to store data for multiple servers. These servers typically connect to the disk array using Fibre Channel.

Fibre Channel (FC): Optical fiber cables transmit data at high speed in a SAN. Fibre Channel is the transport protocol used for this purpose.

Network-Attached Storage (NAS): NAS separates data from applications by storing data on filers attached to the LAN. Filers can share files across multiple applications, platforms, and operating systems.

Internet Small Computer Systems Interface (iSCSI): This standard enables storage and retrieval at high speed (1 GB/second or higher) over regular IP networks.

Basic Strategy -> Keep it Simple

Storage is an immense and complex universe. Once you enter, your mind is soon swimming in strange, even alien concepts. Therefore, it is best to stick to what you know and keep it very simple — especially at the start. One obvious way to avoid complexity is to use the services of a storage service provider. These are firms that lease storage from their own data centers and other services. The advantage of a storage provider is that the vendor provides a variety of storage options for a fixed cost. This is a handy way to add storage capacity or meet regulatory compliance/archiving requirements without having to build new infrastructure.
Of course, simplicity can be taken to extremes (i.e., attempting to pass the entire storage burden to an external source or keeping everything stored on the same old servers using bigger and better disks). Such a strategy eventually runs into a wall; there is so much data stored on so many servers that it becomes impossible to manage.
Beyond DAS, then, where should the rookie storage guy go to ease his woes? Initially, at least, it might be smart to start with NAS and avoid SANs. At its core, a NAS filer is simply a specialized type of server that connects to the network. Storage is rapidly added by plugging the appliance into a network hub or switch. The likelihood is that the server administrator will run into very little that is new to him by buying a NAS box.
The drawback of NAS is that filers and servers share the same LAN. As a result, network performance may eventually be affected. When that juncture is reached, it may be remedied by upgrading the LAN and adding higher-grade NAS equipment. A more long-term solution would be to roll out the first SAN.

Simple SAN

Undoubtedly, the land of the SAN can be forbidding. Continuing with our theme of simplicity, the transition to a SAN can be made smoother by beginning with rapidly maturing iSCSI technology. iSCSI allows the establishment of a SAN over an IP network. Thus, the IT department does not need to learn new protocols or add new skill sets to create a SAN. This also has the advantage of being much less-expensive than an FC SAN.
iSCSI is especially appropriate for companies with IP backbones capable of handling gigabit traffic. While the technology is improving rapidly, it doesn't offer the same speed or capacities as a heavy-duty FC SAN. Similarly, SANs offer higher speeds and throughput than NAS systems. To do this, they offload data traffic to a separate network for storage devices.
On the negative side of the ledger, however, SANs may have difficulty supporting multiple operating systems and platforms. In addition, some users complain about being unable to integrate SAN solutions from different vendors. SANs don't have to be that big to be valuable or affordable, but they are made up of highly specialized components that require strong internal expertise.

Choose Wisely

The basic strategy for storage is to try to stick with the familiar. NAS and iSCSI are good starting points for competent IT departments already familiar with IP networking. FC SANs, on the other hand, should probably be avoided unless you have very large capacity and require the highest possible performance. If so, it is best to recruit a dedicated storage team to wrestle this beast and bend it to your corporate will. Although the cost and complexity are greater in the short term, the potential long-range payoff is greater than with NAS or iSCSI.
And for those that just don't want to involve themselves in yet another IT skill set, managed storage services now cover the entire spectrum. Sometimes it is just less-expensive, easier, or faster to call in the professionals and leave everything to them.

Did You Know?

The basic strategy for storage is to try to stick with the familiar. NAS and iSCSI are good starting points for competent IT departments already familiar with IP networking. FC SANs, on the other hand, should probably be avoided unless you have very large capacity and require the highest possible performance.

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Now let's move forward and learn a bit more about the different types of RAID:


RAID: Redundant Array of Independent Disks, a category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren't generally necessary for personal computers. RAID allows you to store the same data redundantly (in multiple paces) in a balanced ay to improve overall performance.

There are number of different RAID levels:

Level 0 -- Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.
Level 1 -- Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.
Level 2 -- Error-Correcting Coding: Not a typical implementation and rarely used, Level 2 stripes data at the bit level rather than the block level.
Level 3 -- Bit-Interleaved Parity: Provides byte-level striping with a dedicated parity disk. Level 3, which cannot service simultaneous multiple requests, also is rarely used.
Level 4 -- Dedicated Parity Drive: A commonly used implementation of RAID, Level 4 provides block-level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks.
Level 5 -- Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID.
Level 6 -- Independent Data Disks with Double Parity: Provides block-level striping with parity data distributed across all disks.
Level 0+1 -- A Mirror of Stripes: Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them. Used for both replicating and sharing data among disks.
Level 1+0 -- A Stripe of Mirrors: Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.
Level 7: A trademark of Storage Computer Corporation that adds caching to Levels 3 or 4.
RAID S: (also called Parity RAID) EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems.

Never forget to checkout with Wikipedia ;)

Hope it helps and clears your head!