Learning the ABCs for Network Storage - Part One
Basic Network Storage Strategies
Deciding how you're going to start or expand your network storage should be a simple proposition. But spending a day listening to sales pitches riddled with confusing acronyms like DAS, SAN, and NAS will likely result in cranial cramps of the severe variety. Still, as with most things, a little knowledge can help to alleviate the pain of progress. Here's what you need to know.
When purchasing mass storage for your network, there are three basic strategies; DAS (Direct Attached Storage), SAN (Storage Area Network), and NAS (Network Area Storage).
DAS Overview
In the world of network storage, DAS is the king where small networks are concerned. In simplest terms, DAS is like your desktop system with its internal hard disks and tape drives, external peripherals, etc. except that it's also serving a network. The key to the concept is that all the storage is directly controlled by the server, not mounted remotely on a network node à la NAS, or once removed via a Fibre Channel switch as with a SAN.
DAS pros:
DAS is a good for small networks: If the amount of data you're handling is relatively small, typically 500GB or less, and you're not expecting significant growth, then you're fine with DAS. It's relatively simple to upgrade a DAS server or attached RAID box should you want to add more storage later. And you can always move to NAS for additional storage.
DAS is widely understood by IT personnel: Setting up, upgrading, or maintaining a DAS box can be handled by just about anyone with any network experience at all. If your IT guys can't handle a DAS upgrade, you need new ones! By comparison, setting up a SAN will likely entail additional training for your IT personnel.
DAS is fast: Since DAS storage is attached directly to the server; there are no pipes or wires other than local busses for data to travel through.
DAS plays well with others: There's nothing to stop you from attaching either a NAS box or SAN to a DAS server. These are actually your best bets for expansion after your capacity needs top the 500GB mark.
DAS cons:
DAS means downtime: The biggest problem with a DAS server is that to add or repair storage attached to it, you must shut it down. For an in house LAN this can be done in the wee hours without damaging your business. However, for a WAN or Internet server operating around the clock, downtime means lost dollars and consumer confidence. Imagine how much money you could lose while IT takes your server down to make redundant backups of a 500GB (1TB) of data, add new hardware, reconfigure RAID setups, and copy the data back. Costly doesn't even begin to describe it. All kinds of workarounds have been devised, but for many businesses the fact remains that the straight DAS model has been outgrown.
DAS stresses your processor: As a network gets larger, serving files and applications while simultaneously handling networking communications can severely tax a DAS server's CPU.
DAS doesn't scale well: The most common DAS storage technologies --ATA and SCSI-- have limits that require workarounds. ATA allows only 2 devices per channel with a 133MBps maximum transfer rate, and SCSI allows 15 at 160MBps (320MBps SCSI is in the works). A server can only handle so many controller cards -- typically four or five --so your limited in storage scope.
If you’re interested in the DAS solution, here are the two vital questions you should ask yourself before heading in that direction:
1. How much storage do I need? If the answer is 500GB or less DAS alone will do. Anything beyond that and you should be looking at a DAS/NAS or SAN solutions.
2. Can I afford downtime? If the answer to this is no, then DAS isn't your best bet.
Page 2: NAS Overview, Pros and Cons