How can storage improve things?
We now have four different methods of Data Storage & Management - DAS (Direct Attached Storage), SAN (Storage Area Networks using Fibre Channel), iSCSI (SCSI over Ethernet) and NAS (Network Attached Storage).
DAS – Direct Attached Storage
Direct attached storage can be in the form of RAID, tape or optical devices. The most common interface for directly attaching storage is SCSI (Small Computer Systems Interface), this has been around for 20 years and is known and understood by all IT departments. It is also reliable, robust and fast with transfer speeds up to 320MB/s. SCSI is being replaced by SAS (Serial Attached SCSI) which has a more simple cable and is expected to perform at up to 600MB/s. The primary problem of DAS is anything that you attach to the server can only be seen by that server and as such IT departments purchase more disk than normal to avoid the problem of re-building the RAID at a later date.
SAN - Storage Area Networking
A newer way of serving storage is a SAN.
What is a SAN?
Definition 1:
- a high-speed network dedicated to moving information between users and a pool of flexible storage devices
- users no longer compete with server I/O for network bandwidth
Definition 2:
- a high-speed network that establishes a direct connection between heterogeneous storage devices and servers
- a network behind a network
Traditionally, SANs are good at handling data a block at a time rather than a file at a time, so they're ideal for databases. High-speed fibre channel connections give servers quick access to large amounts of data, and specific storage resources can be allocated to specific applications. With fibre-channel switches and management tools, SANs become part of flexible arrays of storage hardware known as storage fabrics. A storage fabric can manage storage for several different applications, as well as linking backup systems to the storage network. SANs are usually an expensive solution.
With the emergence of the SAN - Storage Area Network people for the first time could move large volumes of information very fast 4Gb/s around a dedicated data only network. Whilst this was great news there was also a downside these were:
- Totally new dedicated storage area network using Copper or Fibre cabling
- Additional fibre channel network cards for each server
- Fibre channel switches
- Storage area network management software
- Training costs for deploying and managing new SAN
- Increased storage costs for having fibre channel storage or fibre channel to SCSI bridges
- If redundancy was required then everything would need to be duplicated
The cost of deploying a SAN 4-5 years after they became available is still prohibitively high in relation to other networking infrastructures. An example of this is Fibre Channel cards which are approx £500 - £800 compared to less than £100 for a Gigabit Ethernet card.
As network storage technologies mature, and management tools become more powerful, businesses can begin to take advantage of storage virtualisation. Like virtual servers, storage virtualisation lets you treat storage as a resource pool you can allocate on demand. In the past, SAN architectures have had to be based around single-vendor solutions. That could change with the storage management interface specification that the storage network industry association is pushing as a basis for interoperability. This uses existing management standards like CIM (the common information model) and WBEM (web-based enterprise management), along with web services.
It is important to check whether your applications have been certified for use with NAS or SAN systems before investing in a solution. Some applications, like Microsoft's Exchange 2003 messaging tool, are only certified for use with block-level interfaces.
