SANs and NASs

SAN and NAS

Storage Area Networks and Network Attached Storage - what is the difference?

There are several differences (basing this on more typical setups, there are exceptions)

SAN

Example physical setup: A chassis containing a number of disks, the disks have an FCAL interface connector, which hotplugs into a FCAL backplane, which is connected to one or more FCAL RAID controllers (e.g., dothill/artecon ones I have used have 2 controllers for redundancy). These are connected via FCAL cabling to a FC switch, which have connections to FCAL HBAs in servers. You don't need the RAID controllers in the disk chassis, as you could have JBODs (I think the dothill ones allow a JBOD mode anyway). It seems a bit unusual to have an HBA connect to a RAID controller, at least compared to usual SCSI practice, which would have a RAID controller in the server. I think the idea is that you can define the RAID level on the chassis, and define LUNs on an array which are presented as disks to the server, though I can't remember (5 years ago since I set them up and administered them!). There is presumably some way that allows you to share the array with multiple clients that will only allow one client to access one area of the array. Maybe it uses tags.

In a SAN environment, servers see the SAN storage as local disks, that is, access requests for data on a SAN disk is like any other local disk, at the block level. Typically, this is via SCSI commands, as the attached disks are seen as SCSI devices (I think they use the SCSI command set, not sure. In the Linux kernel source tree, there are FCAL HBA drivers in the SCSI low-level block device drivers section).

NAS

Network Attached Storage

In a NAS environment (which could use a SAN as it's basis), the storage is another host, usually with a full host OS, although Network Appliance boxes that I have run have a special version of BSD on them, with a restricted command set. The NAS host will share the data as per NFS, CIFS etc.

DAS

Direct Attached Storage

Host has storage directly attached