What is the difference between SAN, NAS and DAS?

First it is best to define the difference between a block device and filesystem. This is easier grasped if you are familiar with UNIX because it makes an objective distinction between the two things. Still the same applies to Windows.

  • A block device is a handle to the raw disk.
    • Such as /dev/sda for a disk or /dev/sda1 for a partition on that disk.
  • A filesystem is layered on top of the block device in order to store data. You can then mount this.
    • Such as mount /dev/sda1 /mnt/somepath.

With those terms in mind it’s then easier to see the distinction between the following.

  • DAS is a block device from a disk which is physically [directly] attached to the host machine.
    • You must place a filesystem upon it before it can be used.
    • Technologies to do this include IDE, SCSI, SATA, etc.
  • SAN is a block device which is delivered over the network.
    • Like DAS you must still place a filesystem upon it before it can used.
    • Technologies to do this include FibreChannel, iSCSI, FoE, etc.
  • NAS is a filesystem delivered over the network.
    • It is ready to mount and use.
    • Technologies to do this include NFS, CIFS, AFS, etc.

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