Block devices and filesystems

Most storage devices share the same physical and logical structure, in order to be able to locate the data you want, you need a way to identify where your data resides, so this is the reason of why Hard Disk drives have sectors (or simply “blocks“), in many cases this reflects the layout of the data written into the physical medium. But accessing your data by addressing the sector number while not very complex, it’s an error prone method and you have to keep track yourself of the data you write and the sectors you have written to. So this is where a file-system comes to the rescue, a simple file-system will help you by addressing the blocks used and providing you a common interface to retrieve your data, the most common paradigm used is the folder/file structure (this is why is it called a file-system).

So essentially a file-system leaves you with a very simple interface towards your data, and takes care of the housekeeping and remembering in which blocks/sectors it puts your stuff, normally in a file system you will not refer to block numbers to get your data, but to filenames. This explains the difference between a block device and a filesystem, a filesystem must reside on a block device.

DAS stands for Directly Attached Storage. DAS is generally used to differentiate between storage systems directly attached to a server or workstation and NAS and SAN setups.It 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
  • IDE – Integrated Drive Electronics
  • SCSI – Small Computer System Interface
  • SATA – Serial ATA

NAS stands for Network Attached Storage. It differs from traditional, directly attached storage in that, in NAS, the operating system and other software on the NAS product are dedicated solely to data storage.It is a filesystem delivered over the network.

  • It is ready to mount and use.
  • Technologies to do this include NFS, SMB
  • NFS – Network File System
  • SMB – Server Message Block

SAN stands for Storage Area Network. A SAN is a network designed to attach storage hardware and software to servers. SANs generally come in two forms: as a network primarily dedicated to transferring data between computer systems and storage systems, or as a complete system that includes all of the storage elements and computer systems within the same network. It 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 FC, iSCSI, FCoE
  • FC – Fibre Channel
  • FCoE – Fibre Channel over Ethernet
  • iSCSI – Internet Small Computer System Interface