NAS vs SAN vs DAS
Direct-Attached Storage: Disks Connected to One Server
NAS vs SAN comes down to how storage is shared. Network-attached storage (NAS) serves files over an ordinary network, while a storage area network (SAN) serves raw block devices over a dedicated network. Both are usually compared with direct-attached storage (DAS), which connects disks straight to one server.
DAS is storage in the server chassis or in an external enclosure cabled to that server. Only the server can use those disks, but the connection is simple and has very low latency. DAS fits when one machine needs fast local disks and no other machine needs the data. Its weakness is sharing: if the server fails, its disks are reachable only once that server, or a replacement with the same enclosure, is working again.
NAS: Shared Files Over the Network
Network-attached storage is a device that serves files to many clients over a standard network. The NAS manages the file system itself, and clients request files by name using protocols such as NFS, which Linux and Unix systems commonly use, and SMB, which Windows and macOS use. Because the device understands files, it also handles folders, permissions and file locking.
NAS suits shared documents, home directories, media libraries and backup targets where several users or machines read the same files. It is simpler to set up than a SAN and runs on ordinary Ethernet. File-level access adds overhead, so it is generally less suited to databases that issue many small, time-sensitive reads and writes. Storage that spreads files across many machines is discussed in distributed storage and Ceph.
SAN: Block Storage on a Dedicated Network
A storage area network presents storage to servers as block devices, which behave like local disks. The server sees a logical unit number (LUN), formats it with its own file system and reads and writes blocks directly. The traffic runs over a dedicated network using Fibre Channel or iSCSI, which carries SCSI commands over ordinary IP networks. Block volumes that work this way are described in what block storage is.
Because the server controls the file system, a SAN suits databases, virtualization hosts and clustered applications that need predictable low latency. A hypervisor is the software that runs virtual machines on a host. Only one server should normally mount a given LUN unless the software on top coordinates shared access, because two servers writing to the same file system without coordination can corrupt it.
A Design Studio and a Virtualization Cluster
A design studio with ten staff keeps project files on a NAS. Everyone maps the same shared folder, the device enforces permissions, and one backup job copies one location. A hosting company runs virtual machines across three hypervisors and needs volumes that any host can attach after a failover, so it uses a SAN for those volumes and keeps a local disk in each host for the operating system.
Frequently asked questions
Is NAS or SAN faster?
Neither is faster in every case. A SAN usually gives lower latency for block workloads because it avoids file-level processing and runs on a dedicated network. A NAS can deliver high throughput for large file transfers. Speed depends on the network, the drives and the workload, so measure with the workload you actually run.
When should I use DAS instead of NAS or SAN?
Use direct-attached storage when a single server needs fast disks, sharing is not required, and simplicity and cost matter more than redundancy across machines. If the data must survive a server failure or be reached by several machines, a NAS or SAN is usually the better fit. Object storage, a third option, is compared in block vs file vs object storage.