Reading and writing a disk at the physical level. The disk controller in every computer and server reads and writes the disks at the block level. In storage area networks (SANs), where the disks are in cabinets external to the server, read/write access is also at the block level.
Block Level vs. File Level
An application accesses data by file name (logical), which is translated into block level (physical) for reading and writing by the operating system's file system.
Users always deal with data at the file level; however, database and network administrators may get involved at the block level, because they can stipulate where data are physically stored for performance issues.
NAS vs. SAN
Although a stand-alone unit with its own disks, a network attached storage (NAS) device has its own file system for file level access. Because a SAN uses block level access and has no file system with its associated overhead, a SAN is much faster than a NAS. See SAN
and file system
Remote Block Level Access
With IP storage, block level access can be extended via the IP protocol to remote locations. See SAN
and IP storage
Blocks Are Also Logical
Modern drives maintain their own tables of good and bad sectors and remap requests on the fly. As a result, the physical block reads and writes are translated by the drive into actual sector reads and writes, making the block access somewhat like a logical request, rather than 100% physical.