Network Attached Storage (Nas) - IBM TotalStorage DS300 Best Practices Manual

2.3.1 Network Attached Storage (NAS)

Storage devices which optimize the concept of file sharing across the network
have come to be known as NAS. NAS solutions utilize the mature Ethernet IP
network technology of the LAN. Data is sent to and from NAS devices over the
LAN using TCP/IP protocol. By making storage devices LAN addressable, the
storage is freed from its direct attachment to a specific server, and any-to-any
connectivity is facilitated using the LAN fabric. In principle, any user running any
operating system can access files on the remote storage device. This is done by
means of a common network access protocol, for example, Network File System
(NFS) for UNIX servers, and Common Internet File System (CIFS) for Windows
servers. In addition, a task, such as back-up to tape, can be performed across
the LAN, using software like Tivoli® Storage Manager (TSM), enabling sharing of
expensive hardware resources, such as automated tape libraries, between
multiple servers. A storage device cannot just attach to a LAN. It needs
intelligence to manage the transfer and the organization of data on the device.
The intelligence is provided by a dedicated server to which the common storage
is attached. It is important to understand this concept. NAS comprises a server,
an operating system, plus storage which is shared across the network by many
other servers and clients. So an NAS is a device , rather than a network
infrastructure
, and shared storage is attached to the NAS server.
One of the key differences of an NAS disk device, compared to Direct Attached
Storage or other network storage solutions, such as FC SAN or iSCSI SAN, is
that all I/O operations use file level I/O protocols. File I/O is a high level type of
request that, in essence, specifies only the file to be accessed, but does not
directly address the storage device. This is done later by other operating system
functions in the remote NAS appliance. A file I/O specifies the file. It also
indicates an offset into the file. For instance, the I/O may specify "Go to byte
'1000' in the file, (as though the file was a set of contiguous bytes), and read the
next 256 bytes beginning at that position". Unlike block I/O, there is no
awareness of a disk volume or disk sectors in a file I/O request. Inside the NAS
appliance the operating system keeps tracks of where files are located on disk.
The OS issues a block I/O request to the disks to fulfill the file I/O read and write
requests it receives.
In summary, the network access methods, NFS, CIFS, and NetWare, can only
handle File I/O requests to the remote file system. This is located in the operating
system of the NAS device. I/O requests are packaged by the initiator into TCP/IP
protocols to move across the IP network. The remote NAS file system converts
the request to block I/O and reads or writes the data to the NAS disk storage. To
return data to the requesting client application the NAS appliance software
re-packages the data in TCP/IP protocols to move it back across the network. A
database application which is accessing a remote file located on an NAS device,
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Chapter 2. iSCSI overview