Transcript File Systems and N/W attached storage (NAS)

File Systems and N/W attached
storage (NAS)
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File Systems
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• File systems represent an intermediate layer in the operating system that
provides users with the familiar directories and files and stores these on
block oriented storage media so that they are hidden to the end users.
Also a volume manager is being used between the file system and hard
disk.
• File systems (modern-additional)provide three functions
i. Journaling is a mechanism that guarantees the consistency of the file
system even after a system crash. It Maintains log files.(every change is
written to the file system)
In older file systems like Microsofts FAT32 or unix file system don't have
journaling.
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ii. Snapshots: Similar to instant copy
Snapshots freeze the state of a file system at a given point of
time. Applications and end users can access the frozen copy
through a special path. Snapshot can be realized with any
hardware. Instant copies puts less load on the CPU.
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Volume Manager
The volume manager is an intermediate layer within the OS
between the file system or database and actual hard disks.
It aggregates several hard disks to form a large virtual hard disk
and makes this virtual hard disk visible to higher layers.
Volume manager Provides the same function as that of RAID
controller.
In volume manager functions like RAID, instant copies and
remote mirroring are realized in hardware independent
manner.
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Network File System
• Network file systems are the natural extension of local file system. End
users and applications can access directories and files that are physically
located on a different computer-the file server-over a network file system.
• i.e network file systems give end users and applications access to data
stored on different computer.
• The first network file system was the NFS(Network File System) developed
by SUN Microsystems, which is now the standard network file system on
all Unix systems.
• Microsoft developed its own network file system-Common Internet File
System(CIFS)- for its windows operating system and this is incompatible
with NFS.
• With the aid of network file systems, end users and applications can work
on a common data set from various computers.
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• Before WWW, the FTP provided the mechanism by means of
which users could exchange files over the internet. Access to
the FTP servers is visible to the end user (NFS not visible).
Users require special FTP client for copying between FTP
server and local computer.
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• The HTML and HTTP changed the usage model of the internet.
• In contrast to FTP the data on the internet is linked together by means of
HTML documents.
• The user doesn’t access individual files, instead he surfs the web.
• He views HTML documents on his browser that are statically available on a
HTTP server in the form of files or dynamically generated.
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Network Attached Storage(NAS)
• Network Attached Storage is the name for preconfigured file servers .They
consist of one or more internal servers, disk capacity and a special
operating system.
• NAS servers are usually connected to the LAN, where they provide their
disk space as file servers.
• NAS servers were specially designed for file sharing. This has 2
advantages:
• i. NAS operating systems can be better optimized than generic operating
systems. That is , NAS servers can operate more quickly than file servers
on comparable hardware that are based upon a generic operating system.
• Ii. NAS servers provide Plug and Play file systems. In contrast to a generic
operating system all functions can be removed that are not necessary for
file serving. NAS storage can thus excel due to low installation and
maintenance costs.
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• NAS servers are scalable.
• One disadvantage of NAS servers is the unclear upgrade path. That is, the
internal server cannot simply be replaced by a more powerful server
because this goes against the principle of preconfigured file server. The
upgrade options available in this situation is that of the manufacturer
dependent.
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Performance bottlenecks in file servers.
NFS ,CIFS or FTP and HTTP are not powerful enough for I/O intensive
applications like databases or video processing. I/O intensive databases
draw their storage from disk subsystems and not from file servers.
When a client wants to read a file from a NAS server, the NAS servers OS
loads the file into main memory from the hard disk thru SCSI bus. If the
load on the file server is high its buses becomes a performance
bottleneck.
Thus performance is limited due to:
i. All data accesses to network file systems have to pass through a single file
server.
ii. Current network file systems such as NFS and CIFS and the underlying
network protocols are not suitable for a high throughput.
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• Remote Direct Memory Access (RDMA) is a
direct memory access from the memory of one
computer into that of another without involving
either one's operating system. This permits highthroughput.
• Such transfers require no work to be done by
CPUs, caches, or context switches, and transfers
continue in parallel with other system operations.
When an application performs an RDMA Read or
Write request, the application data is delivered
directly to the network, reducing latency and
enabling fast message transfer.
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Direct Access File System(DAFS)-case study
• DAFS is a newly developed network file system that is meant for the use of
RDMA (Remote Direct Memory Access). It is based upon NFS version 4 and
VI. DAFS makes it possible for several DAFS servers together to provide the
storage space for a large file system. It remains hidden from the
application server .
• The communication between DAFS server and client takes place by means
of RDMA.ie access to data in the DAFS server is very quick. (as that of local
data)
• Here, the important function of file sharing is i.the synchronization of
simultaneous accesses to file entries (ie metadata such as file names,
access rights etc) and file contents in order to protect the consistency of
the data and metadata.
• DAFS makes it possible to cache the locks at the client side so that
subsequent access to the same data requires no interaction with server.
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• ii. also it possesses recovery mechanisms in case the connection between
the DAFS client and DAFS server is interrupted .
DAFS takes over the authentication of client and server and can also
authenticates individual users in relation to a client server session.
DAFS is an interesting approach to the use of NAS servers as storage for I/O
intensive applications.
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Shared Disk File Systems
• With storage networks it is possible for several computers to access a storage
device simultaneously.
• The I/O bottleneck in the file server can be avoided if all clients fetch the files
from the disk directly through the storage network.
• Present day file systems consider their storage devices as local. they
concentrate on caching and aggregation of I/O operations thereby reduces
number of disk accesses.
• In Shared disk file systems there are special algorithms which synchronizes
the simultaneous access of several computers to common disks. Hence here
several computers can access the disk files simultaneously.
• For this to happen Shared disk file systems must synchronize write accesses
in addition to the functions of local file systems. It has to ensure that new
files are written to different areas of hard disk.
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• Advantage: i. here the computers accessing files and the storage devices
communicate with each other directly. (diversion thru central file server is
not required-bottleneck).
• ii. the load on CPU in the accessing machine is reduced because
communication through fiber channel places less load on the processor
.(than thru IP or ethernet)
• Here central file server is not needed. If a machine in the shared disk file
system cluster fails, then the other machines can carry on working. the
availability of the underlying storage devices determines the availability of
shared disk file systems.
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