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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 827
796
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
Client kernel
Server kernel
System call layer
Buffer cache
Buffer cache
Virtual file system layer
Virtual file system layer
Local
FS 1
Local
FS 1
Local
FS 2
Local
FS 2
NFS
client
NFS
server
Driver
Driver
Driver
Driver
Message
to server
Message
from client
Local disks
Local disks
V- node
Figure 10-36.
The NFS layer structure
file system and i-node are recorded because modern Linux systems can support
multiple file systems (e.g., ext2fs, /proc, FAT , etc.). Although VFS was invented to
support NFS, most modern Linux systems now support it as an integral part of the
operating system, even if NFS is not used.
To see how v-nodes are used, let us trace a sequence of
mount
,
open
, and
read
system calls.
To mount a remote file system, the system administrator (or
/etc/rc
)
calls the
mount
program specifying the remote directory, the local directory on
which it is to be mounted, and other information. The
mount
program parses the
name of the remote directory to be mounted and discovers the name of the NFS
server on which the remote directory is located.
It then contacts that machine, ask-
ing for a file handle for the remote directory.
If the directory exists and is available
for remote mounting, the server returns a file handle for the directory. Finally, it
makes a
mount
system call, passing the handle to the kernel.
The kernel then constructs a v-node for the remote directory and asks the NFS
client code in Fig. 10-36 to create an
r-node
(
remote i-node
) in its internal tables
to hold the file handle. The v-node points to the r-node. Each v-node in the VFS
layer will ultimately contain either a pointer to an r-node in the NFS client code, or
a pointer to an i-node in one of the local file systems (shown as dashed lines in
Fig. 10-36).
Thus, from the v-node it is possible to see if a file or directory is local
or remote.
If it is local, the correct file system and i-node can be located.
If it is
remote, the remote host and file handle can be located.
Page 828
SEC. 10.6
THE LINUX FILE SYSTEM
797
When a remote file is opened on the client, at some point during the parsing of
the path name, the kernel hits the directory on which the remote file system is
mounted. It sees that this directory is remote and in the directory’s v-node finds
the pointer to the r-node. It then asks the NFS client code to open the file. The
NFS client code looks up the remaining portion of the path name on the remote
server associated with the mounted directory and gets back a file handle for it.
It
makes an r-node for the remote file in its tables and reports back to the VFS layer,
which puts in its tables a v-node for the file that points to the r-node. Again here
we see that every open file or directory has a v-node that points to either an r-node
or an i-node.
The caller is given a file descriptor for the remote file. This file descriptor is
mapped onto the v-node by tables in the VFS layer.
Note that no table entries are
made on the server side. Although the server is prepared to provide file handles
upon request, it does not keep track of which files happen to have file handles out-
standing and which do not. When a file handle is sent to it for file access, it checks
the handle, and if it is valid, uses it. Validation can include verifying an authentica-
tion key contained in the RPC headers, if security is enabled.
When the file descriptor is used in a subsequent system call, for example,
read
,
the VFS layer locates the corresponding v-node, and from that determines whether
it is local or remote and also which i-node or r-node describes it.
It then sends a
message to the server containing the handle, the file offset (which is maintained on
the client side, not the server side), and the byte count. For efficiency reasons,
transfers between client and server are done in large chunks, normally 8192 bytes,
even if fewer bytes are requested.
When the request message arrives at the server, it is passed to the VFS layer
there, which determines which local file system holds the requested file. The VFS
layer then makes a call to that local file system to read and return the bytes. These
data are then passed back to the client.
After the client’s VFS layer has gotten the
8-KB chunk it asked for, it automatically issues a request for the next chunk, so it
will have it should it be needed shortly. This feature, known as
read ahead
, im-
proves performance considerably.
For writes an analogous path is followed from client to server. Also, transfers
are done in 8-KB chunks here, too.
If a
write
system call supplies fewer than 8 KB
of data, the data are just accumulated locally. Only when the entire 8-KB chunk is
full is it sent to the server.
However, when a file is closed, all of its data are sent to
the server immediately.
Another technique used to improve performance is caching, as in ordinary
UNIX. Servers cache data to avoid disk accesses, but this is invisible to the clients.
Clients maintain two caches, one for file attributes (i-nodes) and one for file data.
When either an i-node or a file block is needed, a check is made to see if it can be
satisfied out of the cache.
If so, network traffic can be avoided.
While client caching helps performance enormously, it also introduces some
nasty problems. Suppose that two clients are both caching the same file block and
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