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Full Discussion: Understanding read/write and kernel interaction
Top Forums Programming Understanding read/write and kernel interaction Post 302354065 by jlliagre on Thursday 17th of September 2009 02:04:27 AM
Old 09-17-2009
Quote:
Originally Posted by Krothos
So once the bytes are transferred stored over in the main memory through the DMA, how is what was read returned to the caller? So if I wanted to read 3 bytes from a file containing "Hello", how is "Hel" returned to the library function? What sees this and how/where is it returned from?
That's the whole purpose of a system call. Passing data from/to the kernel.
Quote:
That was what I was saying. I don't get the process of how write takes place in terms of a file let's say.
That's the picture but the process is quite more complex than your description.
There are several layers crossed by the data between your application and the disk blocks.
As you are talking about a file, the filesystem, file cache and possibly a log are playing a role. Writes are usually delayed so you will need to wait for a flush for the data to be committed on disk. Also, some form of software or hardware raid (mirroring/striping and the likes), checksums or compression might take place, the disk itself will certainly have a cache too.
 

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LEARN ABOUT SUSE

vfs_cacheprime

VFS_CACHEPRIME(8)					    System Administration tools 					 VFS_CACHEPRIME(8)

NAME

       vfs_cacheprime - prime the kernel file data cache

SYNOPSIS

       vfs objects = cacheprime

DESCRIPTION

       This VFS module is part of the samba(7) suite.

       The vfs_cacheprime VFS module reads chunks of file data near the range requested by clients in order to make sure the data is present in
       the kernel file data cache at the time when it is actually requested by clients.

       The size of the disk read operations performed by vfs_cacheprime is determined by the cacheprime:rsize option. All disk read operations are
       aligned on boundaries that are a multiple of this size. Each range of the file data is primed at most once during the time the client has
       the file open.

       This module is stackable.

OPTIONS

       cacheprime:rsize = BYTES
	   The number of bytes with which to prime the kernel data cache.

	   The following suffixes may be applied to BYTES:

	   o   K - BYTES is a number of kilobytes

	   o   M - BYTES is a number of megabytes

	   o   G - BYTES is a number of gigabytes

EXAMPLES

       For a hypothetical disk array, it is necessary to ensure that all read operations are of size 1 megabyte (1048576 bytes), and aligned on 1
       megabyte boundaries:

		[hypothetical]
		vfs objects = cacheprime
		cacheprime:rsize = 1M

CAVEATS

       cacheprime is not a a substitute for a general-purpose readahead mechanism. It is intended for use only in very specific environments where
       disk operations must be aligned and sized to known values (as much as that is possible).

VERSION

       This man page is correct for version 3.0.25 of the Samba suite.

AUTHOR

       The original Samba software and related utilities were created by Andrew Tridgell. Samba is now developed by the Samba Team as an Open
       Source project similar to the way the Linux kernel is developed.

Samba 3.5							    06/18/2010							 VFS_CACHEPRIME(8)
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