According to the man page, we can specify the amount of bytes we want to read from a file descriptor.
But in the read’s implementation, how many read requests will be created to perform a read?
For example, if I want to read 4MB, will it create only one request for 4MB or will it split it into multiple small requests? such as 4KB per request?
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Answer
read(2) is a system call, so it calls the vDSO shared library to dispatch the system call (in very old times it used to be an interrupt, but nowadays there are faster ways of dispatching system calls).
inside the kernel the call is first handled by the vfs (virtual file system); the virtual file system provides a common interface for inodes (the structures that represents open files) and a common way of interfacing with the underlying file system.
the vfs dispatches to the underlying file system (the mount(8) program will tell you which mount point exists and what file system is used there). (see here for more information http://www.inf.fu-berlin.de/lehre/SS01/OS/Lectures/Lecture16.pdf )
the file system can do its own caching, so number of disk reads depends on what is present in the cache and how the file system allocates blocks for storage of a particular file and how the file is divided into disk blocks – all questions to the particular file system)
If you want to do your own caching then open the file with O_DIRECT flag; in this case there is an effort not to use the cache; however all reads have to be aligned to 512 offsets and come in multiples of 512 size (this is in order that your buffer can be transfered via DMA to the backing store http://www.quora.com/Why-does-O_DIRECT-require-I-O-to-be-512-byte-aligned )