Question about hard drive , 'seek' and 'read' in windows OS

Question

Does anyone know when calling 'seek' and 'read' , how is the hard-drive physicly affected?

If i'll be more specific, I know that the harddrive has some kind of a magnetic needle that is used to read the data from the magnetic plates. So my question is , when is the needle actualy moved to the reading location?

Is it moved when we are calling the "seek" windowsApi method (no matter if an actual read performed) , or does "seek" just remember a virtual pointer , and the physical movement of the needle is performed only when the "read" method is called?

Edit: Assume that the data requested from the Hard-Drive doesn't exist in any of the caches (hard-drive cache , Os Cache , Ram and whatever else it could be)

Answer 1

Wanted to break out this question from your post

When is the needle actualy moved to the reading location?

I think the simple answer is "whenever data is requested that is not already present in any number of caches". The problem with predicting hard drive movement is you have to consider all of the different places that cache data read from the hard drive. If the data is present in those caches and accessible in the context requesting the data, the cache will be used instead of actually reading the hard drive. Here are just some of the places that can and do cache hard drive data

• Hard Drive's internal cache
• OS level caches
• Program level caches
• API level cache

In the case where none of the data is present then it will likely be read from the hard drive during a read call. A seek call is unlikely to cause the hard drive to move because you're not changing the physical hard drive pointer but a virtual pointer to the file within your program.

Answer 2

The hard drive head (needle) starts moving and the disk starts spinning up (unless already spinning) at the read operation. There is no head move or spinup at the seek operation.

Please note that the head may move nonsequentially above the disk even if you are reading a file sequentially, i.e. the the read of the 2nd, 3rd etc. 512-byte block may cause the head to move far away as well even if there aren't intervening seeks. This is partially because the file is fragmented on the filesystem, or because the firmware has a sector number remapping (i.e. logical sector 5 is not between logical sectors 4 and 6) to compensate bad-block errors.

Answer 3

The assumption in the question "Assume that the data requested from the Hard-Drive doesn't exist in any of the caches (hard-drive cache , Os Cache , Ram and whatever else it could be)" is difficult to assume and relatively rare. Even in this case, there is only a loose association between user mode file I/O operations and physical storage device operations.

There are many user mode File I/O functions in various windows libraries. Some of the oldest are the C library low level I/O functions. There are also the C library stream I/O functions, the C++ iostreams classes, and the manged I/O classes. There are other I/O interfaces as well that are part of other packages.

In general, all the user mode I/O Libraries are built on top of the Win32 file I/O functions including CreateFile(), SetFilePointer(), ReadFile(), and WriteFile().

Unless a file is opened in unbuffered mode the operating system can cache the files contents. This is done system wide, and not on a per-file basis. So, even if your program had not read or written a file, I/O to a file may be cached and not result in any physical storage device I/Os.

There are many factors that determine how file I/Os map to actual I/O operations on a physical device. This includes, library level bufering, OS cashing, device driver caching, hardware level cashing, device block size, file size, hardware block/sector remapping, and other factors.

The short story here is that you cannot assume that individual file level read or seek operations correspond to physical device operations, such as disk head seeking.

This gets even trickier when writes are considered. Often writes are accompanied by a flush - which the application developer assumes will push the data all the way to the physical media. Developers often assume that when a flush call returns success, that the data is guaranteed to be persistent on the storage device. This is far from true as devices and drivers often ignore flush calls.

There is more complexity with solid state drives which are not mechanical and therefore do not have 'seek' operations. Here, other physical characteristics manifest themselves such as the necessity to erase blocks before they are written to.