Highly efficient filesystem APIs for certain kinds of operations

I occasionally find myself needing certain filesystem APIs which could be implemented very efficiently if supported by the filesystem, but I've never heard of them. For example:

• Truncate file from the beginning, on an allocation unit boundary
• Split file into two on an allocation unit boundary
• Ins开发者_JS百科ert or remove a chunk from the middle of the file, again, on an allocation unit boundary

The only way that I know of to do things like these is to rewrite the data into a new file. This has the benefit that the allocation unit is no longer relevant, but is extremely slow in comparison to some low-level filesystem magic.

I understand that the alignment requirements mean that the methods aren't always applicable, but I think they can still be useful. For example, a file archiver may be able to trim down the archive very efficiently after the user deletes a file from the archive, even if that leaves a small amount of garbage either side for alignment reasons.

Is it really the case that such APIs don't exist, or am I simply not aware of them? I am mostly interested in NTFS, but hearing about other filesystems will be interesting too.

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For NTFS and FAT there are no such APIs. You can obvoiusly truncate the end a file but not the beginning.

Implementing this is unadvisable due to file system caching. Most of the time people implement a layer "on top" of NTFS to support this.

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Raymond Chen has essentially answered this question.

His answer is that no, such APIs don't exist, because there is too little demand for them. Raymond also suggests the use of sparse files and decomitting blocks by zeroing them.