Re: Is there a "make hole" (truncate in middle) syscall?

[Rob Landley <rob@landley.net>]

On Wednesday 10 December 2003 23:13, Hua Zhong wrote:
> Sorry for digging out this old discussion.
>
> This would be a tremendous enhancement to Linux filesystems, and one of
> my current projects actually needs this capability badly.
>
> The project is a lightweight user-space library which implements a
> file-based database. Each database has several files. The files are all
> block-based, and each block is always a multiple of 512 byte (and we
> could make it a multiple of 4K, in case this feature existed).
>
> Blocks are organized as a B+ tree, so we have a root block, which points
> to its child blocks, and in turn they point to the next level. There is
> a free block list too.
>
> The problem is with a lot of add/delete, there are a lot of free blocks
> inside the file. So essentially we'd have to manually shrink these files
> when it grows too big and eats up too much space. If we could just "dig
> a hole", it would be trivial to return those blocks to the filesystem
> without doing an expensive defragmentation.

It could be worse.  Java didn't have a "truncate file" command at all until I 
yelled at Sun about it.  (It was too late to get it into 1.1, but they added 
it to 1.2.  Of course, that was back when I cared about Java... :)

Al Viro mentioned that making hole creation play nice with mmap would be evil.  
I suspect that having the "punch hole" call simply fail if any part of the 
range you're trying to zap is currently mmaped is probably good enough for a 
first pass.  (Maybe some fallback code could write zeroes into the range so 
the behavior was sort of similar in the failure case...)  But I haven't 
looked at the code enough to even know what the issues are, and I certainly 
won't have time this week...

Rob