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

[Hua Zhong]

> The abstract interface for make_hole() is simple, but it turns into a
> pretty expensive filesystem operation, I think.  After many cycles of
> free/allocate, your file would be badly fragmented across the
> filesystem.  

Understood. Two filesystems we are using: tmpfs and ext3. For the
former, fragmentation doesn't matter.

Hey, I think when I get some cycles I can try to implement this for
tmpfs (since it's simpler) myself, and post a patch. :-) But before
that, I want to make sure it's doable.

> You'll probably get better overall performance by keeping
> track of how "sparse" your file is (you could compare st_blocks versus
> how many blocks you have allocated in your tree structure) 
> and re-write
> it when you're wasting more than, say, 20% of the allocated space.
> 
> It turns into an interesting problem if you don't want to double your
> space requirements during the re-write process.  You could write the
> new file "backwards", one MB at a time, truncating the 
> previous file at
> each step to free up the blocks.  You'd end up with contiguous 1MB
> chunks, which given your tree organization is probably good 
> enough.  If
> you wanted really good streaming performance you'd want to do bigger
> chunks (or just write the file from the beginning, or use the
> pre-allocation APIs that I think XFS provides).
> 
> -andy
> 

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