Messages in this thread | | | From | Rob Landley <> | Subject | Re: Is there a "make hole" (truncate in middle) syscall? | Date | Thu, 11 Dec 2003 00:19:28 -0600 |
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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...
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