The ongoing fallocate() story

The proposed fallocate() system call, which exists to allow an application to preallocate blocks for a file, was covered here back in March. Since then there has been quite a bit of discussion, but there is still no fallocate() system call in the mainline - and it's not clear that there will be in 2.6.23 either. There is a new version of the fallocate() patch in circulation, so it seems like a good time to catch up with what is going on.

Back in March, the proposed interface was:

    long fallocate(int fd, int mode, loff_t offset, loff_t len);

It turns out that this specific arrangement of parameters is hard to support on some architectures - the S/390 architecture in particular. Various alternatives were proposed, but getting something that everybody liked proved difficult. In the end, the above prototype is still being used. The S/390 architecture code will have to do some extra bit shuffling to be able to implement this call, but that apparently is the best way to go.

That does not mean that the interface discussions are done, though. The current version of the patch now has four possibilities for mode:

• FA_ALLOCATE will allocate the requested space at the given offset. If this call makes the file longer, the reported size of the file will be increased accordingly, making the allocated blocks part of the file immediately.

• FA_RESV_SPACE preallocates blocks, but does not change the size of the file. So the newly allocated blocks, if past the end of the file, will not appear to be present until the application writes to them (or increases the size of the file in some other way).

• FA_DEALLOCATE returns previously-allocated blocks to the system. The size of the file will be changed if the deallocated blocks are at the end.

• FA_UNRESV_SPACE returns the blocks to the system, but does not change the size of the file.

As an example of how the last two operations differ, consider what happens if an application uses fallocate() to remove the last block from a file. If that block was removed with FA_DEALLOCATE, a subsequent attempt to read that block will return no data - the offset where that block was is now past the end of the file. If, instead, the block is removed with FA_UNRESV_SPACE, an attempt to read it will return a block full of zeros.

It turns out that there are some differing opinions on how this interface should work. A trivial change which has been requested is that the FA_ prefix be changed to FALLOC_ - this change is likely to be made. But it seems there's a number of other flags that people would like to see:

• FALLOC_ZERO_SPACE would write zeros to the requested range - even if that range is already allocated to the file. This feature would be useful because some filesystems can quickly mark the affected range as being uninitialized rather than actually writing zeros to all of those blocks.

• FALLOC_MKSWAP would allocate the space, mark it initialized, but not actually zero out the blocks. The newly-allocated blocks would thus still contain whatever data the previous user left there. This operation, which would clearly have to be privileged, is intended to make it possible to create a swap file in a very quick way. It would require very little in the way of in-kernel memory allocations to implement, making it a useful way to add an emergency swap file to a system which has gone into an out-of-memory condition.

• FALLOC_FL_ERR_FREE would be an additional flag which would affect error handling; in particular, it would control behavior when the filesystem runs out of space part way through an allocation request. If this flag is set, the blocks which were successfully preallocated would be freed; otherwise they would be left in place. There is some opposition to this flag; it may be left out in favor of an official "all or nothing" policy for preallocations.

• FALLOC_FL_NO_MTIME and FALLOC_FL_NO_CTIME would prevent the filesystem from updating the modification times associated with the file.

All told, it's a significant number of new features - enough that some people are starting to wonder if fallocate() is the right approach after all. Christoph Hellwig, in particular, has started to complain; he suggests adding something small which would be able to implement posix_fallocate() and no more. Block deletion, he says, is a different function and should be done with a different system call, and the other features need more thought (and aggressive weeding). So it's unclear where this patch set will go and whether it will be considered ready for 2.6.23.

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