Re: [RFC] Heads up on sys_fallocate()

[Arnd Bergmann]

On Monday 05 March 2007, JÃrn Engel wrote:
> That actually causes an interesting problem for compressing filesystems.
> The space consumed by blocks depends on their contents and how well it
> compresses. ÂAt the moment, the only option I see to support
> posix_fallocate for LogFS is to set an inode flag disabling compression,
> then allocate the blocks.
> 
> But if the file already contains large amounts of compressed data, I
> have a problem. ÂDisabling compression for a range within a file is not
> supported, so I can only return an error. ÂBut which one?

Using the current glibc implementation on a compressed file system ideally
should be a very expensive no-op because you won't actually allocate much
space for a file when writing zeroes to it. You also don't benefit of a
contiguous allocation in logfs, since flash has uniform seek times over
all the medium.

I'd suggest you implement posix_fallocate as an real nop and just return
success without doing anything. You could also return ENOSPC in case
the blocks requested by posix_fallocate don't fit on the medium without
compression, but that is more or less just guesswork (like statfs is).

	Arnd <><
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