Re: fallocate() not "atomic" if insufficient disk space?

[Eric Sandeen <sandeen@redhat.com>]

On 12/28/11 10:09 AM, markk@clara.co.uk wrote:
> Hi,
> 
> I've been experimenting with using fallocate() to pre-allocate space for a
> file on an ext4 partition. I'm testing with Ubuntu kernel
> 3.0.0-14-generic. Does fallocate() behave in the same way on more
> recent/vanilla kernels?
> 
> What I expected to happen is that if fallocate() fails due to lack of disk
> space, no space is allocated, i.e. either nothing happens or the
> allocation succeeds.
> 
> What actually seems to happen is that all remaining space in the partition
> gets allocated to the file. (Thus risking that other programs will fail
> due to lack of disk space until the file is deleted.)

To be honest, I'm not sure how it is _supposed_ to work, but I see this
same behavior with fallocate, with posix_fallocate calling fallocate, and with
posix_fallocate simply writing out data via glibc, (I tested several of those 
combinations on different filesystems, anyway).

Even the posix_fallocate spec doesn't say what is supposed to happen to space
allocated prior to failure, but the implementations seem fairly consistent.
Seems fair to say that callers should check error returns, and unlink or
truncate on error as needed.

-Eric

> If it's relevant, the partition in question has no journal and is mounted
> with barrier=0.
> 
> Example on a partition with ~100MB free:
> 
> $ fallocate -o 0 -l 999999999 blah
> fallocate: blah: fallocate failed: No space left on device
> $ df /
> Filesystem           1K-blocks      Used Available Use% Mounted on
> /dev/sdb1              3849136   3653604         0 100% /
> $ du blah
> 107860	blah
> $ ls -l blah
> -rw-r--r-- 1 mark mark 110444544 2011-12-28 15:51 blah
> $ rm blah
> 
> Same issue when specifying -n to call fallocate() with FALLOC_FL_KEEP_SIZE:
> 
> $ fallocate -n -o 0 -l 999999999 blah
> fallocate: blah: fallocate failed: No space left on device
> $ df /
> Filesystem           1K-blocks      Used Available Use% Mounted on
> /dev/sdb1              3849136   3653604         0 100% /
> $ du blah
> 107860	blah
> $ ls -l blah
> -rw-r--r-- 1 mark mark 0 2011-12-28 15:52 blah
> 
> 
> -- Mark
> 
> 
> --
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