Re: Premature -ENOSPC on btrfs?

[cwillu]

On Fri, Apr 29, 2011 at 5:48 PM, Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
> Hi,
>
> I was giving btrfs (2.6.39-rc4) a quick tryout today and noticed some odd
> behavior when running a rather stupid test.
>
> First, I create a 1GB test fs, format it, and mount it. ÂThen, I run the
> following command to create a huge file, truncate it, rewrite it, and report
> what size file got created.
>
> # while true; do dd if=/dev/zero of=/mnt/testfile bs=1024k; done
>
> This is roughly what I see in terms of file size after filtering out all the
> "dd: writing `/mnt/testfile': No space left on device" messages.
>
> 782237696 bytes (782 MB) copied, 16.5647 s, 47.2 MB/s
> 545259520 bytes (545 MB) copied, 14.061 s, 38.8 MB/s
> 780140544 bytes (780 MB) copied, 15.2959 s, 51.0 MB/s
> 933232640 bytes (933 MB) copied, 15.2241 s, 61.3 MB/s
> 827326464 bytes (827 MB) copied, 14.8383 s, 55.8 MB/s
> 931135488 bytes (931 MB) copied, 15.1554 s, 61.4 MB/s
> 936378368 bytes (936 MB) copied, 2.98301 s, 314 MB/s
> 393216000 bytes (393 MB) copied, 12.9202 s, 30.4 MB/s
> 387973120 bytes (388 MB) copied, 13.4906 s, 28.8 MB/s
> 932184064 bytes (932 MB) copied, 15.3356 s, 60.8 MB/s
> 785383424 bytes (785 MB) copied, 14.6429 s, 53.6 MB/s
> 927989760 bytes (928 MB) copied, 15.4386 s, 60.1 MB/s
> 833617920 bytes (834 MB) copied, 14.6289 s, 57.0 MB/s
> 936378368 bytes (936 MB) copied, 3.33651 s, 281 MB/s
> 393216000 bytes (393 MB) copied, 12.9689 s, 30.3 MB/s
> 389021696 bytes (389 MB) copied, 6.02794 s, 64.5 MB/s
> 294649856 bytes (295 MB) copied, 1.05144 s, 280 MB/s
>
> Strangely, df reports that free space remains, and I can even create a second
> file to fill the empty space, so clearly the fs isn't full yet. ÂIs this the
> intended behavior of btrfs? Âext4/vfat seem to produce the same size file
> every time.

https://btrfs.wiki.kernel.org/index.php/FAQ#if_your_device_is_small

You'll want to use mixed block groups with such a small filesystem.
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