Re: BTRFS free space handling still needs more work: Hangs again

[Robert White <rwhite@pobox.com>]

On 12/27/2014 06:00 AM, Robert White wrote:
> On 12/27/2014 05:16 AM, Martin Steigerwald wrote:
>> It can easily be reproduced without even using Virtualbox, just by a nice
>> simple fio job.
>>
>
> TL;DR: If you want a worst-case example of consuming a BTRFS filesystem
> with one single file...
>
> #!/bin/bash
> # not tested, so correct any syntax errors
> typeset -i counter
> for ((counter=250;counter>0;counter--)); do
>   dd if=/dev/urandom of=/some/file bs=4k count=$counter
> done
> exit 0

Slight correction: you need to prevent the truncate dd performs by 
default, and flush the data and metadata to disk between after each 
invocatoin. So you need the "conv=" flags.

for ((counter=250;counter>0;counter--)); do
dd if=/dev/urandom of=some_file conv=notrunc,fsync bs=4k count=$counter
done



>
>
> Each pass over /some/file is 4k shorter than the previous one, but none
> of the extents can be deallocated. File will be 1MiB in size and usage
> will be something like 125.5MiB (if I've done the math correctly).
> larger values of counter will result in exponentially larger amounts of
> waste.
>
> Doing the bad things is very bad...
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