Re: Healthy amount of free space?

[Chris Murphy <lists@colorremedies.com>]

On Wed, Jul 18, 2018 at 12:01 PM, Austin S. Hemmelgarn
<ahferroin7@gmail.com> wrote:
> On 2018-07-18 13:40, Chris Murphy wrote:
>>
>> On Wed, Jul 18, 2018 at 11:14 AM, Chris Murphy <lists@colorremedies.com>
>> wrote:
>>
>>> I don't know for sure, but based on the addresses reported before and
>>> after dd for the fallocated tmp file, it looks like Btrfs is not using
>>> the originally fallocated addresses for dd. So maybe it is COWing into
>>> new blocks, but is just as quickly deallocating the fallocated blocks
>>> as it goes, and hence doesn't end up in enospc?
>>
>>
>> Previous thread is "Problem with file system" from August 2017. And
>> there's these reproduce steps from Austin which have fallocate coming
>> after the dd.
>>
>>      truncate --size=4G ./test-fs
>>      mkfs.btrfs ./test-fs
>>      mkdir ./test
>>      mount -t auto ./test-fs ./test
>>      dd if=/dev/zero of=./test/test bs=65536 count=32768
>>      fallocate -l 2147483650 ./test/test && echo "Success!"
>>
>>
>> My test Btrfs is 2G not 4G, so I'm cutting the values of dd and
>> fallocate in half.
>>
>> [chris@f28s btrfs]$ sudo dd if=/dev/zero of=tmp bs=1M count=1000
>> 1000+0 records in
>> 1000+0 records out
>> 1048576000 bytes (1.0 GB, 1000 MiB) copied, 7.13391 s, 147 MB/s
>> [chris@f28s btrfs]$ sync
>> [chris@f28s btrfs]$ df -h
>> Filesystem                Size  Used Avail Use% Mounted on
>> /dev/mapper/vg-btrfstest  2.0G 1018M  1.1G  50% /mnt/btrfs
>> [chris@f28s btrfs]$ sudo fallocate -l 1000m tmp
>>
>>
>> Succeeds. If I do it with a 1200M file for dd and fallocate 1200M over
>> it, this fails, but I kinda expect that because there's only 1.1G free
>> space. But maybe that's what you're saying is the bug, it shouldn't
>> fail?
>
> Yes, you're right, I had things backwards (well, kind of, this does work on
> ext4 and regular XFS, so it arguably should work here).

I guess I'm confused what it even means to fallocate over a file with
in-use blocks unless either -d or -p options are used. And from the
man page, I don't grok the distinction between -d and -p either. But
based on their descriptions I'd expect they both should work without
enospc.

-- 
Chris Murphy