Re: bug? fstrim only trims unallocated space, not unused in bg's

[Jeff Mahoney <jeffm@suse.com>]

[-- Attachment #1.1: Type: text/plain, Size: 3022 bytes --]

On 11/20/17 5:59 PM, Chris Murphy wrote:
> On Mon, Nov 20, 2017 at 1:40 PM, Jeff Mahoney <jeffm@suse.com> wrote:
>> On 11/20/17 3:01 PM, Jeff Mahoney wrote:
>>> On 11/20/17 3:00 PM, Jeff Mahoney wrote:
>>>> On 11/19/17 4:38 PM, Chris Murphy wrote:
>>>>> On Sat, Nov 18, 2017 at 11:27 PM, Andrei Borzenkov <arvidjaar@gmail.com> wrote:
>>>>>> 19.11.2017 09:17, Chris Murphy пишет:
>>>>>>> fstrim should trim free space, but it only trims unallocated. This is
>>>>>>> with kernel 4.14.0 and the entire 4.13 series. I'm pretty sure it
>>>>>>> behaved this way with 4.12 also.
>>>>>>>
>>>>>>
>>>>>> Well, I was told it should also trim free space ...
>>>>>>
>>>>>> https://www.spinics.net/lists/linux-btrfs/msg61819.html
>>>>>>
>>>>>
>>>>> It definitely isn't. If I do a partial balance, then fstrim, I get a
>>>>> larger trimmed value, corresponding exactly to unallocated space.
>>>>
>>>>
>>>> I've just tested with 4.14 and it definitely trims within block groups.
>>>
>>> Derp.  This should read 4.12.
>>>
>>>> I've attached my test script and the log of the run.  I'll build and
>>>> test a 4.14 kernel and see if I can reproduce there.  It may well be
>>>> that we're just misreporting the bytes trimmed.
>>
>> I get the same results on v4.14.  I wrote up a little script to parse
>> the btrfs-debug-tree extent tree dump and the discards that are issued
>> after the final sync (when the tree is dumped) match.
>>
>> The script output is also as expected:
>> /mnt2: 95.1 GiB (102082281472 bytes) trimmed
>> # remove every other 100MB file, totalling 1.5 GB
>> + sync
>> + killall blktrace
>> + wait
>> + echo 'after sync'
>> + sleep 1
>> + btrace -a discard /dev/loop0
>> + fstrim -v /mnt2
>> /mnt2: 96.6 GiB (103659962368 bytes) trimmed
>>
>> One thing that may not be apparent is that the byte count is from the
>> device(s)'s perspective.  If you have a file system with duplicate
>> chunks or a redundant RAID mode, the numbers will reflect that.
>>
>> The total byte count should be correct as well.  It's the total number
>> of bytes that we submit for discard and that were accepted by the block
>> layer.
>>
>> Do you have a test case that shows it being wrong and can you provide
>> the blktrace capture of the device(s) while the fstrim is running?
> 
> 
> Further,
> 
> # fstrim -v /
> /: 38 GiB (40767586304 bytes) trimmed
> 
> And then delete 10G worth of files, do not balance, and do nothing for
> a minute before:
> 
> # fstrim -v /
> /: 38 GiB (40767586304 bytes) trimmed
> 
> It's the same value. Free space according to fi us is +10 larger than
> before, and yet nothing additional is trimmed than before. So I don't
> know what's going on but it's not working for me.

What happens if you sync before doing the fstrim again?  The code is
there to drop extents within block groups.  It works for me.  The big
thing is that the space must be freed entirely before we can trim.

-Jeff


-- 
Jeff Mahoney
SUSE Labs


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 854 bytes --]