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

[Jeff Mahoney <jeffm@suse.com>]

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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?

-Jeff

-- 
Jeff Mahoney
SUSE Labs


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