Date:   Wed, 27 May 2020 12:56:44 +0200
From:   Reindl Harald <h.reindl@...lounge.net>
To:     Lukas Czerner <lczerner@...hat.com>
Cc:     Wang Shilong <wangshilong1991@...il.com>,
        linux-ext4@...r.kernel.org, Wang Shilong <wshilong@....com>,
        Shuichi Ihara <sihara@....com>,
        Andreas Dilger <adilger@...ger.ca>
Subject: Re: [PATCH] ext4: introduce EXT4_BG_WAS_TRIMMED to optimize trim



Am 27.05.20 um 12:32 schrieb Lukas Czerner:
> On Wed, May 27, 2020 at 12:11:52PM +0200, Reindl Harald wrote:
>>
>> Am 27.05.20 um 11:57 schrieb Lukas Czerner:
>>> On Wed, May 27, 2020 at 11:32:02AM +0200, Reindl Harald wrote:
>>>>
>>>>
>>>> Am 27.05.20 um 11:19 schrieb Lukas Czerner:
>>>>> On Wed, May 27, 2020 at 04:38:50PM +0900, Wang Shilong wrote:
>>>>>> From: Wang Shilong <wshilong@....com>
>>>>>>
>>>>>> Currently WAS_TRIMMED flag is not persistent, whenever filesystem was
>>>>>> remounted, fstrim need walk all block groups again, the problem with
>>>>>> this is FSTRIM could be slow on very large LUN SSD based filesystem.
>>>>>>
>>>>>> To avoid this kind of problem, we introduce a block group flag
>>>>>> EXT4_BG_WAS_TRIMMED, the side effect of this is we need introduce
>>>>>> extra one block group dirty write after trimming block group.
>>>>
>>>> would that also fix the issue that *way too much* is trimmed all the
>>>> time, no matter if it's a thin provisioned vmware disk or a phyiscal
>>>> RAID10 with SSD
>>>
>>> no, the mechanism remains the same, but the proposal is to make it
>>> pesisten across re-mounts.
>>>
>>>>
>>>> no way of 315 MB deletes within 2 hours or so on a system with just 485M
>>>> used
>>>
>>> The reason is that we're working on block group granularity. So if you
>>> have almost free block group, and you free some blocks from it, the flag
>>> gets freed and next time you run fstrim it'll trim all the free space in
>>> the group. Then again if you free some blocks from the group, the flags
>>> gets cleared again ...
>>>
>>> But I don't think this is a problem at all. Certainly not worth tracking
>>> free/trimmed extents to solve it.
>>
>> it is a problem
>>
>> on a daily "fstrim -av" you trim gigabytes of alredy trimmed blocks
>> which for example on a vmware thin provisioned vdisk makes it down to
>> CBT (changed-block-tracking)
>>
>> so instead completly ignore that untouched space thanks to CBT it's
>> considered as changed and verified in the follow up backup run which
>> takes magnitutdes longer than needed
> 
> Looks like you identified the problem then ;)

well, in a perfect world.....

> But seriously, trim/discard was always considered advisory and the
> storage is completely free to do whatever it wants to do with the
> information. I might even be the case that the discard requests are
> ignored and we might not even need optimization like this. But
> regardless it does take time to go through the block gropus and as a
> result this optimization is useful in the fs itself.

luckily at least fstrim is non-blocking in a vmware environment, on my
physical box it takes ages

this machine *does nothing* than wait to be cloned, 235 MB pretended
deleted data within 50 minutes is absurd on a completly idle guest

so even when i am all in for optimizations thatÄs way over top

[root@...ter:~]$ fstrim -av
/boot: 0 B (0 bytes) trimmed on /dev/sda1
/: 235.8 MiB (247201792 bytes) trimmed on /dev/sdb1

[root@...ter:~]$ df
Filesystem     Type  Size  Used Avail Use% Mounted on
/dev/sdb1      ext4  5.8G  502M  5.3G   9% /
/dev/sda1      ext4  485M   39M  443M   9% /boot

> However it seems to me that the situation you're describing calls for
> optimization on a storage side (TP vdisk in your case), not file system
> side.
> 
> And again, for fine grained discard you can use -o discard

with a terrible performance impact at runtime

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