Re: [PATCH] ext4: introduce EXT4_BG_WAS_TRIMMED to optimize trim

[Lukas Czerner <lczerner@redhat.com>]

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@ddn.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.

> 
> [root@firewall:~]$  fstrim -av
> /boot: 0 B (0 bytes) trimmed on /dev/sda1
> /: 315.2 MiB (330522624 bytes) trimmed on /dev/sdb1

The solution is to run fstrim less often, that's the whole point of the
fstrim. If you need to run it more often, then you probably want to use
-o discard mount option.

-Lukas

> 
> [root@firewall:~]$  df
> Filesystem     Type  Size  Used Avail Use% Mounted on
> /dev/sdb1      ext4  5.8G  463M  5.4G   8% /
> /dev/sda1      ext4  485M   42M  440M   9% /boot
>