RE: [f2fs-dev] [PATCH] f2fs: do not issue small discard commands during checkpoint

[Daejun Park]

Hi Jaegeuk,

> If there're huge # of small discards, this will increase checkpoint latency
> insanely. Let's issue small discards only by trim.
> 
> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> ---
>  fs/f2fs/segment.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 0c0c033c4bdd..ef46bb085385 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -2178,7 +2178,7 @@ void f2fs_clear_prefree_segments(struct f2fs_sb_info *sbi,
>          }
>          mutex_unlock(&dirty_i->seglist_lock);
>  
> -        if (!f2fs_block_unit_discard(sbi))
> +        if (!f2fs_block_unit_discard(sbi) || !force)

If we don't handle the discard entries here, dcc->entry_list will still have them,
so stale discard entries may be handled by trim, causing incorrect discards to be issued.
Therefore, I think this patch should also prevent the creation of discard entries
in add_discard_addrs(), unless it is a checkpoint caused by trim.
This would further reduce the latency caused by the creation of a discard entry.

Thanks,
Daejun