On Nov 3, 2021, at 8:51 AM, Lukas Czerner <lczerner@xxxxxxxxxx> wrote:
>
> Ext4 has an optimization mechanism for batched disacrd (FITRIM) that
> should help speed up subsequent calls of FITRIM ioctl by skipping the
> groups that were previously trimmed. However because the FITRIM allows
> to set the minimum size of an extent to trim, ext4 stores the last
> minimum extent size and only avoids trimming the group if it was
> previously trimmed with minimum extent size equal to, or smaller than
> the current call.
>
> There is currently no way to bypass the optimization without
> umount/mount cycle. This becomes a problem when the file system is
> live migrated to a different storage, because the optimization will
> prevent possibly useful discard calls to the storage.
>
> Fix it by exporting the s_last_trim_minblks via sysfs interface which
> will allow us to set the minimum size to the number of blocks larger
> than subsequent FITRIM call, effectively bypassing the optimization.
>
> By setting the s_last_trim_minblks to ULONG_MAX the optimization will be
> effectively cleared regardless of the previous state, or file system
> configuration.
>
> For example:
> getconf ULONG_MAX > /sys/fs/ext4/dm-1/last_trim_minblks
>
> Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx>
> Reported-by: Laurent GUERBY <laurent@xxxxxxxxxx>
Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx>
> ---
> v2: Remove unnecessary assignment
> v3: s_last_trim_minblks is now unsinged long which simplifies this
>
> fs/ext4/sysfs.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
> index 2314f7446592..95d8a996d2d8 100644
> --- a/fs/ext4/sysfs.c
> +++ b/fs/ext4/sysfs.c
> @@ -245,6 +245,7 @@ EXT4_ATTR(last_error_time, 0444, last_error_time);
> EXT4_ATTR(journal_task, 0444, journal_task);
> EXT4_RW_ATTR_SBI_UI(mb_prefetch, s_mb_prefetch);
> EXT4_RW_ATTR_SBI_UI(mb_prefetch_limit, s_mb_prefetch_limit);
> +EXT4_RW_ATTR_SBI_UL(last_trim_minblks, s_last_trim_minblks);
>
> static unsigned int old_bump_val = 128;
> EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
> @@ -295,6 +296,7 @@ static struct attribute *ext4_attrs[] = {
> #endif
> ATTR_LIST(mb_prefetch),
> ATTR_LIST(mb_prefetch_limit),
> + ATTR_LIST(last_trim_minblks),
> NULL,
> };
> ATTRIBUTE_GROUPS(ext4);
> --
> 2.31.1
>
Cheers, Andreas
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