On 03/25, John wrote:
> On Tue, Mar 24, 2020 at 9:01 PM Chao Yu <yuch...@huawei.com> wrote:
> > I guess ssd should support discard, however I saw sdb was wrapped with
> > device
> > mapper, which may have different character, just want to check that whether
> > device mapper disable the discard.
> >
> > If backup/restore is hard for you, how about let me add some logs on fstrim
> > interface? then we could retry fstrim to see where we actually fail.
>
> I have an older totally different SSD for testing that seems to support
> discard:
> # hdparm -I /dev/sdc | grep TRIM
> * Data Set Management TRIM supported (limit 8 blocks)
> * Deterministic read ZEROs after TRIM
>
> As I test, I created two f2fs partitions on this different SSD:
> 1) /dev/sdc2 normal F2FS
> 2) /dev/sdc3 encrypted with LUKS (dm-crypt) then formatted to F2FS
>
> Results:
> 1) Regular F2FS
> # fstrim -v /regular
> /regular: 0 B (0 bytes) trimmed
This is normal.
>
> 2) Encrypted F2FS
> # fstrim -v /encrypted
> fstrim: /encrypted: the discard operation is not supported
It seems dm-crypt has discard-related support, ie., "allow_discards", in ctrl.
But, FWIW, the target simply doesn't support discard at all.
# grep discards_supported drivers/md/*
drivers/md/dm-cache-target.c: ti->discards_supported = true;
drivers/md/dm-clone-target.c: ti->discards_supported = true;
drivers/md/dm-log-writes.c: ti->discards_supported = true;
drivers/md/dm-table.c: if (!tgt->num_discard_bios && tgt->discards_supported)
drivers/md/dm-table.c: DMWARN("%s: %s: ignoring discards_supported
because num_discard_bios is zero.",
drivers/md/dm-table.c: * 'discards_supported') or it relies on _all_
data devices having
drivers/md/dm-table.c: if (!ti->discards_supported &&
drivers/md/dm-thin.c: * Setting 'discards_supported' circumvents the
normal
drivers/md/dm-thin.c: ti->discards_supported = true;
drivers/md/dm-thin.c: ti->discards_supported = true;
drivers/md/dm-zoned-target.c: ti->discards_supported = true;
>
> Can we conclude that the device mapper is some how spoofing fstrim
> into thinking the device doesn't support trim? Happy to hear your
> thoughts.
>
>
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