Re: do not disable ext4 discards on first discard failure? [was: Re: dm snapshot: ignore discards issued to the snapshot-origin target]

[Eric Sandeen <sandeen@redhat.com>]

On 4/28/11 3:59 PM, Mike Snitzer wrote:
> [cc'ing linux-ext4]
> 
> On Thu, Apr 28 2011 at  3:53am -0400,
> Christoph Hellwig <hch@infradead.org> wrote:
> 
>> On Wed, Apr 27, 2011 at 08:19:13PM -0400, Mike Snitzer wrote:
>>> Discards pose a problem for the snapshot-origin target because they are
>>> treated as writes.  Treating a discard as a write would trigger a
>>> copyout to the snapshot.  Such copyout can prove too costly in the face
>>> of otherwise benign scenarios (e.g. create a snapshot and then mkfs.ext4
>>> the origin -- mkfs.ext4 discards the entire volume by default, which
>>> would copyout the entire origin volume to the snapshot).
>>
>> You also need to make sure that we don't claim discard_zeroes_data for
>> the origin volume in this case.  Especially as ext4 started to rely
>> on this actually working (very bad idea IMHO, but that's another story)
> 
> Eric Sandeen helped me see that having the DM snapshot-origin target
> return success but actually ignore discards is just bad form.
> 
> Especially when you consider that this exercise was motivated by the
> fact that ext4 will disable discards on the first discard failure, see:
> http://www.redhat.com/archives/dm-devel/2011-April/msg00070.html
> 
> Eric and I think it is best to revert this commit:
> a30eec2 ext4: stop issuing discards if not supported by device
> 
> (though ideally ext4 would still WARN_ONCE per superblock with something
>  like: "discard failed, please consider disabling discard support")
> 
> 1) The user asked for discards (with '-o discard' mount option)
>    - what is the real harm in coninuing to issue them even if it _seems_
>      they aren't supported?

TBH I sent a30eec2 on a whim.  Seemed reasonable at the time, but if
discard-ability changes over time, it may not be the best plan.

> 2) assuming the entire block device uniformly supports discards can
>    be flawed (a DM device's discard support can vary based on logical
>    offset).

I still think that concats of floppies, usb disks, and ssds should be rare, so I'm less concerned about that ;)

I think Mike is right though, that if you do not do anything with a discard, you should return -EOPNOTSUPP, and not pretend that you honored it.
 
We should, IMHO, deal with the truth of the matter at the filesystem caller.

-Eric

> Thoughts?