Re: [PATCH] lightnvm: pblk: assume that chunks are closed on 1.2 devices

[Matias BjÃrling]

On 06/26/2018 01:31 PM, Hans Holmberg wrote:
> On Tue, Jun 26, 2018 at 1:38 PM, Matias BjÃrling <mb@xxxxxxxxxxx> wrote:
> > On 06/26/2018 11:37 AM, Javier Gonzalez wrote:
> > > > On 26 Jun 2018, at 10.41, Matias BjÃrling <mb@xxxxxxxxxxx> wrote:
> > > >
> > > > On 06/19/2018 11:06 AM, Hans Holmberg wrote:
> > > > > From: Hans Holmberg <hans.holmberg@xxxxxxxxxxxx>
> > > > > We can't know if a block is closed or not on 1.2 devices, so assume
> > > > > closed state to make sure that blocks are erased before writing.
> > > > > Fixes: 32ef9412c114 ("lightnvm: pblk: implement get log report chunk")
> > > > > Signed-off-by: Hans Holmberg <hans.holmberg@xxxxxxxxxxxx>
> > > > > ---
> > > > > This patch applies on:
> > > > > ssh://github.com/OpenChannelSSD/linux branch for-4.19/core
> > > > >    drivers/lightnvm/pblk-init.c | 5 +++--
> > > > >    1 file changed, 3 insertions(+), 2 deletions(-)
> > > > > diff --git a/drivers/lightnvm/pblk-init.c b/drivers/lightnvm/pblk-init.c
> > > > > index aa24264..3b8aa4a 100644
> > > > > --- a/drivers/lightnvm/pblk-init.c
> > > > > +++ b/drivers/lightnvm/pblk-init.c
> > > > > @@ -717,10 +717,11 @@ static int pblk_setup_line_meta_12(struct pblk
> > > > > *pblk, struct pblk_line *line,
> > > > >                  /*
> > > > >                   * In 1.2 spec. chunk state is not persisted by the
> > > > > device. Thus
> > > > > -                * some of the values are reset each time pblk is
> > > > > instantiated.
> > > > > +                * some of the values are reset each time pblk is
> > > > > instantiated,
> > > > > +                * so we have to assume that the block is closed.
> > > > >                   */
> > > > >                  if (lun_bb_meta[line->id] == NVM_BLK_T_FREE)
> > > > > -                       chunk->state =  NVM_CHK_ST_FREE;
> > > > > +                       chunk->state =  NVM_CHK_ST_CLOSED;
> > > > >                  else
> > > > >                          chunk->state = NVM_CHK_ST_OFFLINE;
> > > >
> > > > pblk should scan (or the lightnvm subsystem) the blocks for their
> > > > state, such that it doesn't have to reinitialize a full drive if it is
> > > > already in a closed state. If marking closed, it does a full erase
> > > > cycle on initialization, which should be avoided since it is a limited
> > > > resource.
> > >
> > >
> > > In 1.2 there is no such state unfortunately. However, pblk will never
> > > attempt to reinitialize the whole drive - metadata for closed blocks
> > > will be recovered and only those going to GC will be erased before
> > > usage. In fact, a full close drive is the state pblk expects.
> > >
> > > This patch only affects "unknown blocks", thus the only case in which
> > > pblk would attempt to double erase is when blocks have been pre-erased
> > > (e.g., factory or through liblightnvm). After an erase round though,
> > > pblk will only erase pre-usage. One thing we could do is attempting to
> > > read the first page of these unknown blocks and mark them as free if
> > > "empty page" is returned. Is this what you mean?
> >
> >
> > Yes, that is what I mean.
> >
> > Note that this can be
> > > costly on large drives; this is the reason we returned to the pre-2.0
> > > behaviour with this patch. We are implementing a log that, among other
> > > things, keeps the state so that pblk can have an accurate state for the
> > > cases this can be a problem.
> >
> >
> > Yep, it will take some time. Good to hear with the log.
>
> Until we have a log in place, this patch unbreaks 1.2 support and has
> no negative impact on performance (as compared to pre 2.0 support), so
> please consider it for the next window.
> The current state is that if a pblk instance is created on a 1.2 disk
> with written blocks, writes will fail.
>
>   / Hans

The negative impact is that all blocks are erased, even if they are in 
free state. This is a showstopper. We cannot throw out 1/X of the 
lifetime of the drive on each initialization. The 1.2 spec is made such 
that a scan can recover the block state accurately.