Re: [Qemu-devel] [PATCH 3/8] block: Support to keep track of I/O status

[Luiz Capitulino <lcapitulino@redhat.com>]

On Tue, 12 Jul 2011 11:12:04 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > Am 12.07.2011 09:45, schrieb Markus Armbruster:
> >> Luiz Capitulino <lcapitulino@redhat.com> writes:
> >> 
> >>> This commit adds support to the BlockDriverState type to keep track
> >>> of the last I/O status. That is, at every I/O operation we update
> >>> a status field in the BlockDriverState instance. Valid statuses are:
> >>> OK, FAILED and ENOSPC.
> >>>
> >>> ENOSPC is distinguished from FAILED because an management application
> >>> can use it to implement thin-provisioning.
> >>>
> >>> This feature has to be explicit enabled by buses/devices supporting it.
> >> 
> >> buses?

I think I should have called it 'interface'.

> >> 
> >>>
> >>> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> >>> ---
> >>>  block.c     |   18 ++++++++++++++++++
> >>>  block.h     |    7 +++++++
> >>>  block_int.h |    2 ++
> >>>  3 files changed, 27 insertions(+), 0 deletions(-)
> >>>
> >>> diff --git a/block.c b/block.c
> >>> index 24a25d5..cc0a34e 100644
> >>> --- a/block.c
> >>> +++ b/block.c
> >>> @@ -195,6 +195,7 @@ BlockDriverState *bdrv_new(const char *device_name)
> >>>      if (device_name[0] != '\0') {
> >>>          QTAILQ_INSERT_TAIL(&bdrv_states, bs, list);
> >>>      }
> >>> +    bs->iostatus_enabled = false;
> >>>      return bs;
> >>>  }
> >>>  
> >>> @@ -2876,6 +2877,23 @@ int bdrv_in_use(BlockDriverState *bs)
> >>>      return bs->in_use;
> >>>  }
> >>>  
> >>> +void bdrv_enable_iostatus(BlockDriverState *bs)
> >>> +{
> >>> +    bs->iostatus_enabled = true;
> >>> +}
> >>> +
> >>> +void bdrv_iostatus_update(BlockDriverState *bs, int error)
> >>> +{
> >>> +    error = abs(error);
> >>> +
> >>> +    if (!error) {
> >>> +        bs->iostatus = BDRV_IOS_OK;
> >>> +    } else {
> >>> +        bs->iostatus = (error == ENOSPC) ? BDRV_IOS_ENOSPC :
> >>> +                                           BDRV_IOS_FAILED;
> >>> +    }
> >>> +}
> >>> +
> >>>  int bdrv_img_create(const char *filename, const char *fmt,
> >>>                      const char *base_filename, const char *base_fmt,
> >>>                      char *options, uint64_t img_size, int flags)
> >>> diff --git a/block.h b/block.h
> >>> index 859d1d9..0dca1bb 100644
> >>> --- a/block.h
> >>> +++ b/block.h
> >>> @@ -50,6 +50,13 @@ typedef enum {
> >>>      BDRV_ACTION_REPORT, BDRV_ACTION_IGNORE, BDRV_ACTION_STOP
> >>>  } BlockMonEventAction;
> >>>  
> >>> +typedef enum {
> >>> +    BDRV_IOS_OK, BDRV_IOS_FAILED, BDRV_IOS_ENOSPC
> >>> +} BlockIOStatus;
> >>> +
> >>> +void bdrv_iostatus_update(BlockDriverState *bs, int error);
> >>> +void bdrv_enable_iostatus(BlockDriverState *bs);
> >>> +void bdrv_enable_io_status(BlockDriverState *bs);
> >>>  void bdrv_mon_event(const BlockDriverState *bdrv,
> >>>                      BlockMonEventAction action, int is_read);
> >>>  void bdrv_info_print(Monitor *mon, const QObject *data);
> >>> diff --git a/block_int.h b/block_int.h
> >>> index 1e265d2..09f038d 100644
> >>> --- a/block_int.h
> >>> +++ b/block_int.h
> >>> @@ -195,6 +195,8 @@ struct BlockDriverState {
> >>>         drivers. They are not used by the block driver */
> >>>      int cyls, heads, secs, translation;
> >>>      BlockErrorAction on_read_error, on_write_error;
> >>> +    bool iostatus_enabled;
> >>> +    BlockIOStatus iostatus;
> >>>      char device_name[32];
> >>>      unsigned long *dirty_bitmap;
> >>>      int64_t dirty_count;
> >> 
> >> Okay, let's see what we got here.
> >> 
> >> The block layer merely holds I/O status, device models set it.
> >> 
> >> Device I/O status is not migrated.  Why?

Bug. :)

> >> 
> >> bdrv_new() creates the BDS with I/O status tracking disabled.  Devices
> >> that do tracking enable it in their qdev init method.  If a device gets
> >> hot unplugged, tracking remains enabled.  If the BDS then gets reused
> >> with a device that doesn't do tracking, I/O status becomes incorrect.
> >> Can't happen right now, because we automatically delete the BDS on hot
> >> unplug, but it's a trap.  Suggest to disable tracking in bdrv_detach().
> >> 
> >> Actually, this is a symptom of the midlayer disease.  I suspect things
> >> would be simpler if we hold the status in its rightful owner, the device
> >> model.  Need a getter for it.  I'm working on a patch series that moves
> >> misplaced state out of the block layer into device models and block
> >> drivers, and a I/O status getter will fit in easily there.

Excellent.

> >
> > This is host state, so the device is not the rightful owner. Devices
> > should not even be involved with enabling it.
> 
> They are because they do the tracking, and thus the tracking only works
> for device models that do it.  Could it be done entirely within the
> block layer?