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

[Kevin Wolf <kwolf@redhat.com>]

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?
> 
>>
>> 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?
> 
> 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.

This is host state, so the device is not the rightful owner. Devices
should not even be involved with enabling it.

Kevin