[dm-devel] [RFC PATCH v2 3/3] dm zoned: add regular device info to metadata

[Hannes Reinecke]

On 3/25/20 9:02 AM, Damien Le Moal wrote:
> On 2020/03/25 15:47, Hannes Reinecke wrote:
>> On 3/25/20 7:29 AM, Damien Le Moal wrote:
>>> On 2020/03/24 20:04, Bob Liu wrote:
>>>> This patch implemented metadata support for regular device by:
>>>>    - Emulated zone information for regular device.
>>>>    - Store metadata at the beginning of regular device.
>>>>
>>>>        | --- zoned device --- | -- regular device ||
>>>>        ^                      ^
>>>>        |                      |Metadata
>>>> zone 0
>>>>
>>>> Signed-off-by: Bob Liu <bob.liu at oracle.com>
>>>> ---
>>>>    drivers/md/dm-zoned-metadata.c | 135 +++++++++++++++++++++++++++++++----------
>>>>    drivers/md/dm-zoned-target.c   |   6 +-
>>>>    drivers/md/dm-zoned.h          |   3 +-
>>>>    3 files changed, 108 insertions(+), 36 deletions(-)
>>>>
>> Having thought about it some more, I think we cannot continue with this
>> 'simple' approach.
>> The immediate problem is that we lie about the disk size; clearly the
>> metadata cannot be used for regular data, yet we expose a target device
>> with the full size of the underlying device.
>> Making me wonder if anybody ever tested a disk-full scenario...
> 
> Current dm-zoned does not do that... What is exposed as target capacity is
> number of chunks * zone size, with the number of chunks being number of zones
> minus metadata zones minus number of zones reserved for reclaim. And I did test
> disk full scenario (when performance goes to the trash bin because reclaim
> struggles...)
> 
Thing is, the second number for the dmsetup target line is _supposed_ to 
be the target size.
Which clearly is wrong here.
I must admit I'm not sure what device-mapper will do with a target 
definition which is larger than the resulting target device ...
Mike should know, but it's definitely awkward.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare at suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer