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

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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@xxxxxxxxxx>
---
   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@xxxxxxx                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer



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