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

[Bob Liu]

On 3/25/20 2:47 PM, 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.

The exposed size is "regular dev size + zoned dev size - metadata size - reserved seq zone size".
I didn't see why there is a lie?

> Making me wonder if anybody ever tested a disk-full scenario...
> The other problem is that with two devices we need to be able to stitch them together in an automated fashion, eg via a systemd service or udev rule.
> But for this we need to be able to identify the devices, which means both need to carry metadata, and both need to have unique identifier within the metadata. Which the current metadata doesn't allow to.
> 
> Hence my plan is to implement a v2 metadata, carrying UUIDs for the dmz set _and_ the component device. With that we can update blkid to create links etc so that the devices can be identified in the system.
> Additionally I would be updating dmzadm to write the new metadata.
> 
> And I will add a new command 'start' to dmzadm which will then create the device-mapper device _with the correct size_. It also has the benefit that we can create the device-mapper target with the UUID specified in the metadata, so the persistent device links will be created automatically.
> 
> Bob, can you send me your improvements to dmzadm so that I can include them in my changes?
> 

Attached, but it's a big patch I haven't split them to smaller one.
The dmz_check/repair can't work neither in current stage.
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