[lvm-devel] [PATCH V2 0/2] Add thin lv and thin pool creation support

[Zdenek Kabelac]

Dne 31.1.2013 22:04, Tony Asleson napsal(a):
> On 01/31/2013 08:25 AM, M. Mohan Kumar wrote:
>> From: "M. Mohan Kumar" <mohan at in.ibm.com>
>>
>> Add thin lv and thin pool creation support to lvm2app.
>>
>> Changes from previous version:
>> * Add support to specify data block size and low water mark thresold for
>> newly created thin pool.
>> * Added support for specifying lowwatermark parameter to thin pool
>> creation in lvcreate command
>
> OK, so the block size if specified as 0 results in a default value being
> selected as before, which is what I believe we want.  The default low
> water mark was 0, which is what we believe disables event creation.
> This doesn't appear to be a safe default behavior?
>
> One option would be to change the type to a signed integer and change
> the meaning from number of blocks free to percentage free and then treat
> -1 as pick a sane default.  Thus 0 would be no events and 30 would
> indicate when 30% blocks free is remaining to generate an event.  We
> could also use an unsigned value and use 100 as please pick a sane
> default as this value and values close to it really wouldn't make much
> sense.
>
> Once we have defined what the default values are, I can modify my python
> bindings patch to use these default values.
>


Currently the lowwatermark setting are unused by dm driver - since we
need to first resolve policies.

Reason why they are not yet support (and dmeventd only does pooling) is,
that we have to make sure during all activations, we are not starting thinpool
which is past the threshold - and there are many ways how to deal with this state.

Also - for now - there is no  per-pool  threshold setting -  IMHO this should 
be improved - and it's the same problem lvm2 has with i.e. mirror/raid,
where many setting are currently set only via  lvm.conf - while there is no 
option for LV override - this should be probably resolved at global level.

Zdenek