Re: [RFC PATCH 2/2] lightnvm: add non-continuous lun target creation support

["Matias Bjørling" <mb@lightnvm.io>]

On 01/21/2016 08:44 AM, Wenwei Tao wrote:
> 2016-01-20 21:19 GMT+08:00 Matias Bjørling <mb@lightnvm.io>:
>> On 01/15/2016 12:44 PM, Wenwei Tao wrote:
>>> When create a target, we specify the begin lunid and
>>> the end lunid, and get the corresponding continuous
>>> luns from media manager, if one of the luns is not free,
>>> we failed to create the target, even if the device's
>>> total free luns are enough.
>>>
>>> So add non-continuous lun target creation support,
>>> thus we can improve the backend device's space utilization.
>>
>> A couple of questions:
>>
>> A user inits lun 3-4 and afterwards another 1-6, then only 1,2,5,6 would
>> be initialized?
>>
>> What about the case where init0 uses 3-4, and init1 uses 1-6, and would
>> share 3-4 with init0?
>>
>> Would it be better to give a list of LUNs as a bitmap, and then try to
>> initialize on top of that? with the added functionality of the user may
>> reserve luns (and thereby reject others attempting to use them)
>>
> 
> I'm not quite understand the bitmap you mentioned.
> This patch do have a bitmap : dev->lun_map and the target creation is
> on top of this bitmap.
> 
> The way how a target gets its LUNs is based on its creation flags.
> If NVM_C_FIXED is set, this means the target wants get its LUNs
> exactly as it specifies from lun_begin to lun_end, if any of them are
> occupied by others, the creation fail.
> If NVM_C_FIXED is not set, the target will get its LUNs from free LUNs
> between  0 and dev->nr_luns, there is no guarantee that final LUNs are
> continuous.
> 
> For the first question, if NVM_C_FIXED is used second creation would
> be fail since 3-4 are already used, otherwise it will success if we
> have enough free LUNs left, but the final LUNs may not from 1 to 6,
> e.g. 1, 2, 5, 6, 7, 11.
> 
> For the second question, from explanation above we know that sharing
> LUNs would not happen in current design.

This is an interesting discussion. This could boil down to a device
supporting either a dense or sparse translation map (or none).

With a dense translation map, there is a 1-to-1 relationship between
lbas and ppas.

With a sparse translation map (or no translation map, handled completely
by the host), we may share luns.

For current implementations, a dense mapping is supported. I wonder the
cost of implementing a sparse map (e.g. b-tree structure) on a device is
a good design choice.

If the device supports sparse mapping, then we should add another bit to
the extension bitmap, and then allow luns to shared. In the current
case, we should properly just deny luns to be shared between targets.

How about extending the functionality to take a bitmap of luns, which
defines the luns that we like to map. Do the necessary check if any of
them is in use, and then proceed if all is available?

That'll remove the ambiguity from selection luns, and instead enable the
user to make the correct decision up front?