[linux-lvm] [PATCH v2] lvs: add -o lv_usable

[Heinz Mauelshagen]

On 9/10/20 8:34 AM, heming.zhao wrote:
> On 9/10/20 1:17 AM, Zdenek Kabelac wrote:
>> Dne 09. 09. 20 v 18:47 Zhao Heming napsal(a):
>>> report LV is usable for upper layer.
>>>
>>> leave issues
>>> - this patch doesn't contain dm table comparison. So if the disk
>>>    is removed then re-inserted, but the re-inserted disk
>>>    major:minor is changed, the code doesn't have ability to detect.
>>> - raid10: removing any 2 disks will think as array broken.
>>>
>>> Signed-off-by: Zhao Heming <heming.zhao at suse.com>
>>> ---
>>> v2:
>>> - remove dm table parsing code in _lv_is_usable()
>>> - add new status bit NOT_USABLE_LV.
>>>    note, I chose the first available bit 0x0000000080000000
>>> - _lvusable_disp() uses lv_is_usable() to return usable status
>>>
>>      dm_list_iterate_items(lvseg, &lv->segments) {
>>>           for (s = 0; s < lvseg->area_count; ++s) {
>>>               if (seg_type(lvseg, s) == AREA_PV) {
>>> -                if (is_missing_pv(seg_pv(lvseg, s)))
>>> +                pv = seg_pv(lvseg, s);
>>> +                if (!(pv->dev) && is_missing_pv(pv)) {
>>>                       lv->status |= PARTIAL_LV;
>>> +                    lv->status |= NOT_USABLE_LV;
>>> +                }
>>>               }
>>>           }
>>>       }
>>
>> Hi
>>
>> As it can be seen here - there is big intersection with meaning of
>> PARTIAL_LV.


The semantics of a usable LV is fuzzy by definition, because for 
instance a multi-segment PARTIAL_LV
linear LV with a subset if its segments missing is still accessible 
relative to the remaining segments
thus doesn't make it unusable.   As a result, LVs failing t o activate 
would be the 'unusable ones'.
The later is given for RAID when it's, e.g.  missing more than its 
maximum number of parity devices
for striped RAID layouts.  So PARTIAL_LV is sufficient to tell that any 
LV is still partially usable.


>>
>> And the question is - what does it mean in the context of various 
>> segment
>> types.
>>
>> I believe we need to discuss with Heinz - whether we want to mark
>> Raid LVs partial in case they are actually 'only leg-pertial' and should
>> be actually activatable without partial activation  - which is ATM 
>> abused for this purpose.

Degraded RAID layouts are always usable unless more than its parity 
devices or all its mirrors failed because of missing PVs.  Hence such 
activatable RaidLVs are not partial at the LV but at the SubLV Level.

>>
>> ATM I'm not sure we want to introduce new flags, which has only slight
>> deviation from current partial flag - which should deserve closer look
>> of its meaning.
>>
>> We'll try to find something with Heinz to agree with.
>>
> Ok, wait for feedback from Heinz.

What are we missing if we define any SubLV partial state with 
PARTIAL_LV/not activatable and
leave it to the specific segment type handlers of the mappings on top of 
such SubLVs
to define their respective PARTIAL_LV state or reject activation. E.g. a 
fully usable RAID6 with a maximimum of 2 missing legs with those missing 
legs either being partial and RAID6 I/O addressing a missing segment 
-or- thise leg SubLVs not having been activated _not_ setting PATIAL_LV 
on the RAID6 LV ('lvs -oname,attr,devices' will show state details on 
the LV tree nodes).

Let's discuss this first before adding MISSING_PV to the picture...

FWIW:
raid0 mappings with a subset of missing segments may not be of much use 
but will provide data still.

Heinz


>
> I agree with you. the PARTIAL_LV is more closer to the new bit 
> NOT_USABLE_LV.
> There is another bit MISSING_PV, which is set when pv is missing or 
> the pv is not workable.

> From my understanding, we could reuse the PARTIAL_LV to show different 
> meaning according to different context. For example, in raid env, the 
> top layer LV will be set PARTIAL_LV when the raid array not usable 
> (e.g. raid0 missing a disk). Other cases, within raid limit, top layer 
> raid LV won't be set. if following the rule, there will no need to set 
> the new bit NOT_USABLE_LV.
>
> Heming
>
>
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