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From: Heinz Mauelshagen <heinzm@redhat.com>
To: LVM general discussion and development <linux-lvm@redhat.com>,
	"heming.zhao" <heming.zhao@suse.com>,
	Zdenek Kabelac <zkabelac@redhat.com>
Cc: teigland@sourceware.org
Subject: Re: [linux-lvm] [PATCH v2] lvs: add -o lv_usable
Date: Thu, 17 Sep 2020 12:18:56 +0200	[thread overview]
Message-ID: <58d180ab-489d-465f-35de-8442dde067b8@redhat.com> (raw)
In-Reply-To: <4a6b7e89-49b9-72a8-6938-a1fc7419ccc1@suse.com>

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@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
>
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

  reply	other threads:[~2020-09-17 10:18 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-09 16:47 Zhao Heming
2020-09-09 17:17 ` Zdenek Kabelac
2020-09-10  6:34   ` heming.zhao
2020-09-17 10:18     ` Heinz Mauelshagen [this message]
2020-09-18  7:07       ` heming.zhao
2020-09-18 11:38         ` Heinz Mauelshagen
2020-09-19  3:58           ` heming.zhao

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