Re: /sys/block and /dev and partitions

[Loic Dachary <loic@dachary.org>]

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On 16/08/2015 00:00, Ilya Dryomov wrote:
> On Sat, Aug 15, 2015 at 11:56 PM, Loic Dachary <loic@dachary.org> wrote:
>> Hi Ilya,
>>
>> On 15/08/2015 19:42, Ilya Dryomov wrote:
>>> On Sat, Aug 15, 2015 at 6:35 PM, Loic Dachary <loic@dachary.org> wrote:
>>>> Hi Sage,
>>>>
>>>> On 15/08/2015 16:28, Sage Weil wrote:
>>>>> On Sat, 15 Aug 2015, Loic Dachary wrote:
>>>>>> Hi,
>>>>>>
>>>>>> Is there a portable and consistent way to figure out if a given /dev/XXX
>>>>>> path (for instance /dev/dm-1) is a partition of a whole device ?
>>>>>> Although checking /sys/block/dm-1/dm/name for a number at the end (like
>>>>>> mpatha1 or mpatha2) would probably work, it feels like a fragile hack.
>>>>>> Looking into /sys/block/dm-1/slaves will lead to
>>>>>> /sys/block/dm-1/slaves/dm-0 and we can check that
>>>>>> /sys/block/dm-*/subsystem is class/block. But that does not necessarily
>>>>>> mean dm-1 is a partition of dm-0, just that it's a slave of dm-0.
>>>>>
>>>>> Take a look at is_partition in ceph-disk, whih is the best I came up with.
>>>>> Basically it checks if the device name appears as /sys/block/*/$foo...
>>>
>>> For regular devices, you can access() /sys/dev/block/maj:min/partition.
>>> If it's there, it's a partition - no need to iterate over /sys/block.
>>
>> I added http://tracker.ceph.com/issues/12706 for when someone has time to rework that part of the code.
>>
>>>
>>>>
>>>> That is consistently updated for /dev/sdb or /dev/vdb but things are different when using multipath. I'll rely on /sys/block/dm-?/dm/name instead until a better solution is found.
>>>
>>> A better way might be to rely on the fact that a dm partition will
>>> necessarily have its uuid prefixed by "part".  In that case, it should
>>> be safe to assume that the thing in slaves is a whole disk - I think
>>> that's what various util-linux tools do.  However, IIRC the dm uuid is
>>> optional, so that won't work on a dm device without a uuid.
>>
>> It looks like multipath on both CentOS 7 and Ubuntu 14.04 set the uuid in this way.
>>
>> Is it also safe to assume that if the uuid is:
>>
>> $ cat /sys/dev/block/253:?/dm/uuid
>> mpath-353333330000007d0
>> part1-mpath-353333330000007d0
>> part2-mpath-353333330000007d0
>>
>> it means these were created by multipath because of the mpath ? When asking dmsetup with:
> 
> Yes, I think so.  I'm pretty sure these "part<id>-" and "mpath-"
> prefixes were devised for exactly this purpose.
> 

Excellent !

> Thanks,
> 
>                 Ilya
> 

-- 
Loïc Dachary, Artisan Logiciel Libre


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