[linux-lvm] Question about filter behavior on pvcreate /dev/md0

[linux-lvm] Question about filter behavior on pvcreate /dev/md0

[Heming Zhao]

Hello List,

I created a md device, and used pvcreate to format it.
But the pvcreate was failed with filter rules.
the filter in /etc/lvm/lvm.conf:
```
filter = [ "r|/dev/.*/by-path/.*|", "r|/dev/.*/by-id/.*|", 
"r|/dev/fd.*|", "r|/dev/cdrom|" ]
```
if I remove the filter, pvcreate will be successful. why?

steps:
```
# rpm -qa | grep lvm2
lvm2-2.02.180-10.el7_6.7.x86_64
lvm2-libs-2.02.180-10.el7_6.7.x86_64
# dd if=/dev/zero of=/dev/sda bs=1M
   ... ...
# dd if=/dev/zero of=/dev/sdb bs=1M
   ... ...
# mdadm -C /dev/md0 -l 0 -n 2 -a yes /dev/sda /dev/sdb
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started
# lsblk
NAME   MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
sda      8:0    0  256M  0 disk
└─md0    9:0    0  508M  0 raid0
sdb      8:16   0  256M  0 disk
└─md0    9:0    0  508M  0 raid0
vda    253:0    0   40G  0 disk
├─vda1 253:1    0    1G  0 part  /boot
├─vda2 253:2    0   38G  0 part  /
└─vda3 253:3    0    1G  0 part  [SWAP]
# pvcreate /dev/md0
   Device /dev/md0 excluded by a filter.
```

Thanks
zhm

Re: [linux-lvm] Question about filter behavior on pvcreate /dev/md0

[Zdenek Kabelac]

Dne 13. 06. 19 v 9:41 Heming Zhao napsal(a):
> Hello List,
> 
> I created a md device, and used pvcreate to format it.
> But the pvcreate was failed with filter rules.
> the filter in /etc/lvm/lvm.conf:
> ```
> filter = [ "r|/dev/.*/by-path/.*|", "r|/dev/.*/by-id/.*|",
> "r|/dev/fd.*|", "r|/dev/cdrom|" ]
> ```


Hi

This filter mostly eliminates ANY block device you have in the system.
The logic of filtering can be seen as puzzling, but please try to read 
carefully comments in lvm.conf about filtering. It's explained there in the 
great detail.

As soon as the 'accept' rule is seen - the device gets accepted.
Otherwise if you remove device path - all it's associated clones will be also 
removed.

So when you block device generates one of /dev/*/by-path access paths (and I'd 
assume /dev/md0 should be) - it gets removed.

As 'quick fix' you can add 'a' rule for /dev/md.*...

Regards

Zdenek

Re: [linux-lvm] Question about filter behavior on pvcreate /dev/md0

[Heming Zhao]

Hello Zdenek,

Thank you for your reply. Your method works well.
I also understand the filter rules gradually.


On 6/14/19 4:51 PM, Zdenek Kabelac wrote:
> Dne 13. 06. 19 v 9:41 Heming Zhao napsal(a):
>> Hello List,
>>
>> I created a md device, and used pvcreate to format it.
>> But the pvcreate was failed with filter rules.
>> the filter in /etc/lvm/lvm.conf:
>> ```
>> filter = [ "r|/dev/.*/by-path/.*|", "r|/dev/.*/by-id/.*|",
>> "r|/dev/fd.*|", "r|/dev/cdrom|" ]
>> ```
> 
> 
> Hi
> 
> This filter mostly eliminates ANY block device you have in the system.
> The logic of filtering can be seen as puzzling, but please try to read 
> carefully comments in lvm.conf about filtering. It's explained there in 
> the great detail.
> 
> As soon as the 'accept' rule is seen - the device gets accepted.
> Otherwise if you remove device path - all it's associated clones will be 
> also removed.
> 
> So when you block device generates one of /dev/*/by-path access paths 
> (and I'd assume /dev/md0 should be) - it gets removed.
> 
> As 'quick fix' you can add 'a' rule for /dev/md.*...
> 
> Regards
> 
> Zdenek
>