[linux-lvm] Question about vgreduce on raid lvm cache

[linux-lvm] Question about vgreduce on raid lvm cache

[Heming Zhao]

Hello list,

I met a lvm cache issue recently.

The machine had an LVM VG configured with:
  dev1 + dev2 -> data_lv with raid 1 mirror
  dev3 + dev4 -> data_cache_lv with raid 1 mirror and type cache

These were assembled to a cached volume with:
"lvconvert --type cache --cachemode writeback --cachepool data/data_cache_lv data/data_lv"

The device dev4 then died and was removed from the system.

When attempting to remove the device using "vgreduce --removemissing", lvm removed both data_cache_lv and data_lv from the data vg.

This is unexpected behaviour, as dev1, dev2, dev3 were all still present.
The expected behaviour should:
```
vgreduce --removemissing
lvconvert -m0 system/lv
```

When it's a non-cached type, and you performe vgreduce, lvs are NOT removed.

the (reproduced) command list:
```
pvcreate /dev/mapper/dev1 /dev/mapper/dev2 /dev/mapper/dev3 /dev/mapper/dev4
vgcreate VG /dev/mapper/dev1 /dev/mapper/dev2 /dev/mapper/dev3 /dev/mapper/dev4
lvcreate --type raid1 -m 1 -L150 -n data_lv VG /dev/mapper/dev1 /dev/mapper/dev2
lvcreate --type raid1 -m 1 -L150 -n data_cache_lv VG /dev/mapper/dev3 /dev/mapper/dev4
lvconvert --type cache --cachemode writeback --cachepool VG/data_cache_lv VG/data_lv
dmsetup remove -f dev4
vgreduce --removemissing --force VG
```

Thanks.

Re: [linux-lvm] Question about vgreduce on raid lvm cache

[Heming Zhao]

Hello list,

I found below question solution, executing removemissing with "--mirrorsonly" is mandatory.
```
vgreduce --removemissing --mirrorsonly --force VG
lvconvert -m0 VG/data_lv
```

But there raise another issue. the default error message is not clear.
When vgreduce on mirrored & cached LVs, user normally runs below command & get below failed messages:
```
# vgreduce --removemissing VG
   WARNING: Couldn't find device with uuid UQZkRE-P8X7-Pkgv-mE5b-q6CY-2DmX-ew2BCn.
   WARNING: VG VG is missing PV UQZkRE-P8X7-Pkgv-mE5b-q6CY-2DmX-ew2BCn.
   WARNING: Couldn't find all devices for LV VG/data_lv_corig_rimage_1 while checking used and assumed devices.
   WARNING: Couldn't find all devices for LV VG/data_lv_corig_rmeta_1 while checking used and assumed devices.
   WARNING: Couldn't find device with uuid UQZkRE-P8X7-Pkgv-mE5b-q6CY-2DmX-ew2BCn.
   WARNING: Partial LV data_lv needs to be repaired or removed.
   WARNING: Partial LV data_lv_corig_rimage_1 needs to be repaired or removed.
   WARNING: Partial LV data_lv_corig_rmeta_1 needs to be repaired or removed.
   WARNING: Partial LV data_lv_corig needs to be repaired or removed.
   There are still partial LVs in VG VG.
   To remove them unconditionally use: vgreduce --removemissing --force.   <===== this command is very dangerous for this mail's scenario
   WARNING: Proceeding to remove empty missing PVs.
   WARNING: Couldn't find device with uuid UQZkRE-P8X7-Pkgv-mE5b-q6CY-2DmX-ew2BCn.
```

Currently the error info are not clear. This very easy to guide user to execute "--removemissing --force", then the LVs will be destroied.

So I filed a patch for this little issue on next mail.

thanks you.

On 11/29/19 4:35 PM, Heming Zhao wrote:
> Hello list,
> 
> I met a lvm cache issue recently.
> 
> The machine had an LVM VG configured with:
>    dev1 + dev2 -> data_lv with raid 1 mirror
>    dev3 + dev4 -> data_cache_lv with raid 1 mirror and type cache
> 
> These were assembled to a cached volume with:
> "lvconvert --type cache --cachemode writeback --cachepool data/data_cache_lv data/data_lv"
> 
> The device dev4 then died and was removed from the system.
> 
> When attempting to remove the device using "vgreduce --removemissing", lvm removed both data_cache_lv and data_lv from the data vg.
> 
> This is unexpected behaviour, as dev1, dev2, dev3 were all still present.
> The expected behaviour should:
> ```
> vgreduce --removemissing
> lvconvert -m0 system/lv
> ```
> 
> When it's a non-cached type, and you performe vgreduce, lvs are NOT removed.
> 
> the (reproduced) command list:
> ```
> pvcreate /dev/mapper/dev1 /dev/mapper/dev2 /dev/mapper/dev3 /dev/mapper/dev4
> vgcreate VG /dev/mapper/dev1 /dev/mapper/dev2 /dev/mapper/dev3 /dev/mapper/dev4
> lvcreate --type raid1 -m 1 -L150 -n data_lv VG /dev/mapper/dev1 /dev/mapper/dev2
> lvcreate --type raid1 -m 1 -L150 -n data_cache_lv VG /dev/mapper/dev3 /dev/mapper/dev4
> lvconvert --type cache --cachemode writeback --cachepool VG/data_cache_lv VG/data_lv
> dmsetup remove -f dev4
> vgreduce --removemissing --force VG
> ```
> 
> Thanks.
> 
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
> 
>