[linux-lvm] "lvconvert --mirrors 1 --stripes 3 vgtest/lvtest" command succeeds, but no stripes

[linux-lvm] "lvconvert --mirrors 1 --stripes 3 vgtest/lvtest" command succeeds, but no stripes

[Gang He]

Hello Guys,

I found lvconvert command (in lvm lvm2-2.02.120) did not handle "--stripes" option correctly.
The reproduce steps are as below,
# vgcreate vgtest /dev/vdb /dev/vdc /dev/vdd /dev/vde /dev/vdf /dev/vdg
# lvcreate -n lvtest -L 8G vgtest
# lvconvert --mirrors 1 --stripes 3 vgtest/lvtest
# lvs -o+stripes
  LV     VG     Attr       LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert #Str
  lvtest vgtest rwi-a-r--- 8.00g                                    12.21               2

But if you create LV using lvcreate command directly, the command succeeds with stripes.
e.g.
# lvcreate --mirrors 1 --stripes 3 -L 4G vgtest
# lvs -o+stripes
  LV     VG     Attr       LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert #Str
  lvol0  vgtest rwi-a-r--- 4.01g                                    8.67                6

I also use lvm2-2.02.180 to reproduce this issue, the lvconvert command works like before,
but the command will print out a message, like that,
# lvconvert --mirrors 1 --stripes 3 vgtest/lvtest
  Command does not accept option: --stripes 3.

Why do we can not use lvconvert to do like that, design issue? but lvcreate can do that.
What limitations are there between mirrors and stripes?

Thanks
Gang

Re: [linux-lvm] "lvconvert --mirrors 1 --stripes 3 vgtest/lvtest" command succeeds, but no stripes

[Zdenek Kabelac]

Dne 16. 05. 19 v 11:03 Gang He napsal(a):
> Hello Guys,
> 
> I found lvconvert command (in lvm lvm2-2.02.120) did not handle "--stripes" option correctly.
> The reproduce steps are as below,
> # vgcreate vgtest /dev/vdb /dev/vdc /dev/vdd /dev/vde /dev/vdf /dev/vdg
> # lvcreate -n lvtest -L 8G vgtest
> # lvconvert --mirrors 1 --stripes 3 vgtest/lvtest
> # lvs -o+stripes
>    LV     VG     Attr       LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert #Str
>    lvtest vgtest rwi-a-r--- 8.00g                                    12.21               2
> 
> But if you create LV using lvcreate command directly, the command succeeds with stripes.
> e.g.
> # lvcreate --mirrors 1 --stripes 3 -L 4G vgtest
> # lvs -o+stripes
>    LV     VG     Attr       LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert #Str
>    lvol0  vgtest rwi-a-r--- 4.01g                                    8.67                6
> 
> I also use lvm2-2.02.180 to reproduce this issue, the lvconvert command works like before,
> but the command will print out a message, like that,
> # lvconvert --mirrors 1 --stripes 3 vgtest/lvtest
>    Command does not accept option: --stripes 3.
> 
> Why do we can not use lvconvert to do like that, design issue? but lvcreate can do that.
> What limitations are there between mirrors and stripes?

Layout of disks with raid target is:

1st. + 2nd.     are paired into raid1

3rd. + 4th.     are paired into raid1

5th. + 6th.     are making 3rd. raid1

then with your --stripe option - above raid1-like-devices are combined into 
one big raid0 array   (aka raid10) (all happens within single device target)

So converting ordinary LV into such beast is not trivial.
Although I believe you should look at reshaping possibilities, but I'm not 
quite sure there is already support for raid10.

In past with old --type mirror target the logic was different - you had 
'individual' mirror legs striped - and then from them it's been mirrored/raid1 
(aka raid01). Clearly old mechanism (where you only say you add new 'striped' 
leg) is way more easier for implementation - you just allocate striped LV and 
add as a new leg to mirror.

Regards

Zdenek