[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