Re: Subject: [001/002 ] raid0 reshape

[Goswin von Brederlow <goswin-v-b@web.de>]

Neil Brown <neilb@suse.de> writes:

> On Monday May 25, goswin-v-b@web.de wrote:
>> That really seems to scream for LVM to support more raid levels. It
>> already has linear, raid0 and raid1 support (although I have no idea
>> how device mapper raid1 compares to md raid1).
>
> Note that LVM (a suite of user-space tools) could conceivably use
> md/raid1, md/raid5 etc. The functionality doesn't have to go in dm.
>
> Neil

How would you do this? Worst case you can have a LV made up of totaly
non linear PEs, meaning lots of 4MB (default PE size) big chunks in
random order on random disks.

Do you create a raid1/5 for each stripe? You surely run out of md
devices.

Create dm mappings for all stripe 0s, stripe 1s, stripe 2s, ... and
then a raid1/5 over those stripe devices?

What if the LV has segments with different raid configurations (number
of disks in a stripe or even different levels)? Create a raid for each
segment and then a dm mapping for a linear raid?


You can get a flood of intermediate devices there. A /proc/mdstat with
200 entries would be horrible. iostat output would be totaly
useless. ...

MfG
        Goswin