Re: Subject: [001/002 ] raid0 reshape

[Neil Brown <neilb@suse.de>]

On Tuesday May 26, goswin-v-b@web.de wrote:
> 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.

We have 2^21 md devices easily (I think that is the number) and it
wouldn't be hard to have more if that were an issue.

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

That might be an option.

> 
> 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?
>

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

Yep, these would be interesting problems to solve.  /proc/mdstat is a
bit of a wart on the design - getting the entry in /proc/mdstat
optional might be a good idea.

As for iostat - where does it get info from ? /proc/partitions? /proc/diskinfo?
Maybe /sys/block?
Either way, we could probably find a way to say "this block device is
'hidden'" .

If you want to be able to slice and dice lot of mini-raid arrays into
an LVM system, then whatever way you implement it you will need to be
keeping track of all those bits.  I think it makes most sense to use
the "block device" as the common abstraction, then if we start finding
issues: solve them.  That way the solutions become available for
others to use in ways we hadn't expected.

 
> MfG
>         Goswin

NeilBrown