Re: [PATCH 0/3] md raid: enhancements to support the device mapper dm-raid target

[NeilBrown <neilb@suse.de>]

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On Fri, 13 Feb 2015 19:47:59 +0100 heinzm@redhat.com wrote:

> From: Heinz Mauelshagen <heinzm@redhat.com>
> 
> I'm enhancing the device mapper raid target (dm-raid) to take
> advantage of so far unused md raid kernel funtionality:
> takeover, reshape, resize, addition and removal of devices to/from raid sets.
> 
> This series of patches remove constraints doing so.
> 
> 
> Patch #1:
> add 2 API functions to allow dm-raid to access the raid takeover
> and resize functionality (namely md_takeover() and md_resize());
> reshape APIs are not needed in lieu of the existing personalilty ones
> 
> Patch #2:
> because device mapper core manages a request queue per mapped device
> utilizing the md make_request API to pass on bios via the dm-raid target,
> no md instance underneath it needs to manage a request queue of its own.
> Thus dm-raid can't use the md raid0 personality as is, because the latter
> accesses the request queue unconditionally in 3 places via mddev->queue
> which this patch addresses.
> 
> Patch #3:
> when dm-raid processes a down takeover to raid0, it needs to destroy
> any existing bitmap, because raid0 does not require one. The patch
> exports the bitmap_destroy() API to allow dm-raid to remove bitmaps.
> 
> 
> Heinz Mauelshagen (3):
>   md core:   add 2 API functions for takeover and resize to support dm-raid
>   md raid0:  access mddev->queue (request queue member) conditionally
>              because it is not set when accessed from dm-raid
>   md bitmap: export bitmap_destroy() to support dm-raid down takover to raid0
> 
>  drivers/md/bitmap.c |  1 +
>  drivers/md/md.c     | 39 ++++++++++++++++++++++++++++++---------
>  drivers/md/md.h     |  3 +++
>  drivers/md/raid0.c  | 48 +++++++++++++++++++++++++++---------------------
>  4 files changed, 61 insertions(+), 30 deletions(-)
> 

Hi Heinz,
 I don't object to these patches if you will find the exported functionality
 useful, but I am a little surprised by them.

 I would expect that dm-raid wouldn't ask md to 'takeover' from one level to
 another, but instead would
   - suspend the dm device
   - dismantle the array using the old level
   - assemble the array using the new level
   - resume the dm device

 The reason md needs 'takeover' is because it doesn't have the same
 device/target separation that dm does.

 I was particularly surprised that you wanted to use md/raid0.c  It is no
 better than dm/dm-stripe.c and managing two different stripe engines under
 LVM doesn't see like a good idea.

 Is there some reason that I have missed which makes it easier to use
 'takeover' rather than suspend/resume?

Thanks,
NeilBrown

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