[dm-devel] [PATCH 0/3] md raid: enhancements to support the device mapper dm-raid target
NeilBrown
neilb at suse.de
Wed Feb 18 02:03:28 UTC 2015
On Fri, 13 Feb 2015 19:47:59 +0100 heinzm at redhat.com wrote:
> From: Heinz Mauelshagen <heinzm at 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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