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Re: [dm-devel] [PATCH 00/12] A dm-raid45 target implemented using md raid5.

Hi Neil,

had a first go reading through your patch series w/o finding any major
issues. The only important feature for an initial release which needs
adding (as you mentioned) is (persistent) dirty log support.

Because you're using a persistent bitmap in the MD RAID personalities,
this looks like a bit more surgery to factor it out to potentially
enhance dm-log.c. For an initial solution we can as well just go with
MDs existing bitmap while keeping the dm-raid456 ctr support for
explicit dirty logging in order to avoid compatibility issues (there's
obviously no parameter to support bitmap chunk sizes so far).

Reshaping could be triggered either preferably via the constructor
involving MD metadata reads to be able to recognize the size change
requested or the message interface. Both ctr/message support could be
implemented sharing the same functions. Enhancements in the status
interface and dm_table_event() throwing on error/finish are mandatory if
we support reshaping.

A shortcoming of this MD wrapping solution vs. dm-raid45 is, that there
is no obvious way to leverage it to be a clustered RAID456 mapping
target. dm-raid45 has been designed with that future enhancement
possibility in mind.

Will try testing your code tomorrow.


On Thu, 2010-04-15 at 16:43 +1000, NeilBrown wrote:
> Greetings Heinz, Alasdair, and all,
> (Alasdair and Heinz cc:ed on this intro, but the patches are
>  only going to the lists).
>  Some months ago I posted a proof-of-concept patch which attempted to
>  provide RAID4/5/6 functionality to 'dm' using md/raid5.c.
>  While it did a least partly work it contained lots of hacks and was
>  very ugly.
>  I finally made time to do the job "properly".
>  The following series, when applied on top of a bunch of patches I
>  just submitted for linux-next, provides a 'dm-raid45' target which is
>  largely compatible with the one that Heinz has written (and several
>  distros are shipping), but which uses md/raid5.c for the core IO
>  processing.
>  I have tried to split the patch up into easy-to-handle pieces.  You
>  will note that some changes to core-dm are required, in particular to
>  pass back 'congestion' information and to handle plugging (which
>  raid5 uses to improve throughput).  I hope the approach I have taken
>  is suitable, but it can obviously be changed if necessary.
>  The create/status/message interface differs from the one in Heinz's
>  patch, but should be close enough to work with current 'dmraid'.
>  If you want to try the patches (rather than just read them) you
>  should probably "git pull" (please don't clone) from
>       git://neil.brown.name/md md-dm-raid45
>  so as to get all the prior refactoring patches in md.
>  Some advantages of this over Heinz's patch (at least as it was
>  when I last looked at it) are:
>   - raid6 support
>   - support for XOR-offload hardware where present
>   - less code duplication
>   - a single dm device can include multiple dm-raid45 targets.
>     (Heinz' code accesses dm_disk(md)->queue directly which
>      is a layering violations and assumes that there is no
>      other target in the mapped_device).
>  There is a lot more that could be done to this such as getting to
>  work with a disk based dirty-log and making the reshape options
>  available.  But this patch set should provide all basic RAID5
>  functionality.
>  Would the dm community be interested in including this work upstream
>  (after suitable review and testing)?
> Thanks,
> NeilBrown
> ---
> NeilBrown (12):
>       md: reduce dependence on sysfs.
>       md/raid5: factor out code for changing size of stripe cache.
>       md/dm: create dm-raid456 module using md/raid5
>       dm-raid456: add support for raising events to userspace.
>       raid5: Don't set read-ahead when there is no queue
>       dm-raid456: add congestion checking.
>       md/raid5: add simple plugging infrastructure.
>       md/plug: optionally use plugger to unplug an array during resync/recovery.
>       dm-raid456: support unplug
>       dm-raid456: add support for setting IO hints.
>       dm-raid456: add suspend/resume method
>       dm-raid456: add message handler.
>  drivers/md/Kconfig            |    8 +
>  drivers/md/Makefile           |    1 
>  drivers/md/dm-raid456.c       |  540 +++++++++++++++++++++++++++++++++++++++++
>  drivers/md/dm-table.c         |   19 +
>  drivers/md/md.c               |  211 ++++++++++------
>  drivers/md/md.h               |   43 +++
>  drivers/md/raid5.c            |  155 +++++++-----
>  drivers/md/raid5.h            |    6 
>  include/linux/device-mapper.h |   13 +
>  9 files changed, 859 insertions(+), 137 deletions(-)
>  create mode 100644 drivers/md/dm-raid456.c

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