On Tue, Mar 30, 2010 at 8:23 AM, Doug Ledford <dledford@redhat.com> wrote:
quoted text
> On 03/29/2010 08:46 PM, Dan Williams wrote:
>> This begs the question, why not change the definition of an imsm
>> container to incorporate anything with imsm metadata?  This definitely
>> would make spare management easier.  This was an early design decision
>> and had the nice side effect that it lined up naturally with the
>> failure and rebuild boundaries of a family.  I could give it more
>> thought, but right now I believe there is a lot riding on this 1:1
>> container-to-family relationship, and I would rather not go there.
>
> I'm fine with the container being family based and not domain based.  I
> just didn't realize that distinction existed.  It's all cleared up now ;-)
>

Great.

quoted text
>>> However, that just means (to me anyway) that I would treat all of the
>>> sata ports as one domain with multiple container arrays in that domain
>>> just like we can have multiple native md arrays in a domain.  If a disk
>>> dies and we hot plug a new one, then mdadm would look for the degraded
>>> container present in the domain and add the spare to it.  It would then
>>> be up to mdmon to determine what logical volumes are currently degraded
>>> and slice up the new drive to work as spares for those degraded logical
>>> volumes.  Does this sound correct to you, and can mdmon do that already
>>> or will this need to be added?
>>
>> This sounds correct, and no mdmon cannot do this today.  The current
>> discussions we (Marcin and I) had with Neil offlist was extending
>> mdadm --monitor to handle spare migration for containers since it
>> already handles spare migration for native md arrays.  It will need
>> some mdmon coordination since mdmon is the only agent that can
>> disambiguate a spare from a stale device at any given point in time.
>
> So we'll need to coordinate on this aspect of things then.  I'll keep
> you updated as I get started implementing this if you want to think
> about how you would like to handle this interaction between mdadm/mdmon.

Ok, that sounds like a good split we'll keep you posted as well,

quoted text
> As far as I can tell, we've reached a fairly decent consensus on things.
>  But, just to be clear, I'll reiterate that concensus here:
>
> Add a new linetype: DOMAIN with options path= (must be specified at
> least once for any domain action other than none and incremental and
> must be something other than a global match for any action other than
> none and incremental) and metadata= (specifies the metadata type
> possible for this domain as one of imsm/ddf/md

Why not 0.90 and 1.x for instead of 'md'?  These match the 'name'
attribute of struct superswitch.

quoted text
> and where for imsm or
> ddf types, we will verify that the path portions of the domain do not
> violate possible platform limitations) and action= (where action is
> none, incremental, readd, safe_use, force_use where action is specific
> to a hotplug when a degraded array in the domain exists and can possibly
> have slightly different meanings depending on whether the path specifies
> a whole disk device or specific partitions on a range of devices

I have been thinking that the path= option specifies controller paths,
not disk devices.  Something like "pci-0000:00:1f.2-scsi-[0-3]*" to
pick the first 4 ahci ports.  This also purposefully excludes virtual
devices dm/md.  I think we want to limit this functionality to
physical controller ports... or were you looking to incorporate
support for any block device?

quoted text
> and
> where there is the possibility of adding more options or a new option
> name for the case of adding a hotplug drive to a domain where no arrays
> are degraded, in which case issues such as boot sectors, partition
> tables, hot spare versus grow, etc. must be addressed).
>
> Modify udev rules files to cover the following scenarios (it's
> unfortunate that we have to split things up like this, but in order to
> deal with either bare drives or drives that have things like lvm data
> and we are using force_use, we must trigger on *all* drive hotplug
> events, we must trigger early, and we must override other subsystem's
> possible hotplug actions, otherwise the force_use option will be a noop):

Can't we limit the scope to the hotplug events we care about by
filtering the udev scripts based on the current contents of the
configuration file?  We already need a step in the process that
verifies if the configuration heeds the platform constraints.  So,
something like mdadm --activate-domains that validates the
configuration, generates the necessary udev scripts and enables
hotplug.

quoted text
>
> 1) plugging in a device that already has md raid metadata present
>   a) if the device has metadata corresponding to one of our arrays,
> attempt to do normal incremental add
>   b) if the device has metadata corresponding to one of our arrays, and
> the normal add failed and the options readd, safe_use, or force_use are
> present in the mdadm.conf file, reattempt to add using readd
>   c) if the device has metadata corresponding to one of our arrays, and
> the readd failed, and the options safe_use or force_use are present,
> then do a regular add of the device to the array (possibly with doing a
> preemptive zero-superblock on the device we are adding).  This should
> never fail.

Yes, but this also reminds me about the multiple superblock case.  It
should usually only happen to people that experiment with different
metadata types, but we should catch and probably ignore drives that
have ambiguous/multiple superblocks.

quoted text
>   d) if the device has metadata that does not correspond to any array
> in the system, and there is a degraded array, and the option force_use
> is present, then quite possibly repartition the device to make the
> partitions match the degraded devices, zero any superblocks, and add the
> device to the arrays.  BIG FAT WARNING: the force_use option will cause
> you to loose data if you plug in an array disk for another machine while
> this machine has degraded arrays.

Let's also limit this to ports that were recently (as specified by a
timeout= option to the DOMAIN) unplugged.  This limits the potential
damage.

quoted text
>
> 2) plugging in a device that doesn't already have md raid metadata
> present but is part of an md domain
>   a) if the device is bare and the option safe_use is present and we
> have degraded arrays, partition the device (if needed) and then add
> partitions to degraded arrays
>   b) if the device is not bare, and the option force_use is present and
> we have degraded arrays, (re)partition the device (if needed) and then
> add partitions to degraded arrays.  BIG FAT WARNING: if you enable this
> mode, and you hotplug say an LVM volume into your domain when you have a
> degraded array, kiss your LVM volume goodbye.
>
> Modify udev rules files to deal with device removal.  Specifically, we
> need to watch for removal of devices that are part of raid arrays and if
> they weren't failed when they were removed, fail them, and then remove
> them from the array.  This is necessary for readd to work.  It also
> releases our hold on the scsi device so it can be fully released and the
> new device can be added back using the same device name.
>

Nod.

quoted text
> Modify mdadm -I mode to read the mdadm.conf file for the DOMAIN lines on
> hotplug events and then modify the -I behavior to suit the situation.
> The majority of the hotplug changes mentioned above will actually be
> implemented as part of mdadm -I, we will simply add a few rules to call
> mdadm -I in a few new situations, then allow mdadm -I (which has
> unlimited smarts, where as udev rules get very convoluted very quickly
> if you try to make them smart) to actually make the decisions and do the
> right thing.  This means that effectively, we might just end up calling
> mdadm -I on every disk hot plug event whether there is md metadata or
> not, but only doing special things when the various conditions above are
> met.

Modulo the ability to have a global enable / disable for domains via
something like --activate-domains

quoted text
>
> Modify mdadm and the spare-group concept of ARRAY lines to coordinate
> spare-group assignments and DOMAIN assignments.  We need to know what to
> do in the event of a conflict between the two.  My guess is that this is
> unlikely, but in the end, I think we need to phase out spare-group
> entirely in favor of domain.  Since we can't have a conflict without
> someone adding domain lines to the config file, I would suggest that the
> domain assignments override spare-group assignments and we complain
> about the conflict.  That way, even though the user obviously intended
> something specific with spare-group, he also must have intended
> something specific with domain assignments, and as the domain keyword is
> the newest and latest thing, honor the latest wishes and warn about it
> in case they misentered something.

Sounds reasonable.

quoted text
>
> Modify mdadm/mdmon to enable spare migration between imsm containers in
> a domain.  Retain mdadm ability to move hot spares between native
> arrays, but make it based on domain now instead of spare-group, and in
> the config settings if someone has spare-group assignments and no domain
> assignments, then create internal domain entries that mimic the
> spare-group layout so that we can modify the core spare movement code to
> only be concerned with domain entries.
>
> I think that covers it.  Do we have a consensus on the general work?

I think we have a consensus.  The wrinkle that comes to mind is the
case we talked about before where some ahci ports have been reserved
for jbod support in the DOMAIN configuration.  If the user plugs in an
imsm-metadata disk into a "jbod port" and reboots the option-rom will
assemble the array across the DOMAIN boundary.  You would need to put
explicit "passthrough" metadata on the disk to get the option-rom to
ignore it, but then you couldn't put another metadata type on that
disk.  So maybe we can't support the subset case and need to force the
platform's full expectation of the domain boundaries or honor the
DOMAIN line and let the user figure out/remember why this one raid
member slot does not respond to hotplug events.

Thanks for the detailed write up.

--
Dan
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