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Re: [dm-devel] [PATCH 00/24] dm-raid456 support using md/raid5.c, now with dirty-log



On Fri, 18 Jun 2010 12:42:46 +0200
Heinz Mauelshagen <heinzm redhat com> wrote:

> On Fri, 2010-06-18 at 13:52 +1000, Neil Brown wrote:
> > On Thu, 17 Jun 2010 12:47:00 +0200
> > Heinz Mauelshagen <heinzm redhat com> wrote:
> > 
> > > On Thu, 2010-06-17 at 15:41 +1000, Neil Brown wrote:
> > > > On Wed, 16 Jun 2010 13:26:27 +0200
> > > > Heinz Mauelshagen <heinzm redhat com> wrote:
> > > > 
> > > > > On Wed, 2010-06-16 at 09:45 +1000, Neil Brown wrote:
> > > > > > Hi Heinz,
> > > > > > 
> > > > > > On Tue, 15 Jun 2010 15:23:26 +0200
> > > > > > Heinz Mauelshagen <heinzm redhat com> wrote:
> > > > > > 
> > > > > > > 
> > > > > > > Neil,
> > > > > > > 
> > > > > > > for missing devices we (re)load the table with error mapped device(s) in
> > > > > > > their place rather than identifying them with a special char/string.
> > > > > > > Did you go for the later in order to avoid some MD hassle with error
> > > > > > > targets being accessed by the MD personalities? If not so, we can drop
> > > > > > > the special '-' char support to identify missing devices.
> > > > > > 
> > > > > > When raid5 determines that a device has failed and so it will not write to it
> > > > > > again, it must be sure that the failure is record in the metadata before it
> > > > > > proceeds with that decision not to even try writing to the failed device.
> > > > > 
> > > > > Yes, that's the mandatory order to avoid data loss.
> > > > > 
> > > > > > Otherwise a crash/restart before the metadata was safely updated would result
> > > > > > in corruption.
> > > > > > 
> > > > > > This means that it must be possible for user-space to explicitly tell the
> > > > > > raid5 that a device is known to be faulty.
> > > > > > Doing that through the constructor seems the natural way to do it.
> > > > > 
> > > > > If /dev/mapper/error with an error target mapping would be used instead,
> > > > > would that cause consistency troubles to the MD personality if accessing
> > > > > those rather than the NULL device solution with the '-' argument you
> > > > > have now?
> > > > > If not so, we could drop the '-' support, which I'm aiming at.
> > > > > 
> > > > 
> > > > I would need to differentiate between /dev/mapper/error (where errors are
> > > > expected) and any other device (where errors are not expected).  How do you
> > > > propose I do that?
> > > 
> > > Is that differentiation mandatory for the MD personality at all?
> > 
> > I thought we had already agreed that it was.
> 
> No ;)
> 
> The following is based on the assumption, that you're refering to MD
> metadata...

Ahhh.... No, I'm not.
My dm-raid45 doesn't see any metadata directly.
Metadata much be handled entirely by user-space, whether lvm or dmraid or
something else.  It will pass the important information from the metadata to
dm-raid45, largely via the target constructor line.

> 
> > 
> > If the metadata records that a device is working, then raid5 will not expect
> > an error and if it gets one it must wait for the metadata to be updated.
> > If the metadata records that a device is not working, then raid5 will never
> > bother even trying to write to that device.
> 
> So that will allow for a /dev/mapper/error device instead of the '-'
> because like you say, RAID5 is not going to write to it anyway.

But RAID5 doesn't *know* that it shouldn't write to it unless you tell it some
how.  That is what "-" means.  It means "the metadata records this device has
failed or missing, and so you should never try to read from or write to it".

Some how the RAID5 module needs to know that the device is failed and the
this status is recorded in the metadata.
There seem to be two proposed ways for it to find this out.

1/ The target constructor line explicitly carries this information, e.g. by
   giving "-" in place of a device name.
2/ RAID5 attempts IO, gets a failure, reports this to user-space, then
   user-space sends the "allow_writes" message which implicitly says "all
   devices which you current think have failed, are recorded as such in the
   metadata".

Both can obviously work, and I can accept that there could be a place for
"allow_writes", but I think that being able to give the information directly
is a good thing.

Thanks,
NeilBrown



> 
> And it shouldn't bother reading neither as long as the metadata tells it
> the drive is dead.
> 
> > 
> > So yes, and important distinction.
> > 
> > I imagine that the dmevent handler would handle a failure by updating the
> > metadata, then dismantling the array and recreating it with the failed device
> > missing.
> 
> Yes, it updates the upspace metadata (ie. for lvm2/dmraid).
> 
> > Given that in some cases it would want to do this anyway to
> > introduce a spare, and as I think this is how dm-raid1 errors are handled, it
> > seems the logical approach.
> > I note that your dm-raid45 code uses a message to "allow_writes".
> 
> Yes.
> 
> > Using that could be racy unless you suspend the device before updating the
> > metadata, and if you do that you may as well use the suspend/resume sequence
> > as part of re-enabling writes. (???)
> 
> The message can be used optionally vs. suspend/resume pairs for
> performance reasons to allow for max reads to go through. Of course it
> needs to be called in the proper sequence (same applies to
> suspend/resume) and then it's not racy.
> 
> > 
> > This is probably just a question of interface design, and we both naturally
> > prefer the one we came up with first :-)
> 
> Well... ;-)
> 
> > 
> > I guess it just feels clumsy for the raid5 driver to have to try to read and
> > fail rather than simply being told up front not to bother trying.
> 
> If I understand you correctly, the state changes in the RAID5
> personalities caused by a bad drive declared dead would lead to
> remembering the bad drive persistently and they (the personalities
> RAID4/5/6) wouldn't even bother accessing it, so it won't make a
> difference to put an error mapped device in place of the dead one.
> 
> > With md/raid5, a read failure will be followed by an attempt to write out the
> > good data in case it was a simple media error.  Having it do that every time
> > you start a degraded array seems extra pointless....
> 
> Hrm, getting confused here: so it'll still try reading even though the
> MD metadata reflects it to be dead?
> 
> /me trying to get twists sorted :)
> 
> Regards,
> Heinz
> 
> > 
> > 
> > > 
> > > You'll get immediate and durable errors from the error target and hence
> > > you'd instantly drop the leg.
> > > 
> > > > 
> > > > > > 
> > > > > > > 
> > > > > > > I'm thinking about adding a ctr wrapper to allow us to keep the "raid45"
> > > > > > > ctr interfaces semantics (ie. the arguments) and the new interface to
> > > > > > > "raid456" if you don't have objections. That would be implemented by 2
> > > > > > > targets being registered implemented in one module.
> > > > > > 
> > > > > > I have no strong objections, though having two targets that do almost the
> > > > > > same things seems rather ugly.
> > > > > 
> > > > > Keep in mind it'll only be another target_type structs, another
> > > > > raid_ctr() function plus another (de)registration of the respective
> > > > > target. The ctr functions for raid45 and raid456 can share moset of the
> > > > > code anyway.
> > > > 
> > > > Sure.
> > > > 
> > > > > 
> > > > > > 
> > > > > > Is there existing published code that uses the extra arguments to raid45?
> > > > > 
> > > > > Yes, dmraid with a list of RAID5 supporting metadata format handlers.
> > > > 
> > > > I just had a look at the latest dmraid from CVS and the only place that I can
> > > > find where a raid45 target is created is in lib/activate/activate.c in
> > > > _dm_raid45_bol.
> > > > This only uses arguments that my code understands.
> > > > What am I missing?
> > > 
> > > I was arguing against breaking API compatibility.
> > > dm-raid45.c is being shipped by distros.
> > > 
> > 
> > Shipped by distros which all have an 'upstream first' policy, and are fully
> > aware of the possible consequences of not following it :-)
> > 
> > I'm not really that fussed about the API.  I don't like seeing APIs that I
> > think are badly designed go into Linux, but plenty of them do and one more
> > isn't going to make much difference.
> > 
> > Approving an API is really a job for the dm maintainer.  Is that Alasdair?
> > Having stated my case I'm happy to accept whatever he prefers.
> > 
> > NeilBrown
> > 
> > > Thanks,
> > > Heinz
> > > 
> > > > Thanks,
> > > > NeilBrown
> > > 
> > 
> 


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