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



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...

> 
> 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.

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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