questions regarding few corner cases of mdadm usage

Hi

I've been doing some tests, mostly for "what would happen if" scenarios
of incremental / foreign metadata assembly - as I avoid those if
possible, still a few things I noticed:

1)

With native metadata, 'mdadm -IRs' can be used to force-run partially
assembled arrays. With external metadata (tested with ddf) though, the
command has no effect. The subarray can be forced into degraded / active
mode through regular 'mdadm -R' - though it also requires manual start
of mdmon (otherwise further operations might end in D state, until the
one is started). For example:

mdadm -C /dev/md/ddf0 -e ddf -n4 /dev/sd[b-e]
mdadm -C /dev/md/test -l5 -n4 /dev/md/ddf0
mdadm -S /dev/md/test /dev/md/ddf0

mdadm -I /dev/sdb
mdadm -I /dev/sdc
mdadm -I /dev/sdd
mdadm -R /dev/md/test

At this point, if the remaining component is added, e.g.
mdadm -I /dev/sde

Then mdmon will have to be started, or any process trying to write will
hang (though mdmon can be started at any moment).

So in short:

- shouldn't -IRs also consider foreign metadata subarrays ?
- shouldn't mdmon be started automatically for run-forced subarrays ?

2) mixing 'mdadm -I' and 'mdadm -As'

If part of an array (possibly runnable) is assembled through 'mdadm -I',
and then 'mdadm -As' is called, it will cause creation of a duplicate
array from the remaining disks. This was true for both native and
external metadata formats. For example:

mdadm -C /dev/md/ddf0 -e ddf -n4 /dev/sd[b-e]
mdadm -C /dev/md/test -l1 -n4 /dev/md/ddf0
mdadm -S /dev/md/test /dev/md/ddf0

mdadm -I /dev/sdb
mdadm: container /dev/md/ddf0 now has 1 devices
mdadm: /dev/md/test assembled with 1 devices but not started

mdadm -I /dev/sdc
mdadm: container /dev/md/ddf0 now has 2 devices
mdadm: /dev/md/test assembled with 1 devices but not started

mdadm -As
mdadm: Container /dev/md/ddf1 has been assembled with 2 drives (out of 4)
mdadm: /dev/md/test_0 assembled with 2 devices but not started

At this point, there're 2 containers + 2 subarrays created, both of
which can be started with 'mdadm -R' to operate independently:

md124 : active raid1 sdd[1] sde[0]
13312 blocks super external:/md125/0 [4/2] [__UU]

md125 : inactive sdd[1](S) sde[0](S)
65536 blocks super external:ddf

md126 : active raid1 sdc[1] sdb[0]
13312 blocks super external:/md127/0 [4/2] [UU__]

md127 : inactive sdc[1](S) sdb[0](S)
65536 blocks super external:ddf


I realize that mixing normal and incremental assembly is at least asking
for problems, though I don't know if the above results fall into "bug"
or "don't do really weird things" scenario.
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Re: questions regarding few corner cases of mdadm usage

On Wed, 10 Aug 2011 00:12:04 +0200 Michal Soltys wrote:

> Hi
>
> I've been doing some tests, mostly for "what would happen if" scenarios
> of incremental / foreign metadata assembly - as I avoid those if
> possible, still a few things I noticed:

Great!! Tests are good - especially testing things that I wouldn't think
of :-)

>
> 1)
>
> With native metadata, 'mdadm -IRs' can be used to force-run partially
> assembled arrays. With external metadata (tested with ddf) though, the
> command has no effect. The subarray can be forced into degraded / active
> mode through regular 'mdadm -R' - though it also requires manual start
> of mdmon (otherwise further operations might end in D state, until the
> one is started). For example:
>
> mdadm -C /dev/md/ddf0 -e ddf -n4 /dev/sd[b-e]
> mdadm -C /dev/md/test -l5 -n4 /dev/md/ddf0
> mdadm -S /dev/md/test /dev/md/ddf0
>
> mdadm -I /dev/sdb
> mdadm -I /dev/sdc
> mdadm -I /dev/sdd
> mdadm -R /dev/md/test
>
> At this point, if the remaining component is added, e.g.
> mdadm -I /dev/sde
>
> Then mdmon will have to be started, or any process trying to write will
> hang (though mdmon can be started at any moment).
>
> So in short:
>
> - shouldn't -IRs also consider foreign metadata subarrays ?

It should. A quick look at the code suggests that it does. So there is
clearly a bug somewhere. I've added it to my list of things to fix.

> - shouldn't mdmon be started automatically for run-forced subarrays ?

Definitely. That code is simply missing. I've added that to my list too.


>
> 2) mixing 'mdadm -I' and 'mdadm -As'
>
> If part of an array (possibly runnable) is assembled through 'mdadm -I',
> and then 'mdadm -As' is called, it will cause creation of a duplicate
> array from the remaining disks. This was true for both native and
> external metadata formats. For example:
>
> mdadm -C /dev/md/ddf0 -e ddf -n4 /dev/sd[b-e]
> mdadm -C /dev/md/test -l1 -n4 /dev/md/ddf0
> mdadm -S /dev/md/test /dev/md/ddf0
>
> mdadm -I /dev/sdb
> mdadm: container /dev/md/ddf0 now has 1 devices
> mdadm: /dev/md/test assembled with 1 devices but not started
>
> mdadm -I /dev/sdc
> mdadm: container /dev/md/ddf0 now has 2 devices
> mdadm: /dev/md/test assembled with 1 devices but not started
>
> mdadm -As
> mdadm: Container /dev/md/ddf1 has been assembled with 2 drives (out of 4)
> mdadm: /dev/md/test_0 assembled with 2 devices but not started
>
> At this point, there're 2 containers + 2 subarrays created, both of
> which can be started with 'mdadm -R' to operate independently:
>
> md124 : active raid1 sdd[1] sde[0]
> 13312 blocks super external:/md125/0 [4/2] [__UU]
>
> md125 : inactive sdd[1](S) sde[0](S)
> 65536 blocks super external:ddf
>
> md126 : active raid1 sdc[1] sdb[0]
> 13312 blocks super external:/md127/0 [4/2] [UU__]
>
> md127 : inactive sdc[1](S) sdb[0](S)
> 65536 blocks super external:ddf
>
>
> I realize that mixing normal and incremental assembly is at least asking
> for problems, though I don't know if the above results fall into "bug"
> or "don't do really weird things" scenario.

This sounds very familiar. I thought I had fixed something similar to that
recently...

If you have two halves of a RAID1 that both look OK but the event counts are
too different, then the first "mdadm -As" would build an md/raid1 with just
one of them, then another "mdadm -As" would build an md/raid1 with just the
other - clearly not what you want, but very similar to what you have.

But I cannot find any evidence that I fixed it.

So I've added this to my list too.

I don't know when I'll get to working on this list, but hopefully within a
month or 2 :-)

Thanks a lot for the report (and sorry for the delay in replying).

NeilBrown


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