[dm-devel] Possible bug in mirror target

[Zdenek Kabelac]

Dne 10. 02. 19 v 22:58 John Stoffel napsal(a):
>>>>>> "Zdenek" == Zdenek Kabelac <zkabelac at redhat.com> writes:
> 
> Zdenek> Dne 05. 02. 19 v 1:47 Drew Hastings napsal(a):
>>> Hi,
>>>
>>> I'm assuming all user space code is expected to use the handle_errors feature,
>>> so this isn't that big of a deal. I'm also using 4.19.13, which I think is
>>> more recent than the latest update to dm-raid1.c
>>>
>>> That said, there may be a bug that causes the entire mirror to crash if there
>>> is an error on the first leg of the mirror, but not on the 2nd leg.
>>>
>>> Works fine if you do cause IO errors on the 2nd leg:
>>>
>>> root at kernel-dev:~# dmsetup create zerodev --table "0 1073741824 zero"
>>> root at kernel-dev:~# dmsetup create errordev --table "0 1073741824 error"
>>>
>>>
>>> root at kernel-dev:~# dmsetup create mirror-table --table "0 1073741824 mirror
>>> core 2 524288 nosync 2 /dev/mapper/zerodev 0 /dev/mapper/errordev 0 0"
> 
> Zdenek> There are 2 operational modes for old dm mirror target.
> Zdenek> One requires to handle errors externally. Please check i.e. the following
> Zdenek> guide for mirror target:
> 
> Zdenek> https://wiki.gentoo.org/wiki/Device-mapper
> 
> Zdenek> Lvm2 is using 'dmeventd' to service this error handling
> Zdenek> (i.e. dropping the mirror leg or allocating replacement one).
> 
> Is it time to remove the dm-mirror target then?  Or to deprecate it?
> Or to just silently replace it with something that does the right
> thing when errors happen?  I can't think of why *anyone* would want to
> use the dm-mirror target as it now seems to work.


Hi

The old dm mirror works differently then new dm raid - so if you have
an old one present/running (i.e. LVM mirrored volume) using this target - you 
need to have this target present to be able to access such volume.

You can (in lvm2) convert such mirror to use newer dm raid target - but
this requires some extra space (even though very small metadata volume) which 
is now required per each mirrored leg.

Both targets are still maintained and bugfixed.

There is also something about very high complexity of mdraid code and
relative simplicity of old mirror target.

As for comment about *anyone* - majority of users are consuming DM targets via 
lvm2 - there you get this transition automatically - when you ask to create 
mirrored  LV - lvm2 will use (if present in kernel)  newer md  raid variant. 
If anyone is using DM directly with 'dmsetup' command - it's assumed these are 
skilled users familiar with kernel doc (dmsetup is low-level admins tool). 
Lvm2 should be seen as 'friendly-face' of these DM targets - which otherwise 
do require pretty complex setup. If you do not want to use lvm2, the tool 
replacing lvm2 needs to reproduce this extra logic.

Regards

Zdenek