[dm-devel] [PATCH 7/7] Hold all write bios when errors are handled

[malahal at us.ibm.com]

Takahiro Yasui [tyasui at redhat.com] wrote:
> > The requirements are:
> > * if one of legs fail or log fails, you must automatically continue 
> > without human intervention
> > * if both legs fail, you must shut it down and not pretend that something 
> > was written when it wasn't (this would break durability requirement of 
> > transactions).
> 
> I agree with this point. lvm mirror could be used on filesystems such as
> ext3 and each filesystem and application needs to take care those situation
> to prevent data corruption. I don't think that it is realistic, and the
> underlying layer should prevent data corruption. I now understand primary
> and secondary disks need to be blocked.

If you think that I/O needs to be blocked for first or second leg
failures, then I am afraid that there is no way to keep the failed
mirror in the system for re-integrating failed devices.

I would like to keep the mirror as mirror when one of the legs fails as
opposed to making it linear now by dmeventd. This is to re-integrate a
failed leg into the mirror to handle transient device failures. Here is
what I am planning to do, let me know if you find any issues:

1. Secondary leg failure. dmeventd will NOT change the meta data at all.
   It will call "lvchange --refresh" that will start resynchronization.
   This will be done a few times. If the device still fails to
   re-integrate, it is left the way it is and no further re-integrations
   attempts are done. The number of attempts can be done at configurable
   intervals.

2. First leg failure (aka Primary leg failure): The kernel would stop
   handling any further I/O. The dmeventd will change mirror meta data
   [it will re-order the legs ] so that the secondary now becomes the
   primary. I will try to re-integrate the failed device few times before
   giving up just as in the case "1" above.

The kernel module will never progress with a first leg failure as you
see above.

Thanks, Malahal.