[PATCH 2 of 2] [RFC] handle a temporary secondary mirror device failure

[Takahiro Yasui <tyasui@redhat.com>]

On 12/02/09 21:36, malahal at us.ibm.com wrote:
> Takahiro Yasui [tyasui at redhat.com] wrote:
>> On 11/30/09 05:22, Malahal Naineni wrote:
>>> If "retry" policy is selected, then the mirror DSO uses "dmsetup"
>>> command to restart a resync. Right now, it is done immediately but we
>>> should do it after a configured time. Also, this is done endlessly but
>>> should be done only few configured times.
>>
>> It sounds good to me. This feature would work in a variety of
>> environments by adding configurable parameter as you mentioned.
>>
>>> Calling 'lvchange --refresh' to re-integrate the secondary mirror upon a
>>> leg failure works if there are no device failures while the "lvchange"
>>> is running.  Otherwise, the failed device is replaced with "error"
>>> target which is not what we want. For now, this patch calls "dmsetup",
>>> but should be replaced with a suitable "LVM" command.
>>
>>> +	snprintf(cmd_str, CMD_SIZE, "dmsetup suspend %s-%s", vg, lv);
>>> +	syslog(LOG_ERR, "Running command: %s", cmd_str);
>>
>> We need to re-queue blocked I/Os instead of finishing them by -EIO.
>> I think that "--noflush" option is necessary for "dmsetup suspend."
> 
> Yes, you are right. I tested this with existing code dm-raid1 module
> which doesn't hold any I/O's, so it worked fine. But I think we should
> use some LVM command (or create one if such a command doesn't exist)
> instead of "dmsetup" command.

lvm commands have an unwelcome behavior to scan all devices except for
ones filtered out by lvm configuration as I described before.

https://www.redhat.com/archives/lvm-devel/2009-April/msg00014.html

If "refresh" is just an action in the device-mapper layer, I rather hope
that it will be implemented without lvm commands as you did. It would
avoid extra time by issuing unnecessary I/Os and achieve quick recovery.

It is best that this device scanning issue of lvm commands is solved. :)

Thanks,
Taka