[Cluster-devel] fence_scsi - Configuration file

[Jan Friesse <jfriesse@redhat.com>]

Ryan O'Hara napsal(a):
> On Thu, Jul 30, 2009 at 04:08:44PM +0200, Jan Friesse wrote:
>> I was working on fence_scsi configuration file and will be very happy
>> for opinions of larger audience for syntax/semantic.
>>
>> Syntax:
>> Basically it's little improved corosync.conf so example:
>>
>> # Comment line
>>
>> # ^^^ white lines ignored
>>
>> # Definition of device
>> /dev/sda
>>
>> # Can be more
>> /dev/sdb
>> /dev/sdc
>>
>> # Configuration section -> everything what doesn't start with /dev/
>> global {
>>   key1: value
>>   key2: value
>> }
>>
>> # Device definition + per device configuration
>> /dev/sdd {
>>   key1: value
>> }
>>
>> ### CUT HERE ####
>>
>> This syntax should give us very flexible background for future
>> additions. On the other hand, it allows user to overwrite vgs
>> auto_detection by simple list of device.
> 
> Right. I'd also like to point out that this config file will need to
> be manually replicated to all node in the cluster if a user wants to
> explicitly list which devices to use.
> 
>> Now something about semantic (what is more important).
>>
>> What I think is good to have is support for hierarchy of options
>> processed in this way:
>> 1) Hardcoded defaults in fence_scsi - Used in case nobody will say
>> something different
>> 2) global device options - Used for all devices listed (and maybe auto
>> detected, will be discussed later)
>> 3) Per device configuration options
>>
>> So I will try to give example for APTPL flag. This is typical flag,
>> where we want to be by default enabled or disabled (really don't care
>> and should be point of discussion) and we want give user ability to
>> change it and maybe change it for some devices differently. So for
>> example, user has:
>> - First array - supports APTPL
>> /dev/sda
>> /dev/sdb
>> - Second array - Doesn't support APTPL
>> /dev/sdc
>>
>> and default hardcoded in fence_scsi is to NOT use APTPL:
>>
>> In my point of view, user can use:
>>
>> ### Cut HERE
>> global {
>>   aptpl: on
>> }
>>
>> /dev/sda
>> /dev/sdb
>> /dev/sdc {
>>   aptpl: off
>> }
>> ### CUT HERE
>>
>> OR
>>
>> ### Cut HERE
>> /dev/sda {
>>   aptpl: on
>> }
>> /dev/sdb {
>>   aptpl: on
>> }
>> # CUT HERE
> 
> I'm ok with this, but I'm not sure that having 2 arrays, one that
> supports APTPL and one that does not, would be useful. Only in the
> case where the array that did support it lost power ... but the one
> that did not support it remained on ... would you see it being
> useful. Still, if people was to do this I suppose it is ok.
>  

It really doesn't make much sense. But I'm really not sure what 
customers will want and this idea can be one of them.

>> Next, in my design is global variable auto_detect. In case, auto_detect
>> is on, we are using OLD method (vgs scanning) and use per device
>> options. So another example. Now user has only one array, supporting
>> APTPL and default is to NOT use APTPL. User DON'T want to enter devices,
>> so can use:
>> ### CUT HERE
>> global {
>>   auto_detect: on
>>   aptpl: on
>> }
>> ### CUT HERE
> 
> What happens if we list devices and have auto_detect set to 'on'? Will
> we ignore the devices? With this auto_detect parameter, it seems that
> it will have to be explicitly set to 'off' and devices will have to be
> listed if we want to avoid auto-detect. Also, what happens if I set
> auto_detect to 'off' and I don't list any devices?
> 

autodetect set to "on" can have one of these two consequences:
- Ignore listed device - read only global sections for parameters
- Ignore listed device but use per device parameters in case we will 
find this device by old method - This is what is implemented now
- Use merge of listed devices and autodetected device

autodetect is by default off. When we not have config file, it will 
become on. When no devices is listed -> we turn it on

I hope nobody will ever set autodetect to off and don't list any device, 
but we can do two things:
- Ignore this flag and use vgs (this is what code does now)
- Do what user want -> no device fencing


> Ryan

(I'm moving discussion to more "visible" place ;) )

Regards,
   Honza