[Gluster-devel] afr logic

[Kevan Benson]

Alexey Filin wrote:
> ops, my English...
>
> the question is: in your terms I have RAID 0+1 (RAID 10 is a trivial case),
> i.e. afr over stripe, if one brick is repaired, is its complement to be
> repaired too? I have some doubts about how stripe algorithm slices files, if
> it depends on parameters not equal for replicas (e.g. load average) then
> complement is to be copied too even if configuration of stripe for bouth
> replicas is the same. Even it is so I want to hear it explicitly from
> glusterfs team with promise don't change the policy!
>
> Regards, Alexey.

http://gluster.org/docs/index.php/GlusterFS_FAQ#Why_is_striping_bad.3F
http://gluster.org/docs/index.php/GlusterFS_Translators_v1.3#Stripe_Translator

Like the AFR translator, the stripe translator lets you specify what 
behavior applies to particular files, so you can specify stripe size per 
file type/name.

I haven't done anything with the striping, and according to the FAQ it's 
not recommended.  Since I haven't done anything with it myself, I can't 
say whether their concerns in the overhead are are dwarfed by the 
bandwidth limitations of the medium when using Ethernet.

As for the repairing question, if you stripe AFR's any brick that had a 
problem would have a problem with a *portion* (stripe) of a file, but 
that portion is AFR'd somewhere else and will be repaired (I assume).  
If you AFR stripes, that means that if there's a problem in a brick, 
it's a portion (stripe) of a file, and there's no copy to repair from.  
There's a copy of the full file, but that doesn't mean the AFR knows one 
of the AFR's files has a problem,it's trusted.afr.version attribure is 
probably the same as the good file.  The good file is there, but it may 
not be what's accessed be default, as the AFr probably has no method of 
determining it's not a valid copy if the trusted.afr.version attributes 
match.

You know, I find myself doing quite a lot of assuming today.  Hopefully 
someone on the gluster team will review all this and let me know where 
I'm correct, clarify where I'm ambiguous, and most importantly, let me 
know when I'm talking out my ass.

-- 

-Kevan Benson
-A-1 Networks