[Gluster-devel] Server-side AFR + Failover and mixed server/client-side AFR

[gordan at bobich.net]

On Tue, 6 May 2008, Krishna Srinivas wrote:

>> >>> If server-side AFR breaks with fail-over, is it possible to use
>> >>> server-side AFR on some nodes and client-side AFR on others, with 
>> >>> the client side AFR nodes talking directly to the same storage 
>> >>> bricks that the server side AFR is unifying for server-side AFR 
>> >>> nodes?
>> >>>
>> >>>  Or would this break things?
>> >>
>> >> It will not break things as long as the afr's subvol listing order are
>> >> same.
>> >
>> > Is this an issue in server-side-only AFR? I have two servers which are also
>> > clients of themselves, and they both list their local subvolume first and
>> > remote subvolume second. Is this a problem? What are the possible
>> > consequences of this?
>>
>>  It will be a problem. The "first" subvol is always the "lock" server.
>>  Consider a case where you are creating a file simultaneously
>>  on two clients, only one of them should succeed. If AFR's
>>  subvols order are not same, chances are that both client
>>  returns success for file creation with same name.
>
> Hence you have "option read-subvlume" to speeden the
> read() calls so that it can be done from local subvol.

The volume order importance may be worth stressing in as many words as 
you just explained in the howto example docs.

Thanks for the clarification, very much appreciated.

Gordan