Fwd: [Gluster-devel] proposals to afr

[Kevan Benson]

Alexey Filin wrote:
> On 10/23/07, Kevan Benson <kbenson at a-1networks.com> wrote:
>   
>> Actually, I just thought of a major problem with this.  I think the
>> extended attributes need to be set as atomic operations.  Imagine the
>> case where two processes are writing the file at the same time, the op
>> counters could get very messed up.
>>     
>
>
> atomic operations is an ideal which is not possible on practice sometimes,
> ideal hardware exists in mind only, developers choose a compromise between
> complexity, performance, reliability, flexibility etc on existing hardware
> always.
>
> to provide operation-counter(or version if it is updated after each
> operation) consistency the concurrent access to the same file is to be done:
>
> * with one thread (to allow concurrent operations with _one_ file to be
> serviced by _one_ thread only) which can provide atomicity with explicit
> queuing
> * or with sync primitive(s) for many threads.
>
> io threads help to decrease latencies when many clients use the same brick
> (as e.g. a glfs doc says) or to overlap network/disk io to increase
> performance per a client (is it implemented in glfs?)
>   

It depends on the context.  Really, an atomic operation just means that 
nothing else can interrupt the action while it's doing this one thing.  
In the case of glusterfs, it could easily be achieved with a short 
thread lock if it isn't already (I suspect it probably is).  I'm 
referring to atomic in the sense that there's only one extended 
attribute, not multiple across threads.  If two separate threads 
(serving two separate requests) are acting on the same file, the op 
counter as you defined it (an extended attribute) could itself become 
inconsistent between different AFR subvolumes, depending on the order 
the write requests are processed.

Imagine the following order of operations:
On subvolume A
First op: request(1).write("this is a request from program a") && 
opCounterIncrement
Second op: request(2).write("this is a request from program b") && 
opCounterIncrement
crash

On subvolume B
First op: request(2).write("this is a request from program b") && 
opCounterIncrement
Second op: request(1).write("this is a request from program a") && 
opCounterIncrement
crash

At this point the opcounters would be the same, the trusted_afr_version 
the same, the data different, and no self-heal would be triggered.

Now, I'm not familiar enough with the internals of GlusterFS to tell you 
whether what I outlines above is even possible, but it is a race 
condition I can see causing problems unless files are implicitly locked 
by AFR writes.  I'm not sure.

> Another solution comes to mind.  Just set another extended attribute
>   
>> denoting that the file is being written to currently (and unset it
>> afterwards).  If the AFR subvolume notices that the file islisted as
>> being written to but no clients have it open (I hope this is easily
>> determinable) a flag is returned for the file.  If all subvolumes return
>> this flag for the file in the AFR (and all the trusted_afr_versions are
>> the same), choose one version of the file (for example from the first
>> AFR subvolume) as the legit copy and copy it to the other AFR nodes.  It
>> doesn't matter which version is the most up to date, they will all be
>> fairly close, and since this is from a failed write operation there was
>> no guarantee the file was in a valid state after the write.  it's
>> doesn't matter which copy you get, as long as it's consistent across AFR
>> members.
>>     
>
>
> I like it more op counter, advantage to op counter is that the flag is set
> only two times (open()/close()) so an overhead is minimal (concurrent access
> to the flag is to be synchronized), the disadvantage is if not closed file
> is enough big it has to be copied sometimes when it is not required, it is
> acceptable if afr crashes rare
>   

Wait, I assumed by operation you meant every specific write to the file, 
so this opcounter could be incremented quite a bit, but you just stated 
it would only be set once as a flag, so maybe I'm misunderstanding you.  
If it's incremented per actual file operation, quite a lot of increments 
might happen.  For example, using wget to save a remote file to disk 
doesn't write everything at once, it does many writes as it's buffer 
fills with enough information to be worth writing to disk.

My thought above was a simple flag as to whether or not the file was 
bing written just to denote whether it should be considered in a 
consistent state if a crash happens.

This whole conversation's gotten into somewhat esoteric territory that 
requires more input from the GlusterFS team on whether it's even worth 
considering doing stuff this way.  Maybe they have a better solution in 
the works?  Any team members care to comment on their thoughts on this?

-- 

-Kevan Benson
-A-1 Networks