Re: [Gluster-devel] Feature requests of glusterfs

[Kevan Benson]

LI Daobing wrote:
> 4. a new AFR model:
>
> Currently, if the AFR have 3 child xlators, and each xlator connect to
> a distinct machine. Then the write speed of this AFR is only 33% of the
> capacity of the network.
>
> Consider a different model, the AFR send data to machine1, machine1 send
> the data to machine2 immediately and then write the data to
> disk. Machine2 also send data to machine3 immediately and then write
> data to disk. Under this model, we can increase the write speed to 3
> times of the previous model (if your switch is good enough and your
> network support full duplex).

This can be achieved through multiple AFR definitions, different for 
each server, that chain the data.  I've discussed it once or twice on 
the list before, but never implemented (time doesn't permit me to follow 
up on lots of ideas).  No need for a new translator, which would be 
complicated (see below).

> In more detail, we need two new kinds of xlators. The first one is the
> combination of the AFR and client-protocol (called *safr*). The second
> one is similar with the `server-protocol'(called *sserver*).
>
> The machine1, machine2, machine3 is set in the option of safr. And safr
> maintain an active-machine list. When safr receive a writev command(or
> other commands), it pick a machine from the active-machine list(for
> exmaple, machine1). then send the data and a list "[machine2, machine3]"
> to machine1. machine1 forward data and list "[machine3]" to machine2
> immediately, machine2 also forward data and an empty list to
> machine3. Machine1, machine2, machine3 also write the data to the disk
> when sending data.
>
> If any machine is down, the afr just remove it from the active-machine
> list. And add it when the machine is up again.
>
> This model is a little far from the current framework, but I think it's
> a good idea to write at 100 MB/s instead of 30+ MB/s in a gigabyte
> network.

This all assumes a lot about the target network, which is a bad thing 
when trying to be as flexible as GlusterFS.  For example, machine1 and 
machine2 may not have an optimized path to each other.  They may be in 
different subnets, off different switches, or in different geographical 
locations, etc.

The only thing your translators provide that isn't already available 
through chained translators is automatic reconfiguration of the chain 
members when a server drops out, which is a good feature, but I would 
rather just add cheap redundant hardware to boost speed, such as extra 
gigabit NICs and switches to allow dedicated paths between select 
systems.  Also, maybe the new switch translator can be added to what's 
already available to achieve what you want, I'm still fuzzy on exactly 
what it can be used for.

> This model is similar with the model in google file system, you can
> check the figure 2 in a paper of google file system[1]. I put a copy of
> this figure at [2].
>
> [1] http://labs.google.com/papers/gfs-sosp2003.pdf
> [2] http://picasaweb.google.com/lidaobing/Public/photo#5150803669289886370

Google has a advantage of designing for a specific usage, which makes 
some design choices better for them than for other projects.

> PS, should I copy this feature request to wiki? Or it's ok to only put
> it here?

OK, now that I've done my best to tear down your proposal and say why 
it's not needed, here's where I put my disclaimer:

1) I'm not a dev, and I haven't really looked into the code, so I don't 
know how easy or hard your proposal is to actually implement.
2) I'm just one person, and even though *I* may think it's not needed, 
others may differ on this point.

--

-Kevan Benson
-A-1 Networks