On 02/24/2015 02:40 AM, Dan Lambright wrote:
----- Original Message -----
From: "Vijay Bellur" <vbellur@xxxxxxxxxx>
To: "Dan Lambright" <dlambrig@xxxxxxxxxx>, "Gluster Devel" <gluster-devel@xxxxxxxxxxx>
Sent: Monday, February 23, 2015 5:26:58 AM
Subject: Re: expanding tiered volumes
On 02/16/2015 08:20 PM, Dan Lambright wrote:
One of the hard parts has been integrating this new volume type with
existing features, such as adding and removing bricks.
Allowing the user to add and remove bricks to both the hot and cold tiers
is a challenge on multiple levels.
The CLI commands would have to specify which tier to add/remove bricks to.
Could you please detail the CLI being considered for defining a hot/cold
tier? I think that will be useful in understanding the problem better.
New phenomena could occur such as rebalancing operations happening
simultanously on both the hot and cold tiers.
In general rebalancing at the same time cache promotion/demotion is
happening is a tricky problem, it requires some tweaks to DHT which are
forthcoming.
To manage the complexity and get this feature done "on time" (per Gluster's
release schedule) we would like to only allow adding and removing bricks
to the cold
tier for the first version. I would like to see if there are any comments
from the upstream community.
We can have some flexibility if the core feature lands by end of this
week. Are there any thoughts on how either tier can be expanded or
shrunk? If it is not very intrusive and seems appropriate to be pulled
in as part of a bug fix, we can look at this beyond feature freeze and
before code freeze.
Attaching a tier is akin to adding bricks:
volume attach-tier <VOLNAME> [<replica> <COUNT>] <NEW-BRICK> ... [force]
Adding bricks to the "cold tier" would be the same CLI as with normal volumes.
volume add-brick <VOLNAME> [<stripe|replica> <COUNT>] <NEW-BRICK> ... [force]
Adding bricks to the "hot tier" could be specified using a modified CLI, e.g. add the keyword "attached-tier".
volume add-brick <VOLNAME> [<stripe|replica> <COUNT>] <NEW-BRICK> ... [force] [attached-tier]
Technically, the ability to expand both the hot and cold tiers is feasible given enough development time. An additional change to DHT is expected from the DHT group to allow rebalancing while promotion and demotion are happening. That fix is the first step.
Thanks for this, Dan!
I think we can add the ability to expand the hot tier after feature
freeze. If we can create cli stubs and call this out as a known issue by
feature freeze, we can add the capability to expand either tier soon
after feature freeze. Providing the ability to expand both tiers is
something that our users would like to see IMO.
-Vijay
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