[Gluster-devel] expanding tiered volumes

[Dan Lambright]

Hello,

An initial version of storage tiering is being developing for Gluster 3.7. It will allow you to add a set of fast bricks to a volume (such as SSDs) which can be used as a cache for slower bricks (HDD). It resembles a feature developed for Ceph last year. 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. 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.

Dan