[Gluster-devel] Gluster tiering feature

Lalatendu Mohanty lmohanty at redhat.com
Mon Oct 20 15:28:28 UTC 2014

On 10/18/2014 01:11 AM, Dan Lambright wrote:
> Myself, Joseph Fernandez, and others have been working on a tiering feature for gluster. We are in the prototype phase and demoed some code recently internally to Red Hat. In its first incarnation it resembles the server side cache tier Ceph has, or dm-cache in the kernel. So, fast storage and slow storage are exposed as a single volume, data that is frequently used makes its way to fast storage, and the system responds dynamically to changing usage. Because migration between tiers is time consuming, a cache tier is a good fit for workloads where the set of hot data is stable. A cache tier can be added or removed at run-time. The tiering logic is very general-purpose infrastructure, and can be used for elaborate data placement graphs or other data migration features.
> The design is in early stages.. but current thinking can be found on the feature page and the links below, and all the code is on the forge.
> http://www.gluster.org/community/documentation/index.php/Features/data-classification
> goo.gl/bkU5qv
> Thanks,
> Dan
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Looks awesome!! Thanks for putting these docs.

However I have a few questions on the following.
/"Current thinking is a snapshot cannot be made of a volume with a cache 
tier, however a cache may be "detached" from a volume and then a 
snapshot could be made//"./

As far as I can understand, detaching cache tier will take a good amount 
of time (of course it depends on the amount of data), so detaching the 
cache tier and then taking snapshot will be time consuming and might not 
be used much (from a user point of view) . However I am wondering if  
pausing cache is less expensive then detaching cache? Also does pause 
cache migrates all data from hot to cold subvol, or just from that time 

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