On Mon, Mar 24, 2014 at 10:08 AM, Jeff Darcy <jda...@redhat.com> wrote: > I was talking to a user about my size-weighted (or optionally > free-space-weighted) rebalance script. This led to thinking about ways to > bring a system back into balance without migrating any old data, as some of > our users already do. Here's the example I was using. > > * Four existing 1TB bricks, which are 90% full. > > * One new 2TB brick, which is empty. > > Therefore, total free space is 2.4TB, of which the new brick has 2.0TB. If > we set up the layouts so that the new brick has 5/6 of the hash space then > as new files are added they should all reach 100% full at the same time > without ever needing to migrate any old data. Yay. > > Unfortunately, there's still a problem. For these kinds of users (e.g. > CDNs) the newest data also tends to remain hottest. What happens when they > want to retire some of their oldest hardware? That *does* involve migrating > old data, and the load for that will disproportionately fall on the newest > servers which really should be spending as much of their time as possible > serving new content. That's not good. > > So (finally) here's the idea. Have a *separate* set of layout values that > are used specifically for rebalance, so that we can rebalance data one way > even as new files are placed another way. Let's consider a slightly > different example.
I think this is a proper clever idea. (Assuming it would work.) One question: would(n't) there be a chance for "thrashing" (maybe there's a better word) where new files are getting put on brick X, but the rebalance is then trying to move them to brick Y? (Well maybe call it a single thrash, not thrashing.) As a side note, I don't see this as a high priority feature that I'm interested in. > > * 4 ancient 1TB bricks, 75% full > > * 16 medium-age 1.5TB bricks, also 75% full > > * 4 new 2TB bricks, empty > > Here's one possible way to use dual layouts: > > * currently 8TB free on the medium bricks, goal is 5TB > > * 4TB free on the new bricks > > * set regular layout to 44% new, 55% medium > > * set rebalance layout to 100% medium > > This way 44% of the new files but *none* of the files from the oldest bricks > will flow toward the newest bricks. 100% of that traffic will be from the > oldest bricks to the medium ones, and shouldn't affect the newest machines > at all. This would all be a lot easier if we had layout inheritance or > default layouts instead of every single directory with its own layout, but > we can probably find ways to deal with that. > > Any reactions? > > > > > _______________________________________________ > Gluster-devel mailing list > Gluster-devel@nongnu.org > https://lists.nongnu.org/mailman/listinfo/gluster-devel _______________________________________________ Gluster-devel mailing list Gluster-devel@nongnu.org https://lists.nongnu.org/mailman/listinfo/gluster-devel
