On Tue, May 12, 2015 at 11:16 PM, Robert LeBlanc <[email protected]> wrote:
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> What is the difference between straw and straw2 buckets? Should we consider
> "upgrading" to straw2 buckets by dumping the CRUSH map and updating them?
Well, straw bucket was supposed to be a general-purpose bucket type,
one where lookups are sufficiently fast and the amount of data that
needs to be moved between the nodes in the event of adding or removing
a node is minimal (data moved only from or to this node and not between
a bunch of other nodes). It was recently discovered that straw buckets
fail to deliver on the minimal data movement front due to a flaw in the
underlying algorithm.
The new straw2 algorithm essentially ensures that when you add or
remove a node we keep the original promise.
Whether or not to replace straw with straw2 is your call. I believe
there are tools in the works that will give you a good estimate of how
much data movement would straw -> straw2 cause, although I don't know
for sure. straw2 is default in hammer if you enable hammer tunables,
so in the long run it probably makes sense to migrate.
Thanks,
Ilya
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