On 2019-09-02 08:43, Wido den Hollander wrote:
On 9/1/19 9:51 PM, Zoltan Arnold Nagy wrote:
On 2019-09-01 05:57, Konstantin Shalygin wrote:
On 8/31/19 4:14 PM, Zoltan Arnold Nagy wrote:
Could you elaborate a bit more? upmap is used to map specific PGs to
specific OSDs
in order to deal with CRUSH inefficiencies.
Why would I want to add a layer of indirection when the goal is to
remove the bucket
entirely?
As I understood you want to make huge CRUSH map changes without huge
data movement.
Upmap can help with this, you map your current PG's to OSD's that
already holds this PG's.
Let's say with upmap I take the current mapping and "override" CRUSH,
then remove the
rack bucket and move the host directly to the root. And then what?
We'd
never, ever
be able to go back, and what's worse, for any new expansions we'd need
to manage the
PG mappings manually.
Unless I'm missing something, while this would solve the problem in
the
very very short
term, it would create horrible issues down the line, and would be
shooting ourselves
in the foot as far as maintainability is concerned.
As I've said we'd been rolling this cluster since at least Firefly,
and
I don't want to
mess with it.
The solution I've outlined in my original mail works (swapping the
bucket IDs) and seems
more maintainable, however, I've been wondering if the bucket types
are
just labels or do
they have any other semantic meaning?
They are just labels. Just like the names of buckets.
Everything is mapped to the IDs you'll find in the CRUSHMap. Negative
IDs for buckets and positive IDs for devices/OSDs.
upmap should be avoided in this case for exactly the reasons you
already
mention: You create some kind of database which is not the solution
here. The only thing you can do with upmap is make this migration
smoother by removing upmap items in batches afterwards.
I wonder if I can swap the types just by editing the crush map, eg
change
a rack into a host without any issues. I'll give this a try :)
If so, then I could just swap the IDs, move the current host into the
place
of the bucket and then delete the host without causing any CRUSH mapping
changes,
eg. no data movement.
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