[tahoe-dev] Tahoe-LAFS and Ceph: Some remarks

[Alex Elsayed]

Alex Elsayed wrote:

> First message: CRUSH and Tahoe-LAFS
> ---cut---
> Since the crushmap is user specified, and the placement is then generated
> based on it, it lets users describe their topology and policies and then
> just lays the data out accordingly.
> ---cut---
> 
> Second message: Overall design similarities between Ceph and Tahoe-LAFS
> In reply to
> https://plus.google.com/108313527900507320366/posts/ZrgdgLhV3NG May ramble
> a bit.
> ---cut---
> It's surprisingly close to Tahoe, as a matter of fact - placement of
> objects can be computed on any node via a function, so the client can know
> where stuff is going without talking to some sort of central server or DHT
> (they have an optimization that the client puts it on one OSD and lets it
> do the distribution, but that's an implementation choice and not a core
> design element).
> ---cut---

I just realized I ought to clarify that in Ceph, the CRUSH layout parameters 
are per-cluster, rather than per client. Some other system using CRUSH could 
do otherwise, but part of why Ceph did it the way they did was so the 
cluster could automatically rereplicate data after a node failed without the 
intervention of the storing client.