Messages in this thread | | | From | Daniel Phillips <> | Subject | Re: [ANNOUNCE] Ramback: faster than a speeding bullet | Date | Sun, 16 Mar 2008 21:52:57 -0800 |
| |
On Sunday 16 March 2008 20:59, david@lang.hm wrote: > On Sun, 16 Mar 2008, Daniel Phillips wrote: > > On Sunday 16 March 2008 18:31, David Newall wrote: > >> Daniel Phillips wrote: > >>> The UPS provides a guarantee of commit to stable storage. No amount of > >>> FUD will change that. > >> > >> What about system crashes? They guarantee that data will be lost. I > > > > Not if it is mirrored and replicated. Also nice if crashes are very > > rare, which they are unless you work at it. > > if you are depending on replication over the network you have just limited > your throughput to your network speed and latency.
Replication does not work that way. On each replication cycle, the differences between the most recent two volume snapshots go over the network. This strategy has the nice effect of consolidating rewrites. There are also excellent delta compression opportunities.
In the worst case, with insufficient bandwidth for the churn rate of the volume, replication rate increases to the time for replicating the full volume. Again, at worst, this would require extra storage for the snapshot to be replicated equivalent to the original volume size, so that the primary volume is not forced to wait synchronously for a replication cycle to complete.
Mirroring on the other hand, makes a realtime copy of a volume, that is never out of date.
I hope this helps.
> on an enterprise level > machine the network can frequently be significantly slower than the disk > array that you are so frantic to avoid waiting for.
Frantic... your word. Designing for dependably high transaction rates requires a different mode of thinking that some traditionalists seem to be having some trouble with.
Daniel
| |