Re: Some very basic questions

[jim owens <jowens@hp.com>]

Avi Kivity wrote:
> jim owens wrote:
>>
>> Remember that the device bandwidth is the limiter so even
>> when each host has a dedicated path to the device (as in
>> dual port SAS or FC), that 2nd host cuts the throughput by
>> more than 1/2 with uncoordinated seeks and transfers.
> 
> That's only a problem if there is a single shared device.  Since btrfs 
> supports multiple devices, each host could own a device set and access 
> from other hosts would be through the owner.  You would need RDMA to get 
> reasonable performance and some kind of dual-porting to get high 
> availability.  Each host could control the allocation tree for its devices.

No.  Every device including a monster $$$ array has the problem.

As I said before, unless the application is partitioned
there is always data host2 needs from host1's disk and that
slows down host1.

If host2 seldom needs any host1 data, then you are describing
a configuration that can be done easily by each host having a
separate filesystem for the device it owns by default.  Each
host nfs mounts the other host's data and if host1 fails, host2
can direct mount host1-fs from the shared array.

Even with multiple disks under the same filesystem as separate
allocated storage there is still the problem of shared namespace
metadata that slows down both hosts.  If you don't need shared
namespaces then you absolutely don't want a cluster fs.

A cluster fs is useful, but the cost can be high so using
it for a single-host fs is not a good idea.

jim

jim