Re: Some very basic questions

[jim owens <jowens@hp.com>]

calin wrote:
>> question is: if you had such an implementation, are there
>> drawbacks expectable for the single-mount case? If not I'd vote for it
>> because there are not really many alternatives "on the market".
> 
> As I understand it, the largest issue is in locking and boundaries. 

Correct, that is the first big issue.  As soon as 2 machines can
access the same device, you must design for distributed locking.
And that means a lot more code, lower performance, and a lot of
things a local-only filesystem could do that must be disallowed.

The second issue is what is the purpose of more than 1 host
accessing the data directly from the device.  There are cases
where this is a good thing because the application is designed
with data partitioning and multi-instance coordination.  It is
a bad thing for random uncoordinated use like backups or fsck.

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.

And if the host device drivers are not designed for multiple
host sharing, this can cause timeouts, resets, and false
device-failed states.

And yes... even read-only access from a 2nd host is trouble
in many parts of the design and does not come for free.

jim