Re: Several questions about enbd

Michael Ulitskiy <[email protected]> wrote:
> I won't comment on failover script transition to mdadm now, as I'm not
> familiar enough with mdadm itself. Nevertheless it's on the list and I'll give it a try

It's a bit more handy.  Using raid* tools means you are playing with the
contents of a raidtab, and relying on the tols interpretation of it.
Mdadm is a direct hands-on experience in comparision!

> as time allows (and sure I'll share the improvements if I come up with any :) ).
> Right now I have just one question left. You said fr1 patch will read from the fastest
> device. How the fastest device is determined? Is device speed constantly monitored

Every 1000 accesses it round-robin reads the other drives for 10
accesses too. It maintains a rolling average of the latencies
experienced in each access. After the short round-robin phase is up it
shoses the drive with the lwest recorded latency for the next 1000
accesses.


> over the course of operation or it takes which device replied first during raid startup

It's retested every 1000 acceses. See above.

> and then go with it? 'Cause in latter case I can imagine it could well choose a network
> device if it happens to reply first (think of busy systems with raid startup on the fly).

Well, I don't know if what it does is the best idea. It's just what I
thought of for it!

About knowing which end was server last after both nodes come up after
a crash ... I'm pretty convinced from thinking about it that it is
heartbeat's business. Can you ask the heartbeat people about it? All
the scripts get told is "start as server" or "start as client", etc.
The decision about their role has already been made by heartbeat.



Peter