Re: endb 101

Peter T. Breuer schrieb:

>>machine A: md0 [hda3+nda0]
>>                       |
>>                       |__________________________
>>                                                  |
>>machine B: md0 [hda3+nda0], enbd-server exports hda3
> 
> 
> I don't understand this diagram very well, because of the "machine B:
> md0 [hda3+nda0]" part. What is that doing there? machine B is exporting
> hda3, not building it into a raid array!

aha. i really though both machines build its own md0 out of
1) a local partition
2) an imported enbd

 > It doesn't make sense. Scrap the "machine B: md0 [hda3+nda0]" part and
 > it will make sense.


so then it's like this:

machine-A: md0 [hda3+nda0]

machine-B:     exporting hda3, but were do the applications
                write to?


so, machine-A has md0, the applications write to md0, if Machine-B is 
down, Machine-A's md0 can still work in degraded mode.

if Machine-A is down, Machine-B has to serve files. does it serve them 
from hda3?

>>if i'm going to use 3 (or more) nodes, where would machine-C import its
>>nda0 from? machine-A or machine-B?
> 
> Nowhere - I'm not sure what you mean ... nda0 is part of a raid array,
> md0 on A, so that's where it stays!

so machine-A is the only machine with a md0 made from a local partition 
and an nda0 from some other machine. i *thought* all machines could 
carry and md0. i was wrong. hm.

> The intention is key. I think you want to build a failover arrangement?

yes.

> In that case, you are doing fine. Now you export md0 from A,

"export md0 from A"? you mean, i "use md0 on machine A, which is made 
from local hda3+nda0", right?

> and when A breaks you fail over to exporting hda3 from B instead. 

if A breaks, B takes over and B's hda3 (which is exported by 
enbd-server, but not used by A now) is serving the files.


> If B breaks instead you still have the hda3 part of the raid on A
 > working.

yes. i think i got it now, if you can please confirm the statement above.

> There are prepared failover scripts for heartbeat in the etc/ directory
> in the archive.

yes, that'll be a tricky part too.

> It's fine - it's even convenient, as you can switch off kernel
> buffering (use the switch that does open O_DIRECT on the server) and
> rely on the db servers to sync between distributed copies of the db.

ok. i'm thinking about one issue here, but it's not really 
enbd-specific: if the db-server (mysqld) is running on A and B all the 
time and now A goes down, B takes over. fine. but in the time A *and* B 
are running, only *one* machines should be allowed to write, yes? (i 
guess i'll only start mysqld on B when A is really down).

thank you for your assistance,
Christian.
-- 
BOFH excuse #389:

/dev/clue was linked to /dev/null