On 12/12/2011 01:17 PM, John R Pierce wrote:
> On 12/12/11 6:43 AM, Digimer wrote:
>> I handle this by setting up two servers running DRBD in active/active
>> with a simple two-node red hat cluster managing a floating IP address.
>> The storage network link uses a simple Active/Passive (mode=1) bond with
>> either link go to separate switches.
>
> DRBD with synchronous writes? doesn't that slow things down
> considerably? if its asychronous, recently written data will be lost
> on a failure.
>
> proper storage appliances implement a shared cache between the master
> and standby storage controllers so that if the master fails, the standby
> has all data, including cached writes. as far as I know, there's no
> way to easily implement this with open source, its part of the secret
> sauce of proper redundant storage.
>
> without this, its not safe to run a transactional database server on
> your storage
With synchronous writes ("protocol C" in DRBD terms), I've found minimal
slow-down. With a little tuning and fast enough networking, it's quite
small, given the HA win. Given this, there is no loss in data, even if a
node fails mid-write as the caller never gets ack and knows to retry.
Shared cache is, I think, a single-point-of-failure. I'll make no
arguments towards transactional DB use and I am not a DBA, but I can
restate that a write is not acknowledged until it's been committed to
both nodes in DRBD, making it very robust with minimal performance hit.
--
Digimer
E-Mail: digimer at alteeve.com
Freenode handle: digimer
Papers and Projects: http://alteeve.com
Node Assassin: http://nodeassassin.org
"omg my singularity battery is dead again.
stupid hawking radiation." - epitron