Re: md with shared disks

[Stan Hoeppner <stan@hardwarefreak.com>]

On 11/13/2014 04:53 PM, Ethan Wilson wrote:
> On 13/11/2014 21:56, Stan Hoeppner wrote:
>> With DRBD and GFS2 it is true active/active at the block level.  You
>> just lose half your disk capacity due to the host-to-host mirroring.
> 
> Sorry but I don't share your definition of active/active.
> 
> Would you say that a raid1 is an active/active thing?
> 
> Doubling the number of disks and repeating the operation on both sides
> is not active/active in the sense that people usually want.
> 
> Active/active commonly means that you have twice the performance of
> active/passive.
> 
> In this sense DRBD not only is an active/passive but it is even way
> below the performances of an active/passive because it has to transmit
> the data to the peer in addition to write to the disks, and this takes
> CPU time for memcpy and interrupts, introduces latency, requires
> additional hardware (= fast networking dedicated to DRBD). An
> active/passive with shared disks is hence "twice" (very roughly) faster
> than DRBD at the same price spent on the head nodes. An active/active
> with shared disks is hence 4 times (again very roughly) faster than
> DRBD, at the same price for the head nodes.
> 
> In addition to this with DRBD you have to buy twice the number of disks,
> which is also an additional expense. Marginally though, because a
> shared-disk infrastructure is way more expensive than a direct-attached
> one, but it has to be planned like that in advance, and not retrofitted
> like you propose.
> 
> His current infrastructure cannot be easily converted to DRBD without
> major losses: if he attempts to do so he will have almost double the
> costs of a basic DRBD shared-nothing direct-attached infrastructure or
> exactly double the cost of a shared-disk infrastructure, intended as
> cost per TB of data. Unfortunately, after this he will still have half
> the performances of an active/passive shared-disk clustered-MD solution.

He doesn't have an infrastructure yet.  He's attempting to build one but
purchased the wrong gear for his requirements.  I presented him with
options to do it the right way, and to salvage what he has already
purchased.  The DRBD active/active option is the latter.  The SAN option
was the former.  You seem to have misunderstood my comments.

Cheers,
Stan