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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
