Hi, I'm not a bcache expert by any means but I might hazard a few comments inline. On 1 February 2012 02:36, infernix <infernix@xxxxxxxxxxxx> wrote: > Hi, > > I've got the following stack, bottom to top: > > - iscsi sans with multiple NICs > - servers with multiple NICs > - open-iscsi cross-connecting to the san over those multiple nics > - multipath to aggregate and load-balance IO over said paths > - LVM on top of multiple mpathX devices > - qemu-kvm running with disks connecting to logical volumes in said LVM > > I am wondering if bcache can fit in, and if so, where? The primary purpose of bcache is to serve as a writeback cache as this provides the most benefit to I/O. However where you place this cache depends on your usage patterns and bottlenecks... If you are bottlenecking purely on random I/O then the easiest and most logical place for bcache is infront of your iSCSI backing store on your SAN. Looking something like this: SAN NICs > IET or SCST > bcache > Backing store This is super simple and means you make the most possible use out of the cache because all of your clients random I/O is gauranteed to make its way through this path. If however you are bottlenecked on the iSCSI interconnect you could feasibly place bcache ontop of the multipath devices (they are just standard dm targets) right below the VMs. This adds the highest possible amount of performance to the VMs at the cost of increased maintaince, complexity of multiple caches and ofcourse cost. > > Ideally I would say it sits between multipath and LVM, e.g. it is active for > the /dev/mapper/mpathX devices. But that doesn't work because there isn't > really a filesystem on it and so it doesn't get assigned an UUID. Even if it > did, that UUID would be the same for all sdX devices in the multipath > device. > > Is there any other way of activating bcache besides passing it an UUID? And > can it even work on top of a dm-multipath device? It works on LVM so it should work fine on multipath targets too. > > Thanks! > -- > To unsubscribe from this list: send the line "unsubscribe linux-bcache" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html Hope that was able to help abit. :) Keep the list in the loop on your progress. Joseph. -- Founder | Director | VP Research Orion Virtualisation Solutions | www.orionvm.com.au | Phone: 1300 56 99 52 | Mobile: 0428 754 846 -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
