On Mon, Jan 16, 2017 at 10:11 AM Jason Dillaman <[email protected]> wrote:
> On Sun, Jan 15, 2017 at 2:56 PM, Shawn Edwards <[email protected]> > wrote: > > If I, say, have 10 rbd attached to the same box using librbd, all 10 of > the > > rbd are clones of the same snapshot, and I have caching turned on, will > each > > rbd be caching blocks from the parent snapshot individually, or will the > 10 > > rbd processes be building up their own cache, ignorant of the other > > process's caching? > > Each process will utilize its own, independent cache for the parent > image. There is no interprocess coordination to share the cache bits. > > That's what I figured. A shared, interprocess cache would be Hard, and not have many use cases. > > The use case I have is a lot of rbd-nbd mounted rbd used as VM boot disks > > all based on the same clone. It would be great if the parent clone > could be > > cached once and then only blocks which were copy-on-write were in each > > individual process's cache. > > If you are worried about the footprint, you could disable the use of > the cache within just the parent image by running "rbd metadata set > <image spec> conf_rbd_cache false" against the parent image. Since you > are using a VM on top of librbd, the librbd cache is effectively a L2 > cache so you probably aren't gaining much by caching reads for the > parent image since the VM will most likely cache the reads as well. > > Ah, interesting. I'll give that a shot and see if that helps. Thanks for the suggestion. > -- > Jason >
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