The wip-rbd-wt branch has some potential changes to the rbd cache code. Currently, each image gets its own separate cache using its own pool of memory. The ObjectCacher module has essentially three knobs: max size -- total bytes we'll cache max dirty -- max dirty bytes before a write will block and wait for writeback target dirty -- threshold at which we'll start writing back older buffers (There's also a hard-coded max age value (~1 second) after which a dirty buffer will be written; that needs to be turned into a tunable.) The first several patches add 'write-thru' caching support. This is done by simply setting the max_dirty value to 0. A write will then block until it is written to the cluster, but we still get the benefits of the read cache. The second set of patches restructure the way the cache itself is managed. One goal is to be able to control cache behavior on a per-image basis (this one write-thru, this was write-back, etc.). Another goal is to share a single pool of memory for several images. The librbd.h calls to do this currently look something like this: int rbd_cache_create(rados_t cluster, rbd_cache_t *cache, uint64_t max_size, uint64_t max_dirty, uint64_t target_dirty); int rbd_cache_destroy(rbd_cache_t cache); int rbd_open_cached(rados_ioctx_t io, const char *name, rbd_image_t image, const char *snap_name, rbd_cache_t cache); Setting the cache tunables should probably be broken out into several different calls, so that it is possible to add new ones in the future. Beyond that, though, the limitation here is that you can set the target_dirty or max_dirty for a _cache_, and then have multiple images share that cache, but you can't then set a max_dirty limit for an individual image. Does it matter? Ideally, I supposed, you could set: - per-cache size - per-cache max_dirty - per-cache target_dirty - per-image max_dirty (0 for write-thru) - per-image target_dirty and then share a single cache for many images, and the flushing logic could observe both sets of dirty limits. That just means calls to set max_dirty and target_dirty for individual images, too. Is it worth the complexity? In the end, this will be wired up to the qemu writeback options, so the range of actual usage will fall within whatever is doable with those options and generic 'rbd cache size = ..' tunables, most likely... sage -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
