Hello Jason, many thanks for your informative response! Am 11.05.2018 um 17:02 schrieb Jason Dillaman: > I cannot speak for Xen, but in general IO to a block device will hit > the pagecache unless the IO operation is flagged as direct (e.g. > O_DIRECT) to bypass the pagecache and directly send it to the block > device. Sure, but it seems that xenserver just forwards io from virtual machines (vm: blkfront, dom-0: blkback) to the ndb device in dom-0. >> Sorry, my question was a bit unprecice: I was searching for usage statistics >> of the rbd cache. >> Is there also a possibility to gather rbd_cache usage statistics as a source >> of verification for optimizing the cache settings? > You can run "perf dump" instead of "config show" to dump out the > current performance counters. There are some stats from the in-memory > cache included in there. Great, i was not aware of that. There are really a lot of statistics which might be useful for analyzing whats going on or if the optimizations improve the performance of our systems. >> Can you provide some hints how to about adequate cache settings for a write >> intensive environment (70% write, 30% read)? >> Is it a good idea to specify a huge rbd cache of 1 GB with a max dirty age >> of 10 seconds? > Depends on your workload and your testing results. I suspect a > database on top of RBD is going to do its own read caching and will be > issuing lots of flush calls to the block device, potentially negating > the need for a large cache. Sure, reducing flushes with the acceptance of a degraded level of reliability seems to be one import key for improved performance. >> >> Our typical workload is originated over 70 percent in database write >> operations in the virtual machines. >> Therefore collecting write operations with rbd cache and writing them in >> chunks to ceph might be a good thing. >> A higher limit for "rbd cache max dirty" might be a adequate here. >> At the other side our read workload typically reads huge files in sequential >> manner. >> >> Therefore it might be useful to do start with a configuration like that: >> >> rbd cache size = 64MB >> rbd cache max dirty = 48MB >> rbd cache target dirty = 32MB >> rbd cache max dirty age = 10 >> >> What is the strategy of librbd to write data to the storage from rbd_cache >> if "rbd cache max dirty = 48MB" is reached? >> Is there a reduction of io operations (merging of ios) compared to the >> granularity of writes of my virtual machines? > If the cache is full, incoming IO will be stalled as the dirty bits > are written back to the backing RBD image to make room available for > the new IO request. Sure, i will have a look at the statistics and the throughput. Is there any consolidation of write requests in rbd cache? Example: If a vm writes small io-requests to the ndb device with belong to the same rados object - does librbd consollidate these requests to a single ceph io? What strategies does librd use for that? Regards Marc _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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