>>> If you try to look at the rbd device under dm-cache from another host, of course >>> any data that was cached on the dm-cache layer will be missing since the >>> dm-cache device itself is local to the host you wrote the data from originally. >> And here it can (and probably will) go horribly wrong. >> If you miss the dm-cache device (cache/hypervisor failure) you will probably end up with an inconsistent filesystem. >> This is because dm-cache is not a ordered write-back cache afaik. >> > >I think that you are twisting in two unrelated points. > >dm-cache does do proper ordering. > >If you use it to cache writes and then take it effectively out of the picture >(i.e., never destage that data from cache), you end up with holes in a file system. > >Nothing to do with ordering, all to do with having a write back cache enabled >and then chopping the write back cached data out of the picture. Way back in this thread it was mentioned you would just lose a few seconds of data when you lose the cache device. My point was that when you lose the cache device you do not just miss x seconds of data but probably lose the whole filesystem. This is because the cache is not “ordered” and random parts, probably the “hot data” you care about, never made it from the cache device into ceph. However, if the write cache would would be "flushed in-order" to Ceph you would just lose x seconds of data and, hopefully, not have a corrupted disk. That could be acceptable for some people. I was just stressing that that isn’t the case. > >If you use dm-cache as a write through cache, this is not a problem (i.e., we >would only be used to cache reads). Caching reads is fine :) _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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