> >My understanding of how a writeback cache should work is that it should only take a few seconds for writes to be streamed onto the network and is focussed on resolving the speed issue of small sync writes. The writes would be bundled into larger writes that are not time sensitive. > >So there is potential for a few seconds data loss but compared to the current trend of using ephemeral storage to solve this issue, it's a major improvement. It think is a bit worse then just a few seconds of data: As mentioned in the blueprint for ceph you would need some kind or ordered write-back cache that maintains checkpoints internally. I am not that familiar with the internals of dm-cache but I do not think it guarantees any write order. E.g. By default it will bypass the cache for sequential IO. So I think it is very likely the “few seconds of data loss" in this case means the filesystem is corrupt and you could lose the whole thing. At the very least you will need to run fsck on it and hope it can sort out all of the errors with minimal data loss. So, for me, it seems conflicting to me to use persistent storage and then hoping your volumes survive a power outage. If you can survive missing that data you are probably better of running fully from ephemeral storage in the first place. Cheers, Robert van Leeuwen _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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