On 03/16/2016 12:15 PM, Van Leeuwen, Robert wrote:
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
Hi Robert,
I might be misunderstanding your point above, but dm-cache provides persistent
storage. It will be there when you reboot and look for data on that same box.
dm-cache is also power failure safe and tested to survive this kind of outage.
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.
In a similar way, using dm-cache for write caching (or any write cache local to
a client) will also mean that your data has a single point of failure since that
data will not be replicated out to the backing store until it is destaged from
cache.
I would note that this is exactly the kind of write cache that is popular these
days in front of enterprise storage arrays on clients so this is not really
uncommon.
Regards,
Ric
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