Zdenek Kabelac schreef op 27-04-2016 8:38:
Yes, it would be seriously bad idea to use 'swap' on cached LV... There are already unsolved issues with plain devices and swapping :) and you want to put caching logic into this stack.
I just think it would never promote it to the cache ;-). Unless the same blocks are written repeatedly with different memory. But typically every write is followed by a read. I don't think the same regions of swap are constantly reused. If your system is not heavily swapping, usually it would be rather stable. On heavy swap though you might find that the cache might start to promote it, but it also means the values constantly change.
Question: if a block has been promoted through reading (for example) (since reading promotion is usually lower (faster)) and is then written to, does this mean the cache for it is updated and then written through (or back)? I suppose it would. Let's assume that is the only sensible thing.
That means you could have swap blocks promoted due to heavy swapping (and hence reading) and if you have writeback, it would still make a modicum of sense since then it would actually cause hits to go to the cache instead of the slower disk.
I mean I could test it. I could just load a few programs heavy on memory, so that I can fully fill my cache (swap I mean) and then I could start alt-tabbing through these programs as a test. And then I would need statistics on lvmcache hits. You say there are issues but I am more than willing to crash my system for it :p.
For example, swap on thin LVM works fine. Although the swap filesystem is not cleared with swapoff. This can be changed with the --discard option, and then it is cleared.
Any discarded swap would instantly discard it from the LVM cache, likely, but not necessarily. If that happened then the promotion counter would never rise.
It is hard to find information on LVMcache. I don't know enough about dmcache to know its relation. I will just ask two short questions in a new email.
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