Details
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Technical task
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Resolution: Unresolved
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Minor
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None
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None
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9223372036854775807
Description
Writeback on close is to piggyback the dirty data for DoM files in the close RPC if the data could fit into the inline buffer. This way it should be able to improve write on small files significantly.
I have had this idea before and I have seen this problem in my recent test. The writeback to small files are really slow and no matter how large the number I set it to max_rpcs_in_flight of mdc, it could simply max out. Small RPCs are expensive. An alternative solution would be to have compound RPC to merge those small RPCs but it would introduce more issues. The easier solution is to have writeback on close.
Attachments
Issue Links
- is related to
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LU-12325 Downgrade lock mode for DOM files when DOM bit gone
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- Open
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So this would cut RPC counts by half and should reduce the RPC processing time for the write since it's inline rather than RDMA, but how big will this effect be relative to the writing itself? It sounds like in your testing, Jinshan, you were unable to keep the MDS busy because of rpc_in_flight limits.
It seems like we should do this, but also raise the RPC in flight limit, unless the MDS CPU/disk was fully busy (which it sounds like it wasn't). And it sounds like raising the RPC in flight limit for the MDS might be a cheap win here.