I am pondering the fact that the txg is part of the hash key for the arc. It seems to me, this has a profound implication: read caching is per txg.
If I read a record into the cache in txg N, and the current txg is closed, this record becomes effectively un-cached in the new (N+1) txg. (Well, there may be some on-going operation that still references the txg N entry, but for read caching I believe this is correct) So the next read will have to read it from disk again, and put it into the arc again, this time with txg N+1. Now we have 2 arc entries for the same piece of data. The entry for txg N will get aged out of the cache eventually, but in theory, there may be potentially many entries of the same (identical data content and location) data block present in the arc, if we keep re-reading the same block. Is my understanding correct? -- This message posted from opensolaris.org
