On Fri, Aug 31, 2018 at 10:24 PM Dilip Kumar <dilip.ku...@enterprisedb.com> wrote: > On Fri, Aug 31, 2018 at 3:08 PM, Dilip Kumar <dilipbal...@gmail.com> wrote: > > As Thomas has already mentioned upthread that we are working on an > > undo-log based storage and he has posted the patch sets for the lowest > > layer called undo-log-storage. > > > > This is the next layer which sits on top of the undo log storage, > > which will provide an interface for prepare, insert, or fetch the undo > > records. This layer will use undo-log-storage to reserve the space for > > the undo records and buffer management routine to write and read the > > undo records.
I have also pushed a new WIP version of the lower level undo log storage layer patch set to a public branch[1]. I'll leave the earlier branch[2] there because the record-level patch posted by Dilip depends on it for now. The changes are mostly internal: it doesn't use DSM segments any more. Originally I wanted to use DSM because I didn't want arbitrary limits, but in fact DSM slots can run out in unpredictable ways, and unlike parallel query the undo log subsystem doesn't have a plan B for when it can't get the space it needs due to concurrent queries. Instead, this version uses a pool of size 4 * max_connections, fixed at startup in regular shared memory. This creates an arbitrary limit on transaction size, but it's a large at 1TB per slot, can be increased, doesn't disappear unpredictably, is easy to monitor (pg_stat_undo_logs), and is probably a useful brake on a system in trouble. More soon. [1] https://github.com/EnterpriseDB/zheap/tree/undo-log-storage-v2 [2] https://github.com/EnterpriseDB/zheap/tree/undo-log-storage -- Thomas Munro http://www.enterprisedb.com
