Good info, thanks for the note. Agreed that it is difficult to pull things apart to isolate these features for offload — so actually running experiments with offload is not possible, as you point out (and for other reasons).
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Maybe I could figure out the lines of code that add versions into a table and then those that collect old versions (they do get collected, right?). Anyway, thought being I could profile while running TPC-C or similar. I was hoping that someone might be able to jump on this with a response that they already did something similar. I know that Stonebraker has done some analysis along these lines, but I’m looking for an independent result that confirms (or not) his work. Thank you, Pete Stevenson > On Jul 7, 2016, at 3:43 PM, Simon Riggs <si...@2ndquadrant.com> wrote: > > On 7 July 2016 at 20:50, Pete Stevenson <etep.nosnev...@gmail.com > <mailto:etep.nosnev...@gmail.com>> wrote: > Hi Simon - > > Thanks for the note. I think it's fair to say that I didn't provide enough > context, so let me try and elaborate on my question. > > I agree, MVCC is a benefit. The research angle here is about enabling MVCC > with hardware offload. Since I didn't explicitly mention it, the offload I > refer to will respect all consistency guarantees also. > > It is the case that for the database to implement MVCC it must provide > consistent read to multiple different versions of data, i.e. depending on the > version used at transaction start. I'm not an expert on postgresql internals, > but this must have some cost. I think the cost related to MVCC guarantees can > roughly be categorized as: creating new versions (linking them in), version > checking on read, garbage collecting old versions, and then there is an > additional cost that I am interested in (again not claiming it is unnecessary > in any sense) but there is a cost to generating the log. > > Thanks, by the way, for the warning about lab vs. reality. That's why I'm > asking this question here. I want to keep the hypothetical tagged as such, > but find defensible and realistic metrics where those exist, i.e. in this > instance, we do have a database that can use MVCC. It should be possible to > figure out how much work goes into maintaining that property. > > PostgreSQL uses a no overwrite storage mechanism, so any additional row > versions are maintained in the same table alongside other rows. The MVCC > actions are mostly mixed in with other aspects of the storage, so not > isolated much for offload. > > Oracle has a different mechanism that does isolate changed row versions into > a separate data structure, so would be much more amenable to offload than > PostgreSQL, in its current form. > > Maybe look at SLRUs (clog etc) as a place to offload something? > > -- > Simon Riggs http://www.2ndQuadrant.com/ > <http://www.2ndquadrant.com/> > PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
