Hello, I was reading through some old threads[1][2][3] while trying to figure out how to add a new GUC to control I/O prefetching for new kinds of things[4][5], and enjoyed Simon Riggs' reference to Jules Verne in the context of RAID spindles.
On 2 Sep 2015 14:54, "Andres Freund" <andres(at)anarazel(dot)de> wrote: > > On 2015-09-02 18:06:54 +0200, Tomas Vondra wrote: > > Maybe the best thing we can do is just completely abandon the "number of > > spindles" idea, and just say "number of I/O requests to prefetch". Possibly > > with an explanation of how to estimate it (devices * queue length). > > I think that'd be a lot better. +many, though I doubt I could describe how to estimate it myself, considering cloud storage, SANs, multi-lane NVMe etc. You basically have to experiment, and like most of our resource consumption limits, it's a per-backend limit anyway, so it's pretty complicated, but I don't see how the harmonic series helps anyone. Should we rename it? Here are my first suggestions: random_page_prefetch_degree maintenance_random_page_prefetch_degree Rationale for this naming pattern: * "random_page" from "random_page_cost" * leaves room for a different setting for sequential prefetching * "degree" conveys the idea without using loaded words like "queue" that might imply we know something about the I/O subsystem or that it's system-wide like kernel and device queues * "maintenance_" prefix is like other GUCs that establish (presumably larger) limits for processes working on behalf of many user sessions Whatever we call it, I don't think it makes sense to try to model the details of any particular storage system. Let's use a simple counter of I/Os initiated but not yet known to have completed (for now: it has definitely completed when the associated pread() complete; perhaps something involving real async I/O completion notification in later releases). [1] https://www.postgresql.org/message-id/flat/CAHyXU0yaUG9R_E5%3D1gdXhD-MpWR%3DGr%3D4%3DEHFD_fRid2%2BSCQrqA%40mail.gmail.com [2] https://www.postgresql.org/message-id/flat/Pine.GSO.4.64.0809220317320.20434%40westnet.com [3] https://www.postgresql.org/message-id/flat/FDDBA24E-FF4D-4654-BA75-692B3BA71B97%40enterprisedb.com [4] https://www.postgresql.org/message-id/flat/CA%2BhUKGJ4VJN8ttxScUFM8dOKX0BrBiboo5uz1cq%3DAovOddfHpA%40mail.gmail.com [5] https://www.postgresql.org/message-id/CA%2BTgmoZP-CTmEPZdmqEOb%2B6t_Tts2nuF7eoqxxvXEHaUoBDmsw%40mail.gmail.com