On Mon, Nov 13, 2017 at 4:36 PM, Alex Rousskov <rousskov@xxxxxxxxxxxxxxxxxxxxxxx> wrote: > On 11/13/2017 02:34 AM, Bike dernikov1 wrote: >> On Fri, Nov 10, 2017 at 4:43 PM, Alex Rousskov wrote: >>> Squid swapping in production is an arguably worse disaster, as you have >>> learned. In many cases, it is better to deal with a lack of swap than to >>> rely on swap's magical effects that most humans poorly understand. YMMV. > >> In this scenario, swap is backup cache (as I understand)? > > In this scenario, swap is not a cache! In fact it is pretty much the > opposite: > > * A cache is, by definition, an optional unreliable "fast" storage meant > to reduce the need to go to some "slow" storage. > > * When in active use, swap is required reliable slow storage meant to > extend fast storage (RAM) capacity. > > Do you see how almost every adjective in the first bullet is replaced > with an antonym in the second one? Yes, definitely mixed up, I meant right but wrote wrong. > Some services, including many databases, over allocate RAM to store > rarely used (computed and/or preloaded) data. When that data is swapped > out, the service often continues to operate normally because the data is > rarely accessed (and/or because swapping it in is still cheaper than > computing it from scratch). > > With Squid, it is very difficult for the OS to correctly identify the > rarely used RAM areas to swap out. When the OS swaps out the wrong area, > Squid slows down (to access that area), which only increases the number > of concurrent transactions and, hence, the amount of RAM Squid needs to > operate, which triggers more wrong swap outs, creating a vicious cycle. That is why best solution would be swap of. Definitely testing swapoff next week, if we won't have new problems. >> Swap could be used to translate back data to mem if used, but it >> stays on disk and purge after some time if not used ? > > The purging bit is wrong. Think of swap as very very very slow RAM. > > Alex. So, when squid need something from swap, it will load that data back to ram. For purge, data then stay in swap forever ? Swap very slow ram, best explanation. Use that metaphor during mentoring younger coworkers :). Thanks for explaining swap memory problems. _______________________________________________ squid-users mailing list squid-users@xxxxxxxxxxxxxxxxxxxxx http://lists.squid-cache.org/listinfo/squid-users
