Greetings, * Sait Talha Nisanci (sait.nisa...@microsoft.com) wrote: > OS version is Ubuntu 18.04.5 LTS. > Filesystem is ext4 and block size is 4KB.
[...] * Sait Talha Nisanci (sait.nisa...@microsoft.com) wrote: > I have run some benchmarks for this patch. Overall it seems that there is a > good improvement with the patch on recovery times: > > The VMs I used have 32GB RAM, pgbench is initialized with a scale factor > 3000(so it doesn’t fit to memory, ~45GB). > > In order to avoid checkpoints during benchmark, max_wal_size(200GB) and > checkpoint_timeout(200 mins) are set to a high value. > > The run is cancelled when there is a reasonable amount of WAL ( > 25GB). The > recovery times are measured from the REDO logs. > > I have tried combination of SSD, HDD, full_page_writes = on/off and > max_io_concurrency = 10/50, the recovery times are as follows (in seconds): > > No prefetch | Default prefetch > values | Default + max_io_concurrency = 50 > SSD, full_page_writes = on 852 301 > 197 > SSD, full_page_writes = off 1642 1359 > 1391 > HDD, full_page_writes = on 6027 6345 > 6390 > HDD, full_page_writes = off 738 275 > 192 > > Default prefetch values: > - Max_recovery_prefetch_distance = 256KB > - Max_io_concurrency = 10 > > It probably makes sense to compare each row separately as the size of WAL can > be different. Is WAL FPW compression enabled..? I'm trying to figure out how, given what's been shared here, that replaying 25GB of WAL is being helped out by 2.5x thanks to prefetch in the SSD case. That prefetch is hurting in the HDD case entirely makes sense to me- we're spending time reading pages from the HDD, which is entirely pointless work given that we're just going to write over those pages entirely with FPWs. Further, if there's 32GB of RAM, and WAL compression isn't enabled and the WAL is only 25GB, then it's very likely that every page touched by the WAL ends up in memory (shared buffers or fs cache), and with FPWs we shouldn't ever need to actually read from the storage to get those pages, right? So how is prefetch helping so much..? I'm not sure that the 'full_page_writes = off' tests are very interesting in this case, since you're going to get torn pages and therefore corruption and hopefully no one is running with that configuration with this OS/filesystem. Thanks, Stephen
signature.asc
Description: PGP signature
