OS Fundamentals - Your SSD lies and that's ok .. I think

fsync is a linux system call that flushes all pages and metadata for a given file to the disk. It is indeed an expensive operation but required for durability especially for database systems. Regular writes that make it to the disk controller are often placed in the SSD local cache to accumulate more writes before getting flushed to the NAND cells. However when the disk controller receives this flush command it is required to immediately persist all of the data to the NAND cells. Some SSDs however don't do that because they don't trust the host and no-op the fsync, in some cases SSD buffers it for few microseconds to receive more writes and hope it doesn't crash in this microsecond. Others battery powered SSDs can completely ignore host flushes because it can survive a crash. In this video I explain this in details and go through details on how postgres provide so many options to fine tune fsync 0:00 Intro 1:00 A Write doesn’t write 2:00 File System Page Cache 6:00 Fsync 7:30 SSD Cache 9:20 SSD ignores the flush 9:30 15 Year old Firefox fsync bug 12:30 What happens if SSD loses power 15:00 What options does Postgres exposes? 15:30 open_sync (O_SYNC) 16:15 open_datasync (O_DSYNC) 17:10 O_DIRECT 19:00 fsync 20:50 fdatasync 21:13 fsync = off 23:30 Don’t make your API simple 26:00 Database on metal? Fundamentals of Backend Engineering Design patterns udemy course (link redirects to udemy with coupon) https://backend.husseinnasser.com Fundamentals of Networking for Effective Backends udemy course (link redirects to udemy with coupon) https://network.husseinnasser.com Fundamentals of Database Engineering udemy course (link redirects to udemy with coupon) https://database.husseinnasser.com Follow me on Medium https://medium.com/@hnasr/membership Introduction to NGINX (link redirects to udemy with coupon) https://nginx.husseinnasser.com Python on the Backend (link redirects to udemy with coupon) https://python.husseinnasser.com Become a Member on YouTube https://www.youtube.com/channel/UC_ML5xP23TOWKUcc-oAE_Eg/join Buy me a coffee if you liked this https://www.buymeacoffee.com/hnasr Arabic Software Engineering Channel https://www.youtube.com/channel/UChWZsjdoRvZ0T9QWZOD6UpA 🔥 Members Only Content https://www.youtube.com/playlist?list=UUMO_ML5xP23TOWKUcc-oAE_Eg 🏭 Backend Engineering Videos in Order https://backend.husseinnasser.com 💾 Database Engineering Videos https://www.youtube.com/playlist?list=PLQnljOFTspQXjD0HOzN7P2tgzu7scWpl2 🎙️Listen to the Backend Engineering Podcast https://husseinnasser.com/podcast Gears and tools used on the Channel (affiliates) 🖼️ Slides and Thumbnail Design Canva https://partner.canva.com/c/2766475/647168/10068 Stay Awesome, Hussein