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Does zram impede disk cache? (swg-empire.de)

submitted 1 week ago* (last edited 1 week ago) by bjoern_tantau@swg-empire.de to c/linux@programming.dev

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In my relentless pursuit of trying to coax more performance out of my Lemmy instance I read that PostgreSQL heavily relies on the OSs disk cache for read performance. I've got 16 GB of RAM and two hdds in RAID 1. I've PostgreSQL configured to use 12 GB of RAM and I've zram swap set up with 8 GB.

But according to htop PostgreSQL is using only about 4 GB. My swap gets hardly touched. And read performance is awful. Opening my profile regularly times out. Only when it's worked once does it load quickly until I don't touch it again for half an hour or so.

Now, my theory is that the zram actually takes available RAM away from the disk cache, thus slowing the whole system down. My googling couldn't bring me the answer because it only showed me how to set up zram in the first place.

Does anyone know if my theory is correct?

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[–] BB_C@programming.dev 2 points 1 week ago

            
              Use zram devices equal to the number of threads in your system.
Use zstd compression.
Mount zram devices as swap with high priority.
Mount disk swap partition(s) with low priority.
Increase swapiness:
   sysctl vm.swappiness=<larger number than default>

Use zramctlto see detailed info about your zram disks.
Check with iotop to see if something unexpected is using a lot of IO traffic.