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

submitted 2 weeks ago* (last edited 2 weeks 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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[–] CondorWonder@lemmy.ca 3 points 2 weeks ago

Based on what I’ve seen with my use of ZRam I don’t think it reserves the total space, but instead consumes whatever is shown in the output of zramctl --output-all. If you’re swapping then yes it would take memory from the system (up to the 8G disk size), based on how compressible the swapped content is (like if you’re getting a 3x ratio it’s 8GB/3=2.6GB). That said - it will take memory from the disk cache if you’re swapping.

Realistically I think your issue is IO and there’s not much you can do with if your disk cache is being flushed. Switching to zswap might help as it should spill more into disk if you’re under memory pressure.