this post was submitted on 17 Sep 2025
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Does zram impede disk cache? (swg-empire.de)
submitted 3 months ago* (last edited 3 months 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 3 months ago

zswap is not better than modern zram in any way. And you can set up the latter with writeback anyway.

But that's not OP's problem since "swap gets hardly touched" in OP's case.