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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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[–] non_burglar@lemmy.world 5 points 2 weeks ago

Zram does not impede disk cache, it's a block device with compression, unavailable to the kernel for anything else.

I do wonder what you're trying to achieve by moving swap to zram? You're potentially moving pages in and out of swap for no real reason, with compression, where the swap wouldn't have occurred if zram weren't in place.