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kgdrenefort
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Joined: 19 Sep 2023
Posts: 315
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 Posted: Mon Jan 13, 2025 12:03 pm Post subject: Questions about ZRAM, ZSwap |
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Hello,
Today I tried to setup some ZRAM into my main system.
I followed this page: https://wiki.gentoo.org/wiki/Zram
And did actually this step: https://wiki.gentoo.org/wiki/Zram#Using_systemd_zramctl
Which on my system, this was made:
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Mephistopheles ~ # modprobe zram
Mephistopheles ~ # zramctl /dev/zram0 --size 4G --algorithm lz4
Mephistopheles ~ # swapoff -a
Mephistopheles ~ # mkswap /dev/zram0
Configure l'espace d'échange (swap) en version 1, taille = 4 GiB (4294963200 octets)
pas d'étiquette, UUID=8b1bddbb-8c9a-4adb-80eb-0d3e2528cc88
Mephistopheles ~ # swapon /dev/zram0
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Previously, I was using a swapfile (/swapfile) of 16GiB, which was as seen disabled (for now) for this process.
Now, I have effectively a 4GiB swap space:
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meself@Mephistopheles ~ $ LC_ALL=C free -m
total used free shared buff/cache available
Mem: 15905 12687 2612 3976 4910 3217
Swap: 4095 4095 0
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First question, is the value showed below normal ?
I tried to check if it was effectively used as intented, as showed on the Zram wiki page:
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$ grep zram /proc/swaps
/dev/zram0 partition 4194300 3391232 -2
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First, I wonder why the last value is -2, while on the wiki it shows 16383, which seems to represent in the example 16GiB of swap. Here, I should not have something like 4096 ?
I'm a bit perplexe, since I see a swap space in /dev/zram0 as set earlier (see my example above).
I guess something is not understood by me.
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Second question, isn't this zram actually zswap ?
As explained on the ZSwap page on the wiki: https://wiki.gentoo.org/wiki/Zswap
| Quote: | | Next to zswap there is something related called zram. Zram can also be used to create a swap device for compressed pages in memory. |
I'm in big doubt, because I followed the ZRam page and ended up with new swap space, as it seems compressed into my RAM.
Could you help me understand this, please ?
Regards,
GASPARD DE RENEFORT Kévin
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Zucca
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Joined: 14 Jun 2007
Posts: 3865
Location: Rasi, Finland
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| Posted: Mon Jan 13, 2025 1:28 pm Post subject: |
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Zswap tries to compress memory pages to aviod them being swapped. Zswap requires swap to function.
Zram is, like you have observed, a method of creating a virtual (compressed) storage device, that always* resides in RAM.
That's their difference.
* you can specify a backing device for zram device, so that its contents can (automatically, dynamically) be moved into non-virtual device.
As for the priority: You can pass --priority/-p for swapon command to adjust the priority of said swap device.
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pingtoo
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Joined: 10 Sep 2021
Posts: 1386
Location: Richmond Hill, Canada
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| Posted: Mon Jan 13, 2025 2:08 pm Post subject: |
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zswap, | Kernel Zswap wrote: | | Zswap is a lightweight compressed cache for swap pages. It takes pages that are in the process of being swapped out and attempts to compress them into a dynamically allocated RAM-based memory pool. zswap basically trades CPU cycles for potentially reduced swap I/O. This trade-off can also result in a significant performance improvement if reads from the compressed cache are faster than reads from a swap device |
zram, | Kernel zram wrote: | | The zram module creates RAM-based block devices named /dev/zram<id> (<id> = 0, 1, ...). Pages written to these disks are compressed and stored in memory itself. These disks allow very fast I/O and compression provides good amounts of memory savings. Some of the use cases include /tmp storage, use as swap disks, various caches under /var and maybe many more |
So as you can see, zswap is cache for swap. Pages that need to store in swap could store in zswap first and would be retrieved faster when needed.
zram is just a disk use your computer ram, you can make a swap on top of that disk (ram disk) or you can make a filesystem on the disk and mount for /var/tmp for example.
I use both. I use zram from multiple nodes over NBD then pool them together with LVM and finially make a filesystem on the entire pool and mount it on /var/tmp for my stage4 build process.
I setup zswap for each nodes that is distcc helper in participle in above setup so I can confident that compiler jobs will not run out of memory. |
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