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How is zram used to speed up swap?
ZRAM creates a block device in RAM where pages that would otherwise be written to swap (disk/ssd) are instead first compressed, then stored. This allows for much faster I/O of swap and also the data compression provides a great amount of memory savings.
What does zswap do for swap in Linux?
Zswap is a Linux kernel feature providing a compressed write-back cache for swapped pages. This increases the performance and decreases the IO-Operations. ZRAM creates a virtual compressed Swap-file in memory as alternative to a swapfile on disk.
Why does zswap use more RAM than zcache?
If the data is compressed, zswap stores it in RAM. If it isn’t, it stores it to the existing swap. This way, zswap takes better advantage of the available RAM and swap space since it won’t, for example, hog the RAM it uses with incompressible data.
Is there a write back cache on zram?
A: Nope, it’s useless, as zram is a compressed RAM DISK, but zswap is a compressed “writeback” CACHE on swap file/disk. Also having both activated can lead to inverse LRU as noted here
How much RAM does 1GB of zram use?
For example, 1GB of ZRAM amounts to around 2 to 3 GB after compression. Also, in our tweaks we encouraged the kernel to swap to ZRAM more frequently, thus freeing up more usable RAM than possible with swap or with no swap.
Are there any downsides to using zram?
A downside of ZRAM is that it does use some CPU for compression but this is usually negated by the gains achieved from avoiding disk swap and also by the overall memory savings of compression. So keep those things in mind per your usage.
How does zram create a virtual block device?
Thanks. zram creates a virtual block device of a size you specify by writing ( for example “512m” ) to /sys/block/zramX/disksize ( where X is the 0 based number of the zram device in question, so 0 for the first one ). You can then format it for swap with mkswap /dev/zram0 and enable swapping on it with swapon /dev/zram0.