The vm.swappiness sysctl controls the kernel's prefernce for swapping
anonymous memory vs page cache. Since Scylla uses very large amounts
of anonymous memory, and tiny amounts of page cache, the correct setting
is to prefer swapping page cache. If the kernel swaps anonymous memory
the reactor will stall until the page fault is satisfied. On the other
hand, page cache pages usually belong to other applications, usually
backup processes that read Scylla files.
This setting has been used in production in Scylla Cloud for a while
with good results.
Users can opt out by not installing the scylla-kernel-conf package
(same as with the other kernel tunables).
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