Thanks for the hint. I wonder where the value of 4 comes from. Is this an old value derived from a (then) fast RAID across many spinning rust disks? As you pointed out, today --- using SSDs -- the cost should be lower; I'd think in some cases (e.g. here a DB used for a catalog of a back-up system) the backing store is exactly one spinning rust drive, where I'd expect a much higher cost for random access (about 80MB/s / (100/s*8KiB)).
Ah, [1] has the answer: they acknowledge that random accesses might be much slower, but expect a good share of accesses to be satisfied by the cache (that'll be a combination of various caches).
Ah, [1] has the answer: they acknowledge that random accesses might be much slower, but expect a good share of accesses to be satisfied by the cache (that'll be a combination of various caches).
[1] https://www.postgresql.org/docs/13/runtime-config-query.html