With LVM it's possible to set up a fast caching SSD partition that speeds up a slow (usually rotating HDD) one: Creating LVM Cache Logical Volumes
Would this alleviate slow performance of Monero running on a HDD?
In other words, does Monero need fast random access to more or less the whole database (which means caching would bring little benefit), or just a certain portion of it at a time (which would be cached efficiently on the smaller SSD caching partition).
The way that Monero's decoy selection works is that each time a wallet constructs a ring signature, it randomly selects requests X ring members from the database and does the necessary proofs. This means that validating each transaction requires random reads throughout the entire database (at least the transaction prefixes database which takes up a large percentage of the space). However, for a full node, basically all prunable data (ring signatures, bulletproofs, etc) is written once and never read again (since it is assumed to be correct by virtue of being placed in the database). In conclusion, SSD caching might be able to speed it up slightly, but probably not enough to be worth it. And that's not even mentioning that you have to do a ton of random writes during initial sync.
Nothing is better than testing the performance for yourself, but I wouldn't hold your breath.
