DoS attack? Nice try.

Performance is subtle, disk iops especially so. I suspect you'd find - if you implemented it - that for the kinds of loads Bitcoin is processing both today and tomorrow prefix filtering either doesn't save any disk seeks or actively makes it worse.

Consider a client that is syncing the last 24 hours of chain. bitcoind pre-allocates space for blocks in large chunks, so most blocks are laid out sequentially on disk. Almost all the cost of a disk read is rotational latency. Once the head is in place data can be read in very fast and modern kernels will attempt to adaptively read ahead in order to exploit this, especially if a program seems to be working through a disk file sequentially. The work of Bloom filtering parts of the chain for this client boils down to a handful of disk seeks at best and the rest of the work is all CPU/memory bound as the block is parsed into objects and tested against the filter. A smarter filtering implementation than ours could do SAX-style parsing of the block and avoid the overhead of turning it all into objects.

Now consider a prefix filtering implementation. You need to calculate a sorted list of all the data elements and tx hashes in the block, that maps to the location in the block where the tx data can be found. These per-block indexes take up extra disk space and, realistically, would likely be implemented using LevelDB as that's a tool which is designed for creating and using these kinds of tables, so then you're both loading the block data itself (blocks are sized about right currently to always fit in the default kernel readahead window) AND also seeking through the indexes, and building them too. A smart implementation might try and pack the index next to each block so it's possible to load both at once with a single seek, but that would probably be more work, as it'd force building of the index to be synchronous with saving the block to disk thus slowing down block relay. In contrast a LevelDB based index would do the bulk of the index-building work on a separate core.

At some block size and client load the additional data storage and increased pressure on the page cache would probably make it worthwhile. But I find it unlikely to be true at current traffic levels, or double or triple today's levels. So I'd rather we spend our very limited collective time on finding ways to increase usage rather than worrying about resources which are not presently scarce.

(as an aside, some of the above analysis would be invalidated if most nodes end up running on SSDs, but I doubt most are. It'd be neat to export storage tech via some kind of stats message - LevelDB is designed for HDDs not SSDs so at some point a new storage subsystem might make sense if the network switched over).