Has anyone considered managing a system like the DragonFLY swapcache for
a DBMS like PostgreSQL?
ie where the admin can assign drives with good random read behaviour
(but perhaps also-ran random write) such as SSDs to provide a cache for
blocks that were dirtied, with async write that hopefully writes them
out before they are forcibly discarded.
And where a cache fail (whether by timeout, hard fail, or CRC fail) just
means having to go back to the real transactional storage.
I'd been thinking that swapcache would help where the working set won't
fit in RAM, also L2ARC on Solaris - but it seems to me that there is no
reason not to allow the DBMS to manage the set-aside area itself where
it is given either access to the raw device or to a pre-sized file on
the device it can map in segments.
While L2ARC is obviously very heavyweight and entwined in ZFS,
Dragonfly's swapcache seems to me remarkably elegant and, it would seem,
very effective.
James
