According to btrfs wiki, some patches have been submitted to support
metadata on different devices than data (i.e. metadata on SSD, data on
HDD):
https://btrfs.wiki.kernel.org/index.php/Project_ideas#Dedicated_metadata_drives
Dedicated metadata drives
Not claimed — submitted — Not in kernel yet
We're able to split data and metadata IO very easily. Metadata tends
to be dominated by seeks and for many applications it makes sense to put
the metadata onto faster SSDs.
This article (almost 2.5 years old) claims one company is already using
either these patches or something similar:
https://lwn.net/Articles/698090/
August 24, 2016
To combat that, he has a set of patches to automatically put the
Btrfs metadata on SSDs. The block layer provides information on whether
the storage is rotational; for now, his patch assumes that if
it is not rotational then it is fast. The patch has made a huge
difference in the latencies and requires less flash storage (e.g. 450GB
for 40TB filesystem) for Facebook's file workload that
consists of a wide variety of file sizes.
Do these patches exist anywhere? I couldn't find them in the list
archive.
Tomasz
