Re: Metadata / Data on Heterogeneous Media

On 02/15/2018 02:06 PM, Adam Borowski wrote:

On Thu, Feb 15, 2018 at 12:15:49PM -0500, Ellis H. Wilson III wrote:

quoted

In discussing the performance of various metadata operations over the past
few days I've had this idea in the back of my head, and wanted to see if
anybody had already thought about it before (likely, I would guess).

It appears based on this page:
https://btrfs.wiki.kernel.org/index.php/Btrfs_design
that data and metadata in BTRFS are fairly well isolated from one another,
particularly in the case of large files.  This appears reinforced by a
recent comment from Qu ("...btrfs strictly
split metadata and data usage...").

Yet, while there are plenty of options to RAID0/1/10/etc across generally
homogeneous media types, there doesn't appear to be any functionality (at
least that I can find) to segment different BTRFS internals to different
types of devices.  E.G., place metadata trees and extent block groups on
SSD, and data trees and extent block groups on HDD(s).

Is this something that has already been considered (and if so, implemented,
which would make me extremely happy)?  Is it feasible it is hasn't been
approached yet?  I admit my internal knowledge of BTRFS is fleeting, though
I'm trying to work on that daily at this time, so forgive me if this is
unapproachable for obvious architectural reasons.

Considered: many times.  It's an obvious improvement, and one that shouldn't
even be that hard to implement.  What remains, it's SMoC then SMoR (Simple
Matter of Coding then Simple Matter of Review), but both of those are in
short supply.

Glad to hear it's been discussed, and I understand the issue of 
resources all too well with the project I'm working on.  Maybe if my 
nights and weekends open up...

After the maximum size of inline extents has been lowered, there's no real
point in putting different types of metadata or not-really-metadata on
different media: thus, existing split of data -vs- metadata block groups is
fine.

That was my thought.  Regarding inlined data, I'm actually quite ok with 
that being on SSD, as that would deliver fast access to tiny objects 
where if you went to HDD you'd spend the great majority of your time 
just seeking to the data in question compared to transfer.

Our existing COW filesystem this is replacing actually did exactly this, 
except it would store the first N KB of each and every object on SSD, so 
even for large files you could get the headers out quickly (as many 
indexing apps want to do).

Best,

ellis