Re: Metadata / Data on Heterogeneous Media

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2018年02月16日 01:15, Ellis H. Wilson III wrote:
> 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).

Just want to point out that, metadata trees, block groups, *AND* tree
blocks of file trees (data trees in your words) are all *METADATA*.

That's to say, btrfs can't isolate tree blocks from file trees and other
trees. They are all tree blocks, thus all metadata.

Real data is, non-inlined, non-hole data. Tree blocks of file tress has
pointers to data, but the tree blocks of file trees are still metadata.


So even on day, btrfs supports to alloc data/meta chunks using different
strategy, we are still a step away from your
different-chunk-for-different-tree idea.

Thanks,
Qu

> 
> 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.
> 
> Best,
> 
> ellis
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