On 01/11/2011 11:52 AM, Jonathan Brassow wrote:
Would it make sense for me to bring up my idea of having the metadata
and data kept separately here? (Perhaps not, as this is related to the
device-mapper/MD integration work; but I'll post it for what it is worth.)
I firmly believe that such schemes - and the idea of metadata in
userspace in general - may be profoundly useful and ought to be
supported to the extent that they are useful, but they shouldn't get in
the way of having something simple and consistent. Emphasis on simple.
Hell, external metadata is basically what bcache does today. But if your
metadata isn't stored with the data it's describing, then... well, it's
not really correct but the way I think of it is whether the data is self
describing or not.
If you've got a filesystem with a cache on another device, then it's not
just a filesystem, it's a filesystem with a cache on another device;
using it as such is critical for consistency, and if you leave that
superblock out then there's no way you can tell them (a plain filesystem
vs. a filesystem with a cache) if you haven't already seen the metadata.
N.B.:
Just because you have a md superblock doesn't mean you can't put all the
important things somewhere else, it's just absolutely critical that you
have some way of noting that in your superblock. Else you can never know
if you have a correct/consistent view of your data.
http://marc.info/?l=linux-raid&m=129434759113635&w=2
brassow
On Jan 11, 2011, at 12:21 PM, Kent Overstreet wrote:
This is, in a roundabout way, an extension of some stuff I was talking
to Neil about - but this is slightly more wild speculation.
Background: bcache
http://bcache.evilpiepirate.org
Bcache currently caches block devices transparently; this is useful
but unsafe. It needs a superblock for the backing device, and it turns
out what it needs out of a superblock is not very dissimilar from what
md does, so I've been thinking about how to best go about using md.
Well, the annoying thing about that for the end user is that if you
want to cache your hard drive safely, you have to plan ahead...
there's no technical reason you shouldn't be able to add a cache to
the filesystem you've already got but you need a place to put the
superblock.
The exact same problem exists with raid: you installed to a single
disk, you decide you want to mirror it - there's no good way of doing
that. There's three different solutions I know of (make a degraded
raid1 on the new disk, copy everything over; use a 1.2 superblock - if
it fits; or when you first install force create a single disk raid1).
They work but they're hacks, it'd be nice to have something better.
The last solution - start with a raid superblock - would be
particularly nice if there was an explicit "noop" raid level; you
could quite conceivable grow from a single disk to a raid6, online.
Trouble is, you could add a cache, create a raid, but not both.
Well, not without a new superblock, which is why I prefaced this by
calling it wild speculation - I really like this solution but it'd be
a fair amount of work.
Change the superblock so it describes a tree structure:
Leaf nodes correspond to component devices. Thus, a superblock that
describes an array with only one component would be a noop superblock.
Then, interior nodes correspond to raid arrays or cache sets. Much of
what's in the start of the version 1 superblock would be here.
Anyways, once you've got that you can have a standard superblock that
you use for everything, and you can safely and easily transition to
whatever you might want to in the future.
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