Re: [GIT] Bcache version 12

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On Thu, Sep 29, 2011 at 04:38:52PM -0700, Dan Williams wrote:
> On Tue, Sep 20, 2011 at 7:54 PM, Kent Overstreet
> > There is (for now) a 1:1 mapping of backing devices to block devices.
> Is that "(for now)" where you see md not being able to model this in the future?

No, the for now was about bcache. I'm planning on adding volume
managament/thin provisioning to bcache, but that may end up being only a
stepping stone to a full fs (i.e. never 

> > Cache devices have a basically identical superblock as backing devices
> > though, and some of the registration code is shared, but cache devices
> > don't correspond to any block devices.
> Just like a raid0 is a virtual creation from two block devices?  Or
> some other meaning of "don't correspond"?


Remember, you can hang multiple backing devices off a cache.

Each backing device shows up as as a new block device - i.e. if you're
caching /dev/sdb, you now use it as /dev/bcache0.

But the SSD doesn't belong to any of those /dev/bcacheN devices.

> > A cache set is a set of cache devices - i.e. SSDs. The primary
> > motivitation for cache sets (as distinct from just caches) is to have
> > the ability to mirror only dirty data, and not clean data.
> >
> > i.e. if you're doing writeback caching of a raid6, your ssd is now a
> > single point of failure. You could use raid1 SSDs, but most of the data
> > in the cache is clean, so you don't need to mirror that... just the
> > dirty data.
> ...but you only incur that "mirror clean data" penalty once, and then
> it's just a normal raid1 mirroring writes, right?

No idea what you mean...

> See, if these things were just md devices multiple cache device would
> already be "done", or at least on its way by just stacking md devices.
>  Where "done" is probably an oversimplification.

No, it really wouldn't save us anything. If all we wanted to do was
mirror everything, there'd be no point in implementing multiple cache
device support, and you'd just use bcache on top of md. We're
implementing something completely new!

You read what I said about only mirroring dirty data... right?

> >> In any case it certainly could be modelled in md - and if the modelling were
> >> not elegant (e.g. even device numbers for backing devices, odd device numbers
> >> for cache devices) we could "fix" md to make it more elegant.
> >
> > But we've no reason to create block devices for caches or have a 1:1
> > mapping - that'd be a serious step backwards in functionality.
> I don't follow that...  there's nothing that prevents having multiple
> superblocks per cache array.

Multiple... superblocks? Do you mean partitioning up the cache, or do
you mean creating multiple block devices for a cache? Either way it's a
silly hack.

> A couple reasons I'm probing the md angle.
> 1/ Since the backing devices are md devices it would be nice if all
> the user space assembly logic that has seeped into udev and dracut
> could be re-used for assembling bcache devices.  As it stands it seems
> bcache relies on in-kernel auto-assembly, which md has discouraged
> with the v1 superblock. 

md was doing in kernel probing, which bcache does not do. What bcache is
doing is centralizing all the code that touches the on disk
superblock/metadata. You want to change something in the superblock -
you just have to tell the kernel to do it for you. Otherwise not only
would there be duplication of code, it'd be impossible to do safely
without races or the userspace code screwing something up; only the
kernel knows and controls the state of everything.

Or do you expect the ext4 superblock to be managed in normal operation
by userspace tools?

> We even have nascent GUI support in
> gnome-disk-utility it would be nice to harness some of that enabling
> momentum for this.

I've got nothing against standardizing the userspace interfaces to make
life easier for things like gnome-disk-utility. Tell me what you want
and if it's sane I'll see about implementing it.

> 2/ md supports multiple superblock formats and if you Google "ssd
> caching" you'll see that there may be other superblock formats that
> the Linux block-caching driver could be asked to support down the
> road.  And wouldn't it be nice if bcache had at least the option to
> support the on-disk format of whatever dm-cache is doing?

That's pure fantasy. That's like expecting the ext4 code to mount a ntfs

There's a lot more to bcache's metadata than a superblock, there's a
journal and a full b-tree. A cache is going to need an index of some

> > The way I see it md is more or less conflating two different things -
> > things that consume block devices
> ...did the interwebs chomp the last part of that thought?

Yeah, was supposed to be "things that consume block devices and things
that provide them".

> Side question, what are the "Change Id:" lines referring to in the git
> commit messages?

Gerrit wants them, and I don't see the point of stripping them out for
the public tree.
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