Hi Kent,

On Tue, Jan 6, 2015 at 5:48 PM, Kent Overstreet [off-list ref] wrote:

On Tue, Jan 06, 2015 at 11:18:10PM +0100, Rolf Fokkens wrote:

quoted

On 01/05/2015 08:47 AM, Slava Pestov wrote:

quoted

The plan is to incrementally backport bug fixes and optimizations from
bcache-dev to upstream for the foreseeable future.

I assume (hope) this won't break bcache w.r.t. the current block device
layout?

quoted

The development branch is going through major changes to support
dynamically adding/removing cache devices

Sound nices, that creates great flexibility. Does this also introduce the
option of storing mirrored writeback data, while storing single copy read
(cached) data (when having multiple SSD's as a cache)?

Yup, and a lot more.

quoted

quoted

and storing data in the btree instead of using it as a cache, enabling
usage without any backing devices.

The needs some explanation. I assume you mean it operates as writeback bache
that never flushes. If that's correct, I can only imagine it's useful when
initially using it. And in that case seems a kind of a time bomb, because
when it's "full" (and there's still no backing device)... what happens then?

What Slava is alluding to is that bcache is becoming a full posix filesystem - I
was actually demoing it... nearly a year ago, I think. With eventual feature
parity with btrfs and much, much better performance.

btrfs use COW b-tree shadowing to support snapshots and clones, what
approach will this new bcache FS use to support those features?

And will this new FS continue to use 2MB buckets without random writes
in them? if that's the case, when this new FS is used on SSDs only, I
guess FS GC has to be turned on for all buckets to reclaim free space.
Since a lot of SSDs use parallel writes, some use compression, it's
not possible to align one logical 2MB bucket at FS level to one single
physical NAND block, then GC will be performed twice for one dirty
bucket at different levels, so SSD life will be cut in half in the
worst case?

None of the existing functionality is going away either, it'll just be faster
and more robust.

quoted

quoted

We're actively working on this code and doing a lot of stress testing of
both the new stuff and backing device support. However, it's its too early
to tell what the new features will look like by the time they're ready to
go upstream, or what a transition plan will look like for existing
installs. I wish we had more time for upstreaming stuff, but I can assure
you its not Kent's intent to just dump bcache-dev as one huge pull request
:-)

As an ethousiastic bcache user myself I would be very happy if there's a
transition plan! :-)

The exact transition plan is really up in the air, but for just caching backing
devices you'll always be able to detach the cache set, reformat the cache set
while using your backing device in passthrough mode, and then reattach.
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