Re: 3.18.1 + latest bcache-dev

On Wed, Jan 07, 2015 at 09:41:19AM -0800, Jianjian Huo wrote:

Hi Kent,

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

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

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

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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?

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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.

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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?

bcache has always been COW for data, and it supports copy offload (i.e. btrfs
reflink) today via just multiple extents pointing to the same data.

I'll write up the plan for snapshots at some point, it's quite different from
the btrfs approach.

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?

Yeah that is a potential issue, for SSDs what we really want is to get raw
access to flash and have bcache be the FTL. Even if that doesn't happen there
are things we can do to mitigate though, suffice it to say this issue has been
discussed a lot :)