linux-kernel - Re: [GIT] Bcache version 12

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Message-ID: <CAA9_cmdCg4UJrzYBzcrhD61PZ6hfrGWbF2A05LPo9WD55874uw@mail.gmail.com>
Date:	Thu, 15 Sep 2011 15:03:08 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Kent Overstreet <kent.overstreet@...il.com>
Cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, rdunlap@...otime.net,
	axboe@...nel.dk, akpm@...ux-foundation.org, neilb@...e.de
Subject: Re: [GIT] Bcache version 12

On Fri, Sep 9, 2011 at 11:45 PM, Kent Overstreet
<kent.overstreet@...il.com> wrote:
> bcache, n.: a cache for arbitrary block devices that uses an SSD
>
> It's probably past time I started poking people to see about getting
> this stuff in. It's synced up with mainline, the documentation is for
> once relatively up to date, and it looks just about production ready.
>
> Suggestions are more than welcome on how to make it easier to review -
> it's entirely too much code, I know (near 10k lines now). I'll be
> emailing the patches that touch other parts of the kernel separately.
>
> Short overview:
> Bcache does both writethrough and writeback caching. It presents itself
> as a new block device, a bit like say md. You can cache an arbitrary
> number of block devices with a single cache device, and attach and
> detach things at runtime - it's quite flexible.
>
> It's very fast. It uses a b+ tree for the index, along with a journal to
> coalesce index updates, and a bunch of other cool tricks like auxiliary
> binary search trees with software floating point keys to avoid a bunch
> of random memory accesses when doing binary searches in the btree. It
> does over 50k iops doing 4k random /writes/ without breaking a sweat,
> and would do many times that if I had faster hardware.

Does it consider the raid5/6 write hole in what it caches?  Guess I
need to take a look at the code, but just wondering if it considers
the need to maintain a consistent strip when writing back to raid5/6
array, or would there still be a need for a separate driver/region of
the SSD for caching that data.
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