linux-kernel - Re: Bcache version 9

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Message-ID: <4CEB7648.8030000@gmail.com>
Date:	Tue, 23 Nov 2010 00:07:36 -0800
From:	Kent Overstreet <kent.overstreet@...il.com>
To:	Greg KH <greg@...ah.com>
CC:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: Bcache version 9

On 11/21/2010 05:09 PM, Greg KH wrote:
> On Sun, Nov 21, 2010 at 06:09:34AM -0800, Kent Overstreet wrote:
>> +++ b/Documentation/bcache.txt
>
> For new sysfs files, please create Documentation/ABI files.
>
>> +All configuration is done via sysfs. To use sde to cache md1, assuming the
>> +SSD's erase block size is 128k:
>> +
>> +  make-bcache -b128k /dev/sde
>> +  echo "/dev/sde">  /sys/kernel/bcache/register_cache
>> +  echo "<UUID>  /dev/md1">  /sys/kernel/bcache/register_dev
>
> /sys/kernel/bcache/?  Really?

That was a completely arbitrary choice dating from when I first started 
hacking on it. No point in moving it when it might be moved again :p

> Come on, shouldn't this be somewhere else?  You only have 1 file here,
> right?

Two files (really three, but the third is for gimpy latency tracing and 
will die eventually). register_dev is there so on bootup you don't have 
to wait for the cache to be discovered - when you add a cache device if 
there's a backing device waiting for a cache, and the cache has seen 
that UUID before it'll do what you want.

> Shouldn't it be a configfs file instead as that is what you are doing?

That was one of the possibilities I had in mind. My main issue with that 
though is I don't see any way to just have a bare config_item - per the 
documentation, the user must do a mkdir() first, which just doesn't make 
any sense for bcache. There's no point in having a persistent object 
besides the one associated with the block device. Maybe there would be 
in the future, with multiple cache devices, but I still think it's a 
lousy interface for that problem - what bcache wants is something more 
like a syscall; you wouldn't use configfs to replace mount(), for example.

There do exist global interfaces in sysfs, not attached to any device - 
besides /sys/kernel, there's /sys/fs which doesn't have any rhyme or 
reason to it I can discern. ecryptfs has /sys/ext4/ecryptfs/version, 
ext4 has per device stuff that you can't find from the device's dir (you 
woludn't know /sys/fs/ext4/md0 exists from looking at /sys/block/md0). 
There's also /sys/fs/cgroup, which is another unique thing as far as I 
can tell...

Then there's /sys/module which has a bunch of ad hoc stuff, but as far 
as I can tell that's all still module parameters and register_cache and 
register_dev certainly aren't module parameters.

So anyways, I absolutely agree that there are better solutions than 
/sys/kernel/bcache but I want to replace it with something correct, not 
something that sucks less. Ideas/flames are of course more than welcome :)
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