Re: [PATCH v5 2/5] lib: Add zstd modules

[Chris Mason <clm@fb.com>]

On 08/10/2017 03:25 PM, Hugo Mills wrote:
> On Thu, Aug 10, 2017 at 01:41:21PM -0400, Chris Mason wrote:
>> On 08/10/2017 04:30 AM, Eric Biggers wrote:
>>>
>>> Theses benchmarks are misleading because they compress the whole file as a
>>> single stream without resetting the dictionary, which isn't how data will
>>> typically be compressed in kernel mode.  With filesystem compression the data
>>> has to be divided into small chunks that can each be decompressed independently.
>>> That eliminates one of the primary advantages of Zstandard (support for large
>>> dictionary sizes).
>>
>> I did btrfs benchmarks of kernel trees and other normal data sets as
>> well.  The numbers were in line with what Nick is posting here.
>> zstd is a big win over both lzo and zlib from a btrfs point of view.
>>
>> It's true Nick's patches only support a single compression level in
>> btrfs, but that's because btrfs doesn't have a way to pass in the
>> compression ratio.  It could easily be a mount option, it was just
>> outside the scope of Nick's initial work.
> 
>     Could we please not add more mount options? I get that they're easy
> to implement, but it's a very blunt instrument. What we tend to see
> (with both nodatacow and compress) is people using the mount options,
> then asking for exceptions, discovering that they can't do that, and
> then falling back to doing it with attributes or btrfs properties.
> Could we just start with btrfs properties this time round, and cut out
> the mount option part of this cycle.
> 
>     In the long run, it'd be great to see most of the btrfs-specific
> mount options get deprecated and ultimately removed entirely, in
> favour of attributes/properties, where feasible.
> 

It's a good point, and as was commented later down I'd just do mount -o 
compress=zstd:3 or something.

But I do prefer properties in general for this.  My big point was just 
that next step is outside of Nick's scope.

-chris