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SubjectRe: [PATCH v5 2/5] lib: Add zstd modules
On Fri, Aug 11, 2017 at 09:20:10AM -0400, Chris Mason wrote:
> 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.

We've solved that already, here's a proposed patch that extends current
mount options,

and a patch that could be backported to older kernels so the new mount
options do not break mounts on older kernels with the new syntax

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