Re: can subvolumes be specified as compressed?

[Chris Ball <cjb@laptop.org>]

Hi,

   > I don't know whether this demand, which specified some subvolumes
   > or directories to be compressed, conflict with the design of
   > btrfs's compression feature, may be they want to do this thing
   > well, and everyone will enable the compression feature, rather
   > than throw the decision to user to determine which should
   > compress.

You can't tell whether you can compress data well until you've already
spent CPU compressing it, though.  Right now we have a useful metric
of "if parts of the file fail to compress, bail out on compressing the
rest of the file, both now and in the future"; but it's only a metric.
There are cases this fails on, such as a tar archive that starts out
with JPEGs and continues with large (compressible) text files.

I think the argument that we don't need to offer user control because
we can just do a good job all the time only works if it's actually
true that we can do a good job all the time, and I don't think that's
true here.  So, I still support user control of the per-inode flag.

(I agree about the subvol flags; I think being able to set quotas and
compression per-subvol is on the roadmap and just not done yet.)

-- 
Chris Ball   <cjb@laptop.org>
One Laptop Per Child