Re: Fedora 34 Change: DNF/RPM Copy on Write enablement for all variants (System-Wide Change)

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On Tue, Dec 22, 2020 at 12:58 PM Matthew Almond via devel
<devel@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> There is also some confusion between compressed data in the rpm and the transcoded one, and filesystem level compression. This proposal affects the former, but not the latter. I'd caution against using btrfs specific attributes to disable compression the dnf/rpm cache directory tree, because then the extents written/shared to the installed file locations will also not be compressed. (this is my interpretation of what I expect to see with FICLONERANGE ioctl etc: it'd be slower if it honored filesystem level compression because it'd need to re-write the data.)

It shouldn't need to rewrite the data. ficlonerange offset and length
is based on the Btrfs logical address space, and this is uncompressed.
That behind the scene it happens to be compressed is a sort of "last
mile" detail, similar to where the file is actually located. Btrfs
logical address for a file suggests there is exactly one copy of the
file and one copy of its metadata, but via chunk tree lookup it may be
that this file has two copies (raid1) or it may be located on any one
of a number of devices. Yet ficlonerange still works as expected
regardless of those details.


-- 
Chris Murphy
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