Re: Why so much "btrfs send" data for "cp -a --reflink"?

[Torsten Bronger <bronger@physik.rwth-aachen.de>]

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Hallöchen!

Filipe Manana writes:

> [...]
>
> But since the files are VM images, very likely what you are seeing
> are writes full of zero bytes.  This is because the current send
> protocol does not have support holes, instead it issues write
> operations with a bunch of zeros.  And holes are very common in VM
> images in general.

Thank you for your answer!  It is plausible: I created a 10G file
with random content, reflink-copied it, and the "btrfs send" is
almost nothing.

> Just look at a few write commands, grab the file name, offset and
> length, then check if after you read the corresponding file range
> you get only zeros.

I haven’t tested it, but the "btrfs send" produces 12G, 1.5G of
which are zeroes, and the source file contains 6.8GB of zeroes.  Can
this be the holes nevertheless?

Regards,
Torsten.

-- 
Torsten Bronger

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