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[Timandahaf <question205270@xxxxxxxxxxxxxxxxxxxxx>]

Question #205270 on Duplicity changed:
https://answers.launchpad.net/duplicity/+question/205270

Timandahaf posted a new comment:
Okay, I took a look. Actually the way it works is:

- Uncompressed data in blocks of 128kB are sent to GPGFile or GZipFile
- the size of the written (compressed) gpg or gzip file is checked against volsize
- this process continues until volsize is hit

Where it gets tricky is this: both Gpgfile and Gzipfile write a footer
when close() called. Unfortunately, I haven't found a way to predict the
size of this footer in advance. The footer size seems to vary quite a
bit depending on the data. I don't even know if I can find a bound on
this footer - if I could, this would solve the problem. As a kludge, I
could assume that the footer will take up to 128kB, and reserve that
much space for it. This will probably work in 99% of the cases, and I've
tried it on volsizes of upto 250MB. What do you think?

As an aside, the GzipWriteFile uses 128 kB blocks as shown above, but
inconsistently uses a value of 32 kB to check for the last block. Fixing
this will essentially apply my idea above to the gzip case. For the gpg
case, we'll simply reduce the specified size by 128 kB.

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