Backup import uses 3 times the space as all the blobs

[link2xt]

When backup is imported, backup is first copied, then blobs are copied to disk, and then VACUUM is executed to delete blobs table from the database. When VACUUM is executed, sqlite copies the database one more time.

A solution for this problem is to use a new backup format based on sqlar format, as proposed by @csb0730. In this format, messages database is stored as another blob/file in the archive, so after unpacking, there is no need to VACUUM it. Only the backup database/archive contains blobs.

See issues deltachat/deltachat-core#261 and #1640 for discussion.

Implementation for non-async core is in csb0730@a87ddf0

[hpk42]

Given storage and other considerations i suggest to close here and rather go for zip-files #1727 -- unless there are advantages of sqlar over zip files that i am missing.

[csb0730]

@hpk42 @link2xt @r10s
First I didn't noticed it, but later I recognized, that sqlar has the possibility to add more tables in file than only sqlar one. See the config table. I use it as in current backup_blobs archive to store some meta information about the backup.

Is this possible with zip too? Do we need it?

And maybe read the pros & cons for sqlar in documentation. Maybe there are some important facts?

What I can say about the story: At first I saw sqlar as a technical compromise but meanwhile I think it is a technical progress related to zip.