[Borgbackup] What to do if borg check fails due to Input/Output Error 5?

[Sascha Ternes]

@ThomasWaldmann: Master branch works with davfs perfectly!

Some additions for the ML archive:

>> So this is again a davfs issue, caused by "borg check" only when davfs
>> cache is too small.
> 
> If you take this to github or to our IRC channel, we could try some code
> modification (disabling our "open files" lru cache), whether that
> improves it for davfs.

We did some experimenting and were successful. First, some words about
my setup:

- WebDAV mounted via davfs2 module
- custom davfs settings: cache_size 50000 MiByte, table_size 4096
- davfs cache on /var/cache/davfs2 with 120 GiB free space
- BorgBackup repo mounted on /srv/borg, 7.63 TB original size, 117 GB
dedup total, 2225 segments

1. BorgBackup 1.1.9
"borg check --debug -p /srv/borg" crashes when reaching segment 103,
which is ~ 50 GB when davfs cache is full.

2. BorgBackup 1.1.10.dev (branch 1.1-maint), lru cache disabled
"borg check --debug -p /srv/borg" does not crash when reaching cache
full. davfs starts to delete data files 1, 2, 3... while the check
continues.
I aborted the check when I was sure that it does not crash, since a
whole check would take several hours.

3. BorgBackup 1.2.0.dev (branch master), unmodified
Same as 2., borg check does not crash anymore.

Sascha