Ytdyklly

I'm currently trying to make a tar backup to a tape with mbuffer acting as a buffer between e.g:

tar --acls -c /var/test | mbuffer -m 8G -P 100% -R 100M | dd of=/dev/nst0 bs=256k

But the buffer is still empty about 10-20 times. So I wanted to use a file based buffer with the following command:

tar --acls -c /var/test | mbuffer -m 60G -T /srv/testbuffer -P 100% -R 100M | dd of=/dev/nst0 bs=256k 

My understanding was, that the option -T would create a file at the given location and use it as a read/write buffer.

But no file is created, and on one system mbuffer just hangs.

• Am I missing something?


• Does mbuffer create the file in a special way?


• Does mbuffer create the file in another directory?


• Does mbuffer extend/use the swap in some way?

As far as I read in the source of mbuffer (settings.c) mbuffer allocs space of the size of the tmpfile via malloc().

Does that mean that I still need the same amount of RAM, or at least swap, as if I would use it without the -T parameter?

no file is created

mbuffer -m 60G -T /srv/testbuffer creates a file, opens it and unlinks it right after. If you check the file descriptors in /proc/<PID of mbuffer>/fd you will find one that points to

/srv/testbuffer (deleted)

This is quite a standard trick when it comes to temporary files. If mbuffer terminates for whatever reason (even if brutally killed), the filesystem will eventually free the space (in the worst case of power failure or kernel panic: after fsck in the future). There's no risk a totally unused, abandoned file will occupy your diskspace forever.

on one system mbuffer just hangs

The tool actually reserves diskspace like fallocate (not just creates a sparse file like truncate) before it starts its main job of buffering data. Allocating a large file may take time. This delay depends on the filesystem type, possible fragmentation etc.

Does that mean that I still need the same amount of RAM, or at least swap, as if I would use it without the -T parameter?

No. The file exists in the filesystem appropriate for the chosen path (/srv/testbuffer in your case), but the path no longer exists. (Note: a new file with the same path will be a separate file with different inode number).