Again at last! Short version: synchronise files with a qcow2 image on a USB 3 connected drive and write the qcow2 image to tape.

For quite some time I’m making backups to LTO tapes; first LTO-1, then LTO-3 and with my Z800 for about 5 years on LTO-4. With an uncompressed capacity of 800 GB.
To identify the tape in the drive library tape tools – ltt – by HPE is used and recorded to a database. For some time the speed wasn’t what it used to be. To rule individual file issues the backup is put on an intermediate qcow2 image and the qcow2 image is written to tape. Updating the qcow2 image is done with rsync. The qcow 2 image is on a NFS share. The NFS share is hosted by a ProLiant DL380p Gen8 running up to date Debian. Still the backup speed is not optimal and the ltt report shows “insufficient margin”. Remarkable during writing to tape is that the network speed is a period almost the full 1 Gbps alternated by a period about half that speed.

The very first qcow2 image was created on a USB 3 connected drive and then copied to the NFS share. In an attempt to see if the USB 3 connected drive would make a difference copying to tape compared with the NFS share I tried that. First i copied the qcow2 image to the USB 3 connected drive with an average speed of 100 MB/s.

Low and behold! Tar reported: Total bytes written: 386708152320 (361GiB, 81MiB/s). The subsequent tape had about same speed. Yes, I’m happy with the backup! Yes, now and then i do a restore test. Also an advantage of the intermediate qcow2 image is that an individual file can be restored faster.

If someone can explain this network speed behavior, you are very welcome! The network speed is between 2 ProLiant DL380p Gen8’s, but also between the Z800 and a ProLiant DL380p Gen8. Every 44th second there is a network speed drop: