/sys/doc/ Documentation archive

Venti Backups to Blu-Ray Discs

Geoff Collyer

Bell Laboratories

Murray Hill, New Jersey 07974


As a precaution against multiple disks in our Plan 9 venti server’s RAID array failing at about the same time, or other catastrophic failure, we record venti arenas, after they are sealed, onto dual-layer Blu-Ray discs (BDs). One could use other large optical discs instead.

The advertised capacity of a dual-layer BD is 50GB, but those aren’t even disk-manufacturer’s (decimal) gigabytes, which would give a capacity of $50 times 10 sup 9$ bytes, which is roughly equivalent to 46.6 gigabytes, as the term is used by everyone but disk manufacturers. In the case of BDs, even that is an exaggeration, with the actual capacity being closer to $48.44 times 10 sup 9$ bytes, so the claimed capacity should be read as ‘50 VAX-gigabytes’, where a VAX-gigabyte is 968,800,338 bytes. The default venti arena size is 512MB, but for some reason our server is configured with 1GB arenas, so we could fit 46 of them on a BD. To leave a little extra room for lead-in, lead-out, inter-track gaps, lossless-linking and the like, we record 45 arenas per BD.

The scripts used and records kept are in /sys/lib/backup and pertain to the file server on which they are stored. You will probably want to edit funcs to set default file server and Blu-ray device, at minimum.

Creating and Updating Backups

We start cdfs after inserting a disc (virgin or partially-written), then use venti/rdarena to copy the next sealed but not backed-up arena to the start of the unwritten portion of the BD by writing to /mnt/cd/wd/x. We don’t fixate data BDs, as that seems to tickle a bug, perhaps in our Sony Blu-Ray burners, that only records the first track in the disc index upon fixation.

One can find out which arenas are sealed and which are open by viewing http://venti-server/index. We also periodically print the most recent fossil dump scores and save the paper with the discs.

All of this is packaged up as scripts in /sys/lib/backup, notably backup. A typical invocation would be just

cd /sys/lib/backup


which will guide its invoker as to which discs to insert into a Blu-ray burner and when. This works for a first full backup and for subsequent incremental backups of just the newly-sealed arenas. There is provision for burning multiple backup sets, the default being set1. To burn a second set, we would

mkdir /sys/lib/backup/set2

backup -s set2

backup and related scripts keep track of which BD is current and how many tracks are recorded, and which arenas have been dumped to BD.

We have recorded two sets of BD backups, one of which will go to Antwerp to seed their venti server and also serve as off-site backup for Murray Hill.

Restoring from Backups

After a disaster, or when setting up a new venti server from BD backups, the first step is to get Plan 9 running on the new venti server. This could be done by installing into a smallish (2GB) fossil partition from a Plan 9 installation CD, if necessary. One would then initialise the new disk partitions per venti-fmt(8) and read all the arenas on all the BDs into the new venti by running venti/wrarena once per BD track (arena).

To add the contents of a backup BD to a (possibly fresh) venti store, shut down the venti server, format the arenas partition, then run

restore first-arena-number

after inserting a BD into the Blu-ray drive. Repeat this for each BD in the backup set. When all the arenas have been restored, it will be necessary to build a new venti index, the usual steps being to run checkarenas, fmtisect, fmtbloom, fmtindex, and buildindex -b, all from venti-fmt(8). Then the venti server may be restarted.

Once the venti storage has been restored, a new fossil partition (perhaps the existing one or another one) can be initialised from the last fossil dump score corresponding to the last arena on BD (see fossil/flfmt in fossil(4)):

fossil/flfmt -v c388...32b5 /dev/sdC0/fossil