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.HTML "Venti Backups to Blu-Ray Discs"
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.EQ
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delim $$
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.EN
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.
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.TL
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Venti Backups to Blu-Ray Discs
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.AU
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Geoff Collyer
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.AI
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.MH
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.SH
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Overview
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.PP
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As a precaution against multiple disks in our
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Plan 9
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.I venti
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server's RAID array failing at about the same time,
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or other catastrophic failure, we record
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.I venti
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arenas,
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after they are sealed,
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onto dual-layer Blu-Ray discs (BDs).
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One could use other large optical discs instead.
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.PP
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The advertised capacity of a dual-layer BD is 50GB,
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but those aren't even disk-manufacturer's (decimal) gigabytes,
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which would give a capacity of
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$50 times 10 sup 9$ bytes,
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which is roughly equivalent to 46.6 gigabytes,
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as the term is used by everyone but disk manufacturers.
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In the case of BDs,
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even that is an exaggeration, with the actual capacity being
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closer to $48.44 times 10 sup 9$ bytes,
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so the claimed capacity should be read as `50 BD-gigabytes',
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where a
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.I BD-gigabyte
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is 968,800,338 bytes.
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The default
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.I venti
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arena size is 512MB, but for some reason our server is configured
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with 1GB arenas, so we could fit 46 of them on a BD.
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To leave a little extra room for lead-in, lead-out, inter-track gaps,
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lossless-linking and the like,
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we record 45 arenas per BD.
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.PP
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The scripts used and records kept are in
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.CW /sys/lib/backup
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and pertain to the file server on which they are stored.
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You will probably want to edit
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.CW funcs
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to set default file server and Blu-ray device, at minimum.
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.SH
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Creating and Updating Backups
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.PP
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We start
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.I cdfs
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after inserting a disc (virgin or partially-written),
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then use
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.I venti/rdarena
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to copy the next sealed but not backed-up arena
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to the start of the unwritten portion of the BD by writing to
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.CW /mnt/cd/wd/x .
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We don't fixate data BDs, as that seems to tickle a bug,
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perhaps in our Sony Blu-Ray burners, that only records the first track
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in the disc index upon fixation.
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.PP
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One can find out which arenas are sealed and which are open by viewing
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.CW http://\fIventi-server\fP/index .
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We also periodically print the most recent
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.I fossil
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dump scores and save the paper with the discs.
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.PP
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All of this is packaged up as scripts in
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.CW /sys/lib/backup ,
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notably
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.CW backup .
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A typical invocation would be just
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.DS
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.ft CW
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cd /sys/lib/backup
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backup
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.ft
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.DE
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which will guide its invoker as to which discs to insert into a Blu-ray
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burner and when.
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This works for a first full backup and for subsequent incremental backups
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of just the newly-sealed arenas.
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There is provision for burning multiple backup sets, the default being
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.CW set1 .
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To burn a second set, we would
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.DS
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.ft CW
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mkdir /sys/lib/backup/set2
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backup -s set2
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.ft
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.DE
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.CW backup
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and related scripts keep track of which BD is current and how many
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tracks are recorded, and which arenas have been dumped to BD.
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.PP
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We have recorded two sets of BD backups, one of which will go to
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Antwerp to seed their
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.I venti
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server and also serve as off-site backup for Murray Hill.
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.SH
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Restoring from Backups
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.PP
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After a disaster, or when setting up a new
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.I venti
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server from BD backups,
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the first step is to get Plan 9 running on the new
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.I venti
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server.
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This could be done by installing into a smallish (2GB)
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.I fossil
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partition from a Plan 9 installation CD, if necessary.
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One would then initialise the new disk partitions per
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.I venti-fmt (8)
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and read all the arenas on all the BDs into the new
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.I venti
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by running
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.I venti/wrarena
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once per BD track (arena).
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.br
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.ne 3
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.PP
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To add the contents of a backup BD to a (possibly fresh)
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.I venti
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store,
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shut down the
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.I venti
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server,
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format the arenas partition, then
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run
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.DS
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.ft CW
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restore \fIfirst-arena-number\fP
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.ft
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.DE
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after inserting a BD into the Blu-ray drive.
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Repeat this for each BD in the backup set.
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When all the arenas have been restored,
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it will be necessary to build a new
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.I venti
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index,
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the usual steps being to run
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.I checkarenas ,
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.I fmtisect ,
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.I fmtbloom ,
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.I fmtindex ,
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and
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.I "buildindex -b" ,
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all from
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.I venti-fmt (8).
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Then the
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.I venti
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server may be restarted.
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.PP
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Once the
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.I venti
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storage has been restored,
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a new
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.I fossil
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partition (perhaps the existing one or another one)
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can be initialised from the last
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.I fossil
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dump score corresponding to the last arena on BD
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(see
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.I fossil/flfmt
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in
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.I fossil (4)):
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.DS
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.ft CW
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fossil/flfmt -v c388...32b5 /dev/sdC0/fossil
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.ft
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.DE