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backup-snapshot-file-structure.md

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Backup snapshot file structure

Backup snapshots are stored in rotating increment directories named after the date and time the snapshot was taken. Each snapshot directory contains a full backup snapshot of all relevant data stores. Repository, Search, and Pages data is stored efficiently via hard links.

Please note Symlinks must be maintained when archiving backup snapshots. Dereferencing or excluding symlinks, or storing the snapshot contents on a filesystem which does not support symlinks will result in operational problems when the data is restored.

The following example shows a snapshot file hierarchy for hourly frequency. There are five snapshot directories, with the current symlink pointing to the most recent successful snapshot:

./data
   |- 20180124T010000
   |- 20180125T010000
   |- 20180126T010000
   |- 20180127T010000
   |- 20180128T010000
      |- audit-log
      |- benchmarks
      |- elasticsearch
      |- git-hooks
      |- hookshot
      |- pages
      |- repositories
      |- storage
      |- authorized-keys.json
      |- enterprise.ghl
      |- es-scan-complete
      |- manage-password
      |- mssql
      |- mysql.sql.gz
      |- redis.rdb
      |- settings.json
      |- ssh-host-keys.tar
      |- ssl-ca-certificates.tar
      |- strategy
      |- uuid
      |- version
   |- current -> 20180128T010000

Note: the GHE_DATA_DIR variable set in backup.config can be used to change the disk location where snapshots are written.

MS SQL Server backup structure

Actions service uses MS SQL Server as backend data store. Each snapshot includes a suite of backup files for MS SQL Server database(s).

To save time in backup, a three-level backup strategy is implemented. Based on the GHE_MSSQL_BACKUP_CADENCE setting, at each snapshot, either a (F)ull backup, a (D)ifferential or a (T)ransaction log backup is taken.

As a result, a suite always contains following for each database: a full backup, possibly a differential backup and at least one transaction log backup. Their relationship with timeline is demonstrated below:

M---8:00--16:00---T---8:00--16:00---W... (timeline)

F-----------------F-----------------F... (full backup)
#-----D-----D-----#-----D-----D-----#... (differential backup)
T--T--T--T--T--T--T--T--T--T--T--T--T... (transaction log backup)

To save disk space, at each snapshot, hard links are created to point to previous backup files. Only newly-created backup files are transferred from appliance to backup host. When a new full/differential backup is created, they become the new source for hard links and new base line for transaction log backups, for subsequent snapshots.

During restore, a suite of backup files are restored in the sequence of full -> differential -> chronological transaction log.

Benchmark data

Benchmark data for each snapshot is stored as a log file within the benchmarks directory within a snapshot directory. The benchmark log can be used to determine the duration of each backup step. For example:

ghe-backup-store-version took 0s
ghe-backup-settings took 2s
ghe-export-authorized-keys took 0s
ghe-export-ssh-host-keys took 0s
ghe-backup-mysql-binary took 9s
ghe-backup-mysql took 9s
ghe-backup-minio took 0s
ghe-backup-redis took 1s
ghe-backup-es-audit-log took 1s
ghe-backup-repositories - Generating routes took 3s
ghe-backup-repositories - Fetching routes took 0s
ghe-backup-repositories - Processing routes took 0s
ghe-backup-pages - hostname took 1s
ghe-backup-pages took 1s
ghe-backup-storage - Generating routes took 2s
ghe-backup-storage - Fetching routes took 0s
ghe-backup-storage - Processing routes took 0s
ghe-backup-git-hooks took 0s
ghe-backup-es-rsync took 2s