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[Discuss] Backing up LVM partitions using snapshots

On Dec 13, 2011, at 11:11 PM, markw at wrote:
> Using a block level store, an incremental backup is no different than a full backup.

You're not following me.  You have a full dump.  You have a first differential against that dump.  You have a second differential made against the first differential.  If you lose the first differential then you are screwed.

A way to work around this is to make your second differential against the full dump rather than against the first differential.  In this case, loss of the first differential is not a disaster.  On the other hand, a file system with a high churn rate, like a mail spool or data collection volume, is eventually going to have differentials nearly as large as the data itself, at which point it will be no better than a file system dump as far as efficiency is concerned.  It will, however, be worse in terms of recovery since it can't be used to restore arbitrary files.  The whole dump chain needs to be unwound -- and that's a lot of time for a 16TB volume.

> It appears that you are thinking of using tape or something.
> tape is dead. The backup medium is a combination of RAID and/or cloud.

[expletive deleted].  Cloud storage is unreliable.  If your provider is down then your backups are unavailable.  If your provider goes out of business then your backups are *gone*.  A RAID cage in your or your cloud storage's data center isn't proof against a flood or fire.  Tape is slow and cumbersome and costly but it still beats everything else for archival storage.  If it were otherwise then we'd have long since abandoned magtape for any of the plethora of other "tape killer" technologies that have come done the 'pike.  Remember when CD-R was going to make tape obsolete?  Remember when MO media like DVD-RAM was going to make tape obsolete?  Remember when DVD-R was going to make tape obsolete?  None of these are widely used for large scale (enterprise) backups.  What do we use?  Magnetic tape.  We use disk-based archives for quick recovery, but our long-term archives are on magnetic tape -- at least, they are in the real world.

ObTechnialPoint: /dev/st0 is a block device.  It's a sequential access block device rather than a random access block device, but it's still a block device.

--Rich P.

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