Backup in a snap: A guide to snapshot technologies


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A series of steps are required to initiate a snapshot:

  1. It starts with a command that a backup is about to occur.
  2. This command tells the system to quiesce the file system and apps running at that point in time.
  3. The file system is then flushed so that any pending file transactions are completed.
  4. The snapshot is then created.
  5. Afterwards, the file system and applications are released to resume normal operations.

Snapshot technology has also moved beyond just data protection. Snapshots are an efficient and non-disruptive way to test application software against real data without endangering live production data. They're also ideal for data mining and ediscovery. Snapshots have also evolved into a very effective -- even preferred -- disaster recovery methodology that protects against malware, human errors and data corruption.

Where snapshot technology lives

The common perception may be that snapshotting is a storage system feature, but that's only one place that the technology may reside. Snapshot technologies are generally available in seven different types of implementations:

  1. File systems of servers, desktops and laptops
  2. Logical volume managers (LVMs)
  3. Network-attached storage (NAS)

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  1. Storage arrays
  2. Storage virtualization appliances
  3. Server virtualization hypervisors
  4. SQL databases

File system-based snapshots

File system-based snapshots are available in Microsoft Corp.'s Windows NTFS via Volume Shadow Copy Services (Shadow Copy in Vista); Novell Storage Services (NSS) on NetWare 4.11 or better; Novell's OES-Linux in SUSE Linux; and the Zettabyte File System (ZFS) on Sun Microsystems Inc.'s Solaris and Apple Mac OS X 10.6 (Snow Leopard).

One of the advantages of file system-based snapshot is that it tends to be "free" because it comes with the file system. It also works well and the latest file systems make it pretty easy to use. On the downside, each file system must be managed separately, which can become onerous as the number of systems proliferates. It also means that if snapshot replication is required, each file system must be set up to replicate its own snapshots. In addition, different file systems will likely vary in the kinds of snapshots they provide; snapshot frequency; the amount of capacity that must be reserved (if capacity must be reserved); as well as snapshot set up, operations and manageability. The complexity increases as more servers and file systems must be managed.

LVM snapshots

LVM snapshot technology is available with Hewlett-Packard (HP) Co.'s HP-UX Logical Volume Manager, Linux Logical Volume Manager and Linux Enterprise Volume Management System; Microsoft's Logical Disk Manager for Windows 2000 and later; Sun Solaris 10 ZFS; and Symantec Corp.'s Veritas Volume Manager (part of Symantec Veritas Storage Foundation).

Logical volume manager snapshot technology can sometimes run across a number of file systems; for example, Symantec's Veritas Volume Manager can function with most common operating systems. LVMs also usually include storage multi-pathing and storage virtualization features.

When using LVMs, there are typically additional costs per server for license/maintenance fees. You may also confront the same issues of coordination and complicated implementations found with file system-based snapshots.

This was first published in October 2009

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