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Eliminate backup windows
A different approach to solving the backup window problem is to create a snapshot or PIT copy of data used for backup purposes. Once the PIT copy is created, normal data processing can resume because an image of the quiesced system has been captured.

Snapshots create a virtual copy of data. It's called "virtual" because the second copy is only created if blocks are changed after the copy was initiated. Because most data in a volume doesn't change daily, these snapshot copies don't take up a large amount of disk space. The additional disk space required is equal to the amount of changed data. Snapshot creation is typically scripted; it takes less than a minute to create the virtual copy of a volume. Once the copy is made, the primary data volume is available again for changes without any impact to the backups. Snapshots address two critical backup window issues:

  • They provide the ability to resume data processing without fear of having open files skipped because a quiesced system is only required while the snapshot is created.
  • You can start the next night's backup even though the backup from the night before is still running because each night's backup image is captured on a separate snapshot image.
Snapshot creation commands are often integrated into applications, such as databases, to

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allow for temporary quiescing of the application, creation of the snapshot and resumption of the application.

PIT copies, sometimes referred to as clones, are similar to snapshots because they also quickly create a copy of data that can be used for static backup images. The advantages of PIT copies are that a full copy of the data resides on a completely separate volume, while snapshots only copy the changed portion of a volume. This helps address the issue of resource contention caused by backups on production servers. The PIT copy volumes can then be mounted to surrogate clients. Disk drives and all client system resources driving the backups are separate from production disk drives and system resources. Like snapshots, PIT copy creation can be integrated into applications, ensuring clean copies of data quickly.

The size of the volumes typically has minimal impact on how long it takes to create snapshots or PIT copies; therefore, this method scales easily as storage capacity grows. The downside of PIT copies is the higher cost associated with the increased disk capacity needed to create full copies of data on different drives.

Although snapshots and PIT copies reduce backup duration, another big benefit is the effect on restores: If a user is looking to retrieve data from last night's backups, the data doesn't have to be retrieved from tape because all the information resides on disk.

Restore performance issues
In optimizing data transfer for quick backups, administrators may unknowingly create restore performance issues. This is particularly true when trying to reduce backup window durations by decreasing the ratio of fulls to incrementals, or by increasing the number of streams multiplexed to a tape drive target.

When decreasing the full to incremental ratio, restores may require that more tape volumes have to be mounted and read. In one incremental-forever scenario, we saw a customer recall more than 1,000 tape volumes to restore a single system. Most incremental-forever systems have controls to limit the number of tapes that a single file system can be spread across, but this requires additional configuration and data movement cycles. Fortunately, this data movement isn't typically associated with a backup window because it doesn't impact client operations. In addition, the proliferation of disk-based targets makes reading from a large number of incremental backups a non-issue (due to the random access nature of disk).

Multiplexing a large number of backup streams to a single tape drive causes restores to degrade. Because multiplexing intersperses data from one client with another, the sequential nature of tape often requires reading all data on the tape to retrieve the bits associated with a single file system. Depending on the tape technology and priority or restore speed, it's not uncommon to have four or more streams multiplexed simultaneously to a single tape drive.

Where to start
If a large percentage of backups are running slowly, causing them to run beyond the desired backup window, look at components shared by all of the clients. This includes the backup network, backup servers and backup target device (tape or disk). If only a small percentage of clients are exceeding the desired backup window, look at client-side issues first. Start with just a few clients. What you learn from the first few will likely help troubleshoot the others.

This was first published in August 2004

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