FreeBSD Mastery: ZFS (IT Mastery Book 7) by Michael W Lucas & Allan Jude

Author:Michael W Lucas & Allan Jude [Lucas, Michael W]
Language: eng
Format: mobi
Tags: nonfiction
Publisher: Tilted Windmill Press
Published: 2015-05-21T04:00:00+00:00

Metadata Redundancy

Each dataset stores an extra copy of its internal metadata, so that if a single block is corrupted, the amount of user data lost is limited. This extra copy is in addition to any redundancy provided at the VDEV level (e.g., by mirroring or RAID-Z). It’s also in addition to any extra copies specified by the copies property (below), up to a total of three copies.

The redundant_metadata property lets you decide how redundant you want your dataset metadata to be. Most users should never change this property.

When redundant_metadata is set to all (the default), ZFS stores an extra copy of all metadata. If a single on-disk block is corrupt, at worst a single block of user data can be lost.

When you set redundant_metadata to most, ZFS stores an extra copy of only most types of metadata. This can improve performance of random writes, because less metadata must be written. When only most metadata is redundant, at worst about 100 blocks of user data can be lost if a single on-disk block is corrupt. The exact behavior of which metadata blocks are stored redundantly may change in future releases.

If you set redundant_metadata to most and copies to 3, and the dataset lives on a mirrored pool, then ZFS stores six copies of most metadata, and four copies of data and some metadata.

This property was designed for specific use cases that frequently update metadata, such as databases. If the data is already protected by sufficiently strong fault tolerance, reducing the number of copies of the metadata that must be written each time the database changes can improve performance. Change this value only if you know what you are doing.

Now that you have a grip on datasets, let’s talk about pool maintenance.

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