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Turning on ZFS compression can produce a surprisingly large space-saving figure, but the setting alone does not tell you why. Compression applies to newly written data, zero-filled blocks can become holes, and ZFS space figures measure different scopes. To explain what happened on a particular pool, you need its before-and-after command output—not an assumed compression rate.
What turning on ZFS compression does—and does not do
ZFS compression is transparent and operates per record. The compression property belongs to a dataset, so enabling it on one dataset does not establish that every dataset in the pool uses compression. Child datasets can inherit the property from a parent unless they override it.
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Changing the property affects only newly written data. Existing blocks are not automatically recompressed when you change the setting. If you need existing data to be compressed under the new property, it must be rewritten—for example, by sending and receiving it, copying it, or using zfs-rewrite. That is a separate operation with its own space and operational consequences, not an automatic result of enabling compression. OpenZFS documents the compression property and its behavior.
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Your newly written data may compress well
How much ordinary data compresses depends on its contents and record geometry. A measured ratio from one pool is not a prediction for another workload, and OpenZFS documentation does not provide a general-purpose percentage to expect.
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Zero-filled blocks can be stored as holes
OpenZFS states that any compression setting other than off detects all-zero blocks and stores them as holes, regardless of the selected algorithm. This can contribute to a larger apparent gain when newly written data contains zero-filled blocks. See the OpenZFS compression documentation.
You may be comparing different accounting scopes
used is allocated or charged space, while logicalused represents logical use by a dataset and its descendants and includes metadata. The ratio properties also differ in scope: for a non-snapshot, compressratio includes used space for descendants, while refcompressratio covers referenced space. These are useful ZFS accounting measures, not direct measurements of application file sizes. OpenZFS defines these dataset properties.
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Pool-level used and available figures are estimates adjusted for RAIDZ parity overhead. Descendants, reservations, clones, and snapshots can affect where space is charged. Snapshot space is shared rather than exclusively charged to every snapshot; destroying one snapshot can change the accounting shown for others. A change in pool free space therefore cannot be read as the compression ratio of one dataset. OpenZFS explains pool properties and accounting.
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Capture the output for the same dataset scope before and after a workload or rewrite. OpenZFS provides these commands:
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zfs get compression,compressratio,refcompressratio pool/data
zfs list -o name,used,logicalused,compressratio -r poolReplace pool/data with the dataset you want to inspect. The recursive listing shows datasets beneath pool; compare like-for-like scopes and record the full output and timing. Check each relevant dataset’s property rather than assuming a pool-wide change reached every child.
Interpret the figures together: compressratio on a non-snapshot includes descendants, refcompressratio is for referenced space, and logicalused includes descendant usage and metadata. None is a clean substitute for measuring the sizes of application files. For pool-level figures, account for the RAIDZ and snapshot caveats described above.
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What does compression=on select?
on selects the current default compression algorithm; it is not a fixed algorithm name. OpenZFS documents that the default is lzjb or lz4 depending on the pool’s enabled feature state. The exact behavior can therefore depend on the pool and OpenZFS release. Check the documentation for the installed release rather than treating a rolling manual as version-specific guidance. The current OpenZFS property manual describes the default.
The title alone cannot establish which algorithm was selected, the amount of data affected, or whether the space change came from compression rather than accounting scope or snapshots. To explain an individual pool’s result, report its OpenZFS version, pool layout, exact datasets changed, prior and new property values, snapshot state, measurement commands, timing, and complete before-and-after output.
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