42 lines
2.1 KiB
ReStructuredText
42 lines
2.1 KiB
ReStructuredText
Trim/discard
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============
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Trim or discard is an operation on a storage device based on flash technology
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(SSD, NVMe or similar), a thin-provisioned device or could be emulated on top
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of other block device types. On real hardware, there's a different lifetime
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span of the memory cells and the driver firmware usually tries to optimize for
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that. The trim operation issued by user provides hints about what data are
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unused and allow to reclaim the memory cells. On thin-provisioned or emulated
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this is could simply free the space.
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There are three main uses of trim that BTRFS supports:
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synchronous
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enabled by mounting filesystem with ``-o discard`` or ``-o
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discard=sync``, the trim is done right after the file extents get freed,
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this however could have severe performance hit and is not recommended
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as the ranges to be trimmed could be too fragmented
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asynchronous
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enabled by mounting filesystem with ``-o discard=async``, which is an
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improved version of the synchronous trim where the freed file extents
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are first tracked in memory and after a period or enough ranges accumulate
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the trim is started, expecting the ranges to be much larger and
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allowing to throttle the number of IO requests which does not interfere
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with the rest of the filesystem activity
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manually by fstrim
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the tool :command:`fstrim` starts a trim operation on the whole filesystem, no
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mount options need to be specified, so it's up to the filesystem to
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traverse the free space and start the trim, this is suitable for running
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it as periodic service
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The trim is considered only a hint to the device, it could ignore it completely,
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start it only on ranges meeting some criteria, or decide not to do it because of
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other factors affecting the memory cells. The device itself could internally
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relocate the data, however this leads to unexpected performance drop. Running
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trim periodically could prevent that too.
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When a filesystem is created by :doc:`mkfs.btrfs` and is capable
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of trim, then it's by default performed on all devices.
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