Red Hat Summit19장. Discarding unused blocks
Discard operations improve storage performance and lifespan by informing the storage device which blocks are no longer in use. It allows SSDs to optimize wear leveling and enabling thin-provisioned storage to reclaim space.
Requirements
The block device underlying the file system must support physical discard operations.
Physical discard operations are supported if the value in the
/sys/block/<device>/queue/discard_max_bytesfile is not zero.
19.1. Types of block discard operations
Block discard operations can be performed by using batch, online, or periodic methods, each with specific use cases and performance recommendations.
The following list describes the various discard operations:
- Batch discard
-
This type of discard is part of the
fstrimcommand. It discards all unused blocks in a file system that match criteria specified by the administrator. Red Hat Enterprise Linux 10 supports batch discard on XFS and ext4 formatted devices that support physical discard operations. - Online discard
This type of discard operation is configured at mount time with the discard option, and runs in real time without user intervention. However, it only discards blocks that are transitioning from used to free. Red Hat Enterprise Linux 10 supports online discard on XFS and ext4 formatted devices.
Use batch discard, except when online discard is required to maintain performance, or when batch discard is not feasible for the workload of the system.
- Periodic discard
-
Batch operations that are run regularly by a
systemdservice.
All types are supported by the XFS and ext4 file systems.
Recommendations
Use batch or periodic discard.
Use online discard only if:
- the system’s workload is such that batch discard is not feasible, or
- online discard operations are necessary to maintain performance.
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