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Chapter 11. Read-ahead Settings

RBD supports read-ahead/prefetching to optimize small, sequential reads. This should normally be handled by the guest OS in the case of a VM, but boot loaders may not issue efficient reads. Read-ahead is automatically disabled if caching is disabled.

rbd readahead trigger requests

Description
Number of sequential read requests necessary to trigger read-ahead.
Type
Integer
Required
No
Default
10

rbd readahead max bytes

Description
Maximum size of a read-ahead request. If zero, read-ahead is disabled.
Type
64-bit Integer
Required
No
Default
512 KiB

rbd readahead disable after bytes

Description
After this many bytes have been read from an RBD image, read-ahead is disabled for that image until it is closed. This allows the guest OS to take over read-ahead once it is booted. If zero, read-ahead stays enabled.
Type
64-bit Integer
Required
No
Default
50 MiB