The origin is divided up into blocks of a fixed size. This block size
is configurable when you first create the cache. Typically we've been
using block sizes of 256KB - 1024KB. The block size must be between 64
-(32KB) and 2097152 (1GB) and a multiple of 64 (32KB).
+sectors (32KB) and 2097152 sectors (1GB) and a multiple of 64 sectors (32KB).
Having a fixed block size simplifies the target a lot. But it is
something of a compromise. For instance, a small part of a block may be
For the time being, a message "migration_threshold <#sectors>"
can be used to set the maximum number of sectors being migrated,
-the default being 204800 sectors (or 100MB).
+the default being 204800 sectors (100MB).
Updating on-disk metadata
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