nbdkit-cacheextents-filter - cache extents
nbdkit --filter=cacheextents pluginnbdkit-cacheextents-filter is a filter that caches the result of last extents() call.
A common use for this filter is to improve performance when using a client performing a linear pass over the entire image while asking for only one extent at a time (such as qemu-img convert), but where the plugin can provide multiple extents for the same high latency as a single extent (such as nbdkit-vddk-plugin(1)). For example:
nbdkit --filter=cacheextents --run 'qemu-img map "$uri"' vddk ...For files with big extents (when it is unlikely for one extents() call to return multiple different extents) this does not slow down the access.
This filter only caches image metadata; to also cache image contents, place this filter between nbdkit-cache-filter(1) and the plugin.
There are no parameters specific to nbdkit-cacheextents-filter. Any parameters are passed through to and processed by the underlying plugin in the normal way.
The filter.
Use nbdkit --dump-config to find the location of $filterdir.
nbdkit-cacheextents-filter first appeared in nbdkit 1.14.
nbdkit(1), nbdkit-cache-filter(1), nbdkit-extentlist-filter(1), nbdkit-readahead-filter(1), nbdkit-scan-filter(1), nbdkit-vddk-plugin(1), nbdkit-filter(3), qemu-img(1).
Martin Kletzander
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