Re: [Libguestfs] [PATCH v4 07/12] v2v: nbdkit: Add the readahead filter unconditionally if it is available.

On 9/20/19 9:04 AM, Martin Kletzander wrote: Checking for file existence for filters is somewhat less fragile than for plugins, because all filters are built in-tree (we've specifically documented that we don't provide ABI guarantees for filters, so the only sane way to use a filter is to compile it at the same time/version as the nbdkit binary that will load it). But you do have a point that checking for files is still more fragile than just asking nbdkit whether a given filter exists: $ nbdkit --dump-plugin --filter=nosuch null nbdkit: error: cannot open filter 'nosuch': /usr/lib64/nbdkit/filters/nbdkit-nosuch-filter.so: cannot open shared object file: No such file or directory $ echo $? 1 $ nbdkit --dump-plugin --filter=cache null path=/usr/lib64/nbdkit/plugins/nbdkit-null-plugin.so name=null ... $ echo $? 0 Similarly for --help. But notice: $ diff -u <(nbdkit --dump-plugin --filter=cache null) \ <(nbdkit --dump-plugin null) $ diff -u <(nbdkit --help --filter=cache null) <(nbdkit --help null) --- /dev/fd/63 2019-09-20 09:31:43.184428823 -0500 +++ /dev/fd/62 2019-09-20 09:31:43.185428824 -0500 @@ -23,15 +23,6 @@ Please read the nbdkit(1) manual page for full usage. -filter: cache (nbdkit caching filter) -(/usr/lib64/nbdkit/filters/nbdkit-cache-filter.so) -cache=MODE Set cache MODE, one of writeback (default), - writethrough, or unsafe. -cache-on-read=BOOL Set to true to cache on reads (default false). -cache-max-size=SIZE Set maximum space used by cache. -cache-high-threshold=PCT Percentage of max size where reclaim begins. -cache-low-threshold=PCT Percentage of max size where reclaim ends. - plugin: null (/usr/lib64/nbdkit/plugins/nbdkit-null-plugin.so) size=<SIZE> Size of the backing disk So we could probably improve --dump-plugin to ALSO show details about the filter (such as a second path=), the way --help does, or even add a 'nbdkit --dump-filter filtername' to work in isolation. But while you'd have to wait for a future nbdkit release to get more details from --dump-plugin, and/or a way to list all available filters (think search permissions on a directory), you already have a way with current nbdkit to check existence of a filter (think read permissions on a directory). -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org