Re: [Libguestfs] [nbdkit PATCH] noextents: Document use case with tmpfs

tmpfs has a known bug of O(n^2) behavior with lseek(SEEK_HOLE); this is one situation where the noextents filter can come in handy to avoid the performance penalty of exposing accurate extents. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- filters/noextents/nbdkit-noextents-filter.pod | 7 ++++++- plugins/file/nbdkit-file-plugin.pod | 9 ++++++++- 2 files changed, 14 insertions(+), 2 deletions(-) diff --git a/filters/noextents/nbdkit-noextents-filter.pod b/filters/noextents/nbdkit-noextents-filter.pod index aa2e9e2..46f6bdb 100644 --- a/filters/noextents/nbdkit-noextents-filter.pod +++ b/filters/noextents/nbdkit-noextents-filter.pod @@ -13,6 +13,10 @@ client to detect sparse regions of the underlying disk. C<nbdkit-noextents-filter> disables this so that the plugin appears to be fully allocated. +This filter can be useful when combined with L<nbdkit-file-plugin(1)> +serving a file from a file system known to have poor C<lseek(2)> +performance (C<tmpfs> is known to be one such system). + =head1 PARAMETERS There are no parameters specific to nbdkit-noextents-filter. Any @@ -23,7 +27,8 @@ plugin in the normal way. L<nbdkit(1)>, L<nbdkit-filter(3)>, -L<nbdkit-nozero-filter(1)>. +L<nbdkit-nozero-filter(1)>, +L<nbdkit-file-plugin(1)>. =head1 AUTHORS diff --git a/plugins/file/nbdkit-file-plugin.pod b/plugins/file/nbdkit-file-plugin.pod index d34a638..9241418 100644 --- a/plugins/file/nbdkit-file-plugin.pod +++ b/plugins/file/nbdkit-file-plugin.pod @@ -15,6 +15,12 @@ It serves the named C<FILENAME> over NBD. Local block devices To concatenate multiple files, use L<nbdkit-split-plugin(1)>. +If you want to expose a file that resides on a file system known to +have poor C<lseek(2)> performance when searching for holes (C<tmpfs> +is known to be one such file system), you can use +L<nbdkit-noextents-filter(1)> to avoid the penalty of probing for +holes. + =head1 PARAMETERS =over 4 @@ -88,7 +94,8 @@ or block device efficiently or not. L<nbdkit(1)>, L<nbdkit-plugin(3)>, L<nbdkit-split-plugin(1)>, -L<nbdkit-partitioning-plugin(1)>. +L<nbdkit-partitioning-plugin(1)>, +L<nbdkit-noextents-filter(1)>. =head1 AUTHORS