Re: [Libguestfs] [nbdkit PATCH] RFC: noextents: Add extentmode=data config parameter

While writing tests for an nbdcopy bug, I found myself wanting a way to easily view an entire image as data, but without disabling extents support altogether. The existing extentlist filter can do this, but requires a secondary file. Still an RFC because I need testsuite coverage similar to test-nozero.sh (eek - we don't have any direct test of the noextent filter, but only indirect coverage through other tests). Also, are there any other extentmode=MODE values that might make sense? --- filters/noextents/nbdkit-noextents-filter.pod | 32 ++++++++++-- filters/noextents/noextents.c | 50 ++++++++++++++++++- 2 files changed, 75 insertions(+), 7 deletions(-) diff --git a/filters/noextents/nbdkit-noextents-filter.pod b/filters/noextents/nbdkit-noextents-filter.pod index 891b197d..aac2f097 100644 --- a/filters/noextents/nbdkit-noextents-filter.pod +++ b/filters/noextents/nbdkit-noextents-filter.pod @@ -4,7 +4,7 @@ nbdkit-noextents-filter - disable extents in the underlying plugin =head1 SYNOPSIS - nbdkit --filter=noextents plugin + nbdkit --filter=noextents plugin [plugin-args...] [extentmode=MODE] =head1 DESCRIPTION @@ -23,9 +23,31 @@ performance (C<tmpfs> is known to be one such system). =head1 PARAMETERS -There are no parameters specific to nbdkit-noextents-filter. Any -parameters are passed through to and processed by the underlying -plugin in the normal way. +The parameter C<extentmode> is optional, and controls which mode the +filter will use. + +=over 4 + +=item B<extentmode=mask> + +Extent support is not advertised to the client; clients should not +query for extent information, and must assume the entire disk is +allocated. + +This is the default if the C<extentmode> parameter is not specified. + +=item B<extentmode=data> + +(nbdkit E<ge> 1.30) + +Extent support is advertised, but extent requests from the client will +be answered with a claim that the entire disk forms a single allocated +data extent. + +=back + +All other parameters are passed through to and processed by the +underlying plugin in the normal way.

=head1 FILES @@ -61,4 +83,4 @@ Richard W.M. Jones =head1 COPYRIGHT -Copyright (C) 2019 Red Hat Inc. +Copyright (C) 2019-2022 Red Hat Inc. diff --git a/filters/noextents/noextents.c b/filters/noextents/noextents.c index f3044809..36231a35 100644 --- a/filters/noextents/noextents.c +++ b/filters/noextents/noextents.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2019 Red Hat Inc. + * Copyright (C) 2019-2022 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -32,19 +32,65 @@ #include <config.h> +#include <string.h> +#include <assert.h> + #include <nbdkit-filter.h> +static enum ExtentMode { + MASK, + DATA, +} extentmode; + +static int +noextents_config (nbdkit_next_config *next, nbdkit_backend *nxdata, + const char *key, const char *value) +{ + if (strcmp (key, "extentmode") == 0) { + if (strcmp (value, "mask") == 0) + extentmode = MASK; + else if (strcmp (value, "data") == 0) + extentmode = DATA; + else { + nbdkit_error ("unknown extentmode '%s'", value); + return -1; + } + return 0; + } + + return next (nxdata, key, value); +} + +#define noextents_config_help \ + "extentmode=<MODE> One of 'mask' (default), 'data'.\n" + +/* Advertise desired extents support. */ static int noextents_can_extents (nbdkit_next *next, void *handle) { - return 0; + return extentmode == DATA; +} + +/* Produce single data extent. */ +static int +noextents_extents (nbdkit_next *next, + void *handle, uint32_t count, uint64_t offset, + uint32_t flags, + struct nbdkit_extents *ret_extents, + int *err) +{ + assert (extentmode == DATA); + return nbdkit_add_extent (ret_extents, offset, count, 0); }

static struct nbdkit_filter filter = { .name = "noextents", .longname = "nbdkit noextents filter", + .config = noextents_config, + .config_help = noextents_config_help, .can_extents = noextents_can_extents, + .extents = noextents_extents, }; NBDKIT_REGISTER_FILTER(filter)