[Libguestfs] [nbdkit PATCH 5/9] plugins: Implement no-op .cache for in-memory plugins

For our plugins which have no backing file but generate everything on the fly or store things in memory, falling back to .pread on a cache request is just wasted work. Implement a no-op .cache callback for the drivers where there is no benefit to trying to cache anything. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- plugins/data/data.c | 10 ++++++++++ plugins/full/full.c | 12 +++++++++++- plugins/memory/memory.c | 10 ++++++++++ plugins/null/null.c | 12 +++++++++++- plugins/pattern/pattern.c | 12 +++++++++++- plugins/random/random.c | 12 +++++++++++- plugins/streaming/streaming.c | 11 +++++++++++ plugins/zero/zero.c | 12 +++++++++++- 8 files changed, 86 insertions(+), 5 deletions(-) diff --git a/plugins/data/data.c b/plugins/data/data.c index 55380c6..aaa3d2d 100644 --- a/plugins/data/data.c +++ b/plugins/data/data.c @@ -377,6 +377,15 @@ data_extents (void *handle, uint32_t count, uint64_t offset, return sparse_array_extents (sa, count, offset, extents); } +/* Cache. */ +static int +data_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags) +{ + /* Everything is already in memory, falling back to .pread is + actually slower than treating this as a no-op. */ + return 0; +} + static struct nbdkit_plugin plugin = { .name = "data", .version = PACKAGE_VERSION, @@ -394,6 +403,7 @@ static struct nbdkit_plugin plugin = { .zero = data_zero, .trim = data_trim, .extents = data_extents, + .cache = data_cache, /* In this plugin, errno is preserved properly along error return * paths from failed system calls. */ diff --git a/plugins/full/full.c b/plugins/full/full.c index 7661856..51a9d67 100644 --- a/plugins/full/full.c +++ b/plugins/full/full.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2017-2018 Red Hat Inc. + * Copyright (C) 2017-2019 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -137,6 +137,15 @@ full_extents (void *handle, uint32_t count, uint64_t offset, uint32_t flags, NBDKIT_EXTENT_HOLE | NBDKIT_EXTENT_ZERO); } +/* Cache. */ +static int +full_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags) +{ + /* Everything is already in memory, falling back to .pread is + actually slower than treating this as a no-op. */ + return 0; +} + /* Note that we don't need to handle flush: If there has been previous * write then we have already returned an error. If there have been * no previous writes then flush can be ignored. @@ -156,6 +165,7 @@ static struct nbdkit_plugin plugin = { .zero = full_zero, .trim = full_trim, .extents = full_extents, + .cache = full_cache, /* In this plugin, errno is preserved properly along error return * paths from failed system calls. */ diff --git a/plugins/memory/memory.c b/plugins/memory/memory.c index 90fa99e..baa29e2 100644 --- a/plugins/memory/memory.c +++ b/plugins/memory/memory.c @@ -172,6 +172,15 @@ memory_extents (void *handle, uint32_t count, uint64_t offset, return sparse_array_extents (sa, count, offset, extents); } +/* Cache. */ +static int +memory_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags) +{ + /* Everything is already in memory, falling back to .pread is + actually slower than treating this as a no-op. */ + return 0; +} + static struct nbdkit_plugin plugin = { .name = "memory", .version = PACKAGE_VERSION, @@ -189,6 +198,7 @@ static struct nbdkit_plugin plugin = { .zero = memory_zero, .trim = memory_trim, .extents = memory_extents, + .cache = memory_cache, /* In this plugin, errno is preserved properly along error return * paths from failed system calls. */ diff --git a/plugins/null/null.c b/plugins/null/null.c index 518b63b..5e40868 100644 --- a/plugins/null/null.c +++ b/plugins/null/null.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2017-2018 Red Hat Inc. + * Copyright (C) 2017-2019 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -140,6 +140,15 @@ null_extents (void *handle, uint32_t count, uint64_t offset, uint32_t flags, NBDKIT_EXTENT_HOLE | NBDKIT_EXTENT_ZERO); } +/* Cache. */ +static int +null_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags) +{ + /* Everything is already in memory, falling back to .pread is + actually slower than treating this as a no-op. */ + return 0; +} + static struct nbdkit_plugin plugin = { .name = "null", .version = PACKAGE_VERSION, @@ -155,6 +164,7 @@ static struct nbdkit_plugin plugin = { .can_fua = null_can_fua, .flush = null_flush, .extents = null_extents, + .cache = null_cache, /* In this plugin, errno is preserved properly along error return * paths from failed system calls. */ diff --git a/plugins/pattern/pattern.c b/plugins/pattern/pattern.c index 115bd96..e25da36 100644 --- a/plugins/pattern/pattern.c +++ b/plugins/pattern/pattern.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2017-2018 Red Hat Inc. + * Copyright (C) 2017-2019 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -117,6 +117,15 @@ pattern_pread (void *handle, void *buf, uint32_t count, uint64_t offset, return 0; } +/* Cache. */ +static int +pattern_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags) +{ + /* Everything is already in memory, falling back to .pread is + actually slower than treating this as a no-op. */ + return 0; +} + static struct nbdkit_plugin plugin = { .name = "pattern", .version = PACKAGE_VERSION, @@ -127,6 +136,7 @@ static struct nbdkit_plugin plugin = { .get_size = pattern_get_size, .can_multi_conn = pattern_can_multi_conn, .pread = pattern_pread, + .cache = pattern_cache, /* In this plugin, errno is preserved properly along error return * paths from failed system calls. */ diff --git a/plugins/random/random.c b/plugins/random/random.c index 7fb42c8..6219169 100644 --- a/plugins/random/random.c +++ b/plugins/random/random.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2017-2018 Red Hat Inc. + * Copyright (C) 2017-2019 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -146,6 +146,15 @@ random_pread (void *handle, void *buf, uint32_t count, uint64_t offset, return 0; } +/* Cache. */ +static int +random_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags) +{ + /* Everything is already in memory, falling back to .pread is + actually slower than treating this as a no-op. */ + return 0; +} + static struct nbdkit_plugin plugin = { .name = "random", .version = PACKAGE_VERSION, @@ -157,6 +166,7 @@ static struct nbdkit_plugin plugin = { .get_size = random_get_size, .can_multi_conn = random_can_multi_conn, .pread = random_pread, + .cache = random_cache, /* In this plugin, errno is preserved properly along error return * paths from failed system calls. */ diff --git a/plugins/streaming/streaming.c b/plugins/streaming/streaming.c index 4ca3e76..505ed91 100644 --- a/plugins/streaming/streaming.c +++ b/plugins/streaming/streaming.c @@ -246,6 +246,16 @@ streaming_pread (void *handle, void *buf, uint32_t count, uint64_t offset) return -1; } +/* Cache. */ +static int +streaming_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags) +{ + /* nbdkit's default of falling back to .pread is pointless: it will + * fail for addresses already written, and waste a memset() for + * addresses not yet reached. Treat this as a no-op instead. */ + return 0; +} + static struct nbdkit_plugin plugin = { .name = "streaming", .longname = "nbdkit streaming plugin", @@ -259,6 +269,7 @@ static struct nbdkit_plugin plugin = { .get_size = streaming_get_size, .pwrite = streaming_pwrite, .pread = streaming_pread, + .cache = streaming_cache, .errno_is_preserved = 1, }; diff --git a/plugins/zero/zero.c b/plugins/zero/zero.c index 49ce08e..be8fc2a 100644 --- a/plugins/zero/zero.c +++ b/plugins/zero/zero.c @@ -1,5 +1,5 @@ /* nbdkit - * Copyright (C) 2017-2018 Red Hat Inc. + * Copyright (C) 2017-2019 Red Hat Inc. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are @@ -76,6 +76,15 @@ zero_pread (void *handle, void *buf, uint32_t count, uint64_t offset, return -1; } +/* Cache. */ +static int +zero_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags) +{ + /* Everything is already in memory, falling back to .pread is + actually slower than treating this as a no-op. */ + return 0; +} + static struct nbdkit_plugin plugin = { .name = "zero", .version = PACKAGE_VERSION, @@ -83,6 +92,7 @@ static struct nbdkit_plugin plugin = { .open = zero_open, .get_size = zero_get_size, .pread = zero_pread, + .cache = zero_cache, /* In this plugin, errno is preserved properly along error return * paths from failed system calls. */ -- 2.20.1