Because these filters perform a read-modify-write cycle for requests which are smaller than the block size of the filter, we can adjust or set the preferred block size to the block size of the filter or the preferred block size of the underlying plugin, whichever is larger. We're careful not to set a preferred block size which is larger than the maximum block size. After this change: $ ./nbdkit --filter=cow floppy /usr/share/doc/nbdkit-devel \ cow-block-size=128k \ --run 'nbdinfo $uri' protocol: newstyle-fixed without TLS export="": export-size: 1458176 (1424K) content: DOS/MBR boot sector; partition 1 : ID=0xc, start-CHS (0x3ff,254,63), end-CHS (0x3ff,254,63), startsector 2048, 800 sectors uri: nbd://localhost:10809/ contexts: base:allocation is_rotational: false is_read_only: false can_cache: true can_df: true can_fast_zero: false can_flush: true can_fua: true can_multi_conn: true can_trim: true can_zero: false block_size_minimum: 1 block_size_preferred: 131072 <--- note block_size_maximum: 4294967295 --- filters/cache/cache.c | 21 +++++++++++++++++++++ filters/cow/cow.c | 21 +++++++++++++++++++++ 2 files changed, 42 insertions(+) diff --git a/filters/cache/cache.c b/filters/cache/cache.c index c912c5fb..f0ee42f1 100644 --- a/filters/cache/cache.c +++ b/filters/cache/cache.c @@ -260,6 +260,26 @@ cache_get_size (nbdkit_next *next, return size; } +/* Block size constraints. */ +static int +cache_block_size (nbdkit_next *next, void *handle, + uint32_t *minimum, uint32_t *preferred, uint32_t *maximum) +{ + if (next->block_size (next, minimum, preferred, maximum) == -1) + return -1; + + if (*minimum == 0) { /* No constraints set by the plugin. */ + *minimum = 1; + *preferred = blksize; + *maximum = 0xffffffff; + } + else if (*maximum >= blksize) { + *preferred = MAX (*preferred, blksize); + } + + return 0; +} + /* Force an early call to cache_get_size because we have to set the * backing file size and bitmap size before any other read or write * calls. @@ -716,6 +736,7 @@ static struct nbdkit_filter filter = { .get_ready = cache_get_ready, .prepare = cache_prepare, .get_size = cache_get_size, + .block_size = cache_block_size, .can_cache = cache_can_cache, .can_fast_zero = cache_can_fast_zero, .can_flush = cache_can_flush, diff --git a/filters/cow/cow.c b/filters/cow/cow.c index e111c60f..cdc5b44f 100644 --- a/filters/cow/cow.c +++ b/filters/cow/cow.c @@ -182,6 +182,26 @@ cow_get_size (nbdkit_next *next, return size; } +/* Block size constraints. */ +static int +cow_block_size (nbdkit_next *next, void *handle, + uint32_t *minimum, uint32_t *preferred, uint32_t *maximum) +{ + if (next->block_size (next, minimum, preferred, maximum) == -1) + return -1; + + if (*minimum == 0) { /* No constraints set by the plugin. */ + *minimum = 1; + *preferred = blksize; + *maximum = 0xffffffff; + } + else if (*maximum >= blksize) { + *preferred = MAX (*preferred, blksize); + } + + return 0; +} + /* Force an early call to cow_get_size because we have to set the * backing file size and bitmap size before any other read or write * calls. @@ -762,6 +782,7 @@ static struct nbdkit_filter filter = { .get_ready = cow_get_ready, .prepare = cow_prepare, .get_size = cow_get_size, + .block_size = cow_block_size, .can_write = cow_can_write, .can_flush = cow_can_flush, .can_trim = cow_can_trim, -- 2.35.1