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qemu-nbd export img and detect block if is zero with libnbd
Description of problem:
In our project,we use qemu-nbd to export a img,and use libnbd to read/write data.if the img is preallocated,we wonder the data block if is zero,we use api nbd_block_status in libnbd to get the block status,but it shows server does not support structured replies: Operation not supported.I know our qemu is too old.so,i want to know how can i know if the block in preallocated is zero or not in nbd client.
Steps to reproduce:
1.qemu-nbd -p 8889 -f raw a.img
2.the nbd client use libnbd,code is:
```c
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <assert.h>
#include <stdbool.h>
#include <errno.h>
#include <libnbd.h>
static const char *bitmap;
struct data {
bool req_one; /* input: true if req_one was passed to request */
int count; /* input: count of expected remaining calls */
bool fail; /* input: true to return failure */
bool seen_base; /* output: true if base:allocation encountered */
bool seen_dirty; /* output: true if qemu:dirty-bitmap encountered */
};
static int
cb (void *opaque, const char *metacontext, uint64_t offset,
uint32_t *entries, size_t len, int *error)
{
struct data *data = opaque;
/* libnbd does not actually verify that a server is fully compliant
* to the spec; the asserts marked [qemu-nbd] are thus dependent on
* the fact that qemu-nbd is compliant. Furthermore, qemu 5.2 and
* 6.0 disagree on whether base:allocation includes the hole bit for
* the zeroes at 512k (both answers are compliant); but we care more
* that the zeroes show up in the dirty bitmap
*/
assert (offset == 0);
assert (!*error || (data->fail && data->count == 1 && *error == EPROTO));
assert (data->count-- > 0); /* [qemu-nbd] */
if (strcmp (metacontext, LIBNBD_CONTEXT_BASE_ALLOCATION) == 0) {
assert (!data->seen_base); /* [qemu-nbd] */
data->seen_base = true;
if (data->req_one)
assert (len == 2); /* [qemu-nbd] */
else
assert ((len & 1) == 0 && len > 2); /* [qemu-nbd] */
/* Data block offset 0 size 128k */
assert (entries[0] == 131072); assert (entries[1] == 0);
if (!data->req_one) {
if (len == 4) {
/* hole|zero offset 128k size 896k */
assert (entries[2] == 917504);
assert (entries[3] == (LIBNBD_STATE_HOLE|
LIBNBD_STATE_ZERO));
}
else {
assert (len == 8);
/* hole|zero offset 128k size 384k */
assert (entries[2] == 393216);
assert (entries[3] == (LIBNBD_STATE_HOLE|
LIBNBD_STATE_ZERO));
/* allocated zero offset 512k size 64k */
assert (entries[4] == 65536);
assert (entries[5] == LIBNBD_STATE_ZERO);
/* hole|zero offset 576k size 448k */
assert (entries[6] == 458752);
assert (entries[7] == (LIBNBD_STATE_HOLE|
LIBNBD_STATE_ZERO));
}
}
}
else if (strcmp (metacontext, bitmap) == 0) {
assert (!data->seen_dirty); /* [qemu-nbd] */
data->seen_dirty = true;
assert (len == (data->req_one ? 2 : 10)); /* [qemu-nbd] */
assert (entries[0] == 65536); assert (entries[1] == 0);
if (!data->req_one) {
/* dirty block offset 64K size 64K */
assert (entries[2] == 65536); assert (entries[3] == 1);
assert (entries[4] == 393216); assert (entries[5] == 0);
/* dirty block offset 512K size 64K */
assert (entries[6] == 65536); assert (entries[7] == 1);
assert (entries[8] == 458752); assert (entries[9] == 0);
}
}
else {
fprintf (stderr, "unexpected context %s\n", metacontext);
exit (EXIT_FAILURE);
}
if (data->fail) {
/* Something NBD servers can't send */
*error = data->count == 1 ? EPROTO : ECONNREFUSED;
return -1;
}
return 0;
}
int
main (int argc, char *argv[])
{
struct nbd_handle *nbd;
int64_t exportsize;
struct data data;
char c;
if (argc < 3) {
fprintf (stderr, "%s bitmap qemu-nbd [args ...]\n", argv[0]);
exit (EXIT_FAILURE);
}
bitmap = argv[1];
nbd = nbd_create ();
if (nbd == NULL) {
fprintf (stderr, "%s\n", nbd_get_error ());
exit (EXIT_FAILURE);
}
nbd_add_meta_context (nbd, LIBNBD_CONTEXT_BASE_ALLOCATION);
nbd_add_meta_context (nbd, bitmap);
if (nbd_connect_tcp (nbd, argv[2],argv[3]) == -1) {
fprintf (stderr, "%s\n", nbd_get_error ());
exit (EXIT_FAILURE);
}
exportsize = nbd_get_size (nbd);
if (exportsize == -1) {
fprintf (stderr, "%s\n", nbd_get_error ());
exit (EXIT_FAILURE);
}
data = (struct data) { .count = 2, };
if (nbd_block_status (nbd, exportsize, 0,
(nbd_extent_callback) { .callback = cb, .user_data = &data },
0) == -1) {
fprintf (stderr, "%s\n", nbd_get_error ());
exit (EXIT_FAILURE);
}
assert (data.seen_base && data.seen_dirty);
data = (struct data) { .req_one = true, .count = 2, };
if (nbd_block_status (nbd, exportsize, 0,
(nbd_extent_callback) { .callback = cb, .user_data = &data },
LIBNBD_CMD_FLAG_REQ_ONE) == -1) {
fprintf (stderr, "%s\n", nbd_get_error ());
exit (EXIT_FAILURE);
}
assert (data.seen_base && data.seen_dirty);
/* Trigger a failed callback, to prove connection stays up. */
data = (struct data) { .count = 2, .fail = true, };
if (nbd_block_status (nbd, exportsize, 0,
(nbd_extent_callback) { .callback = cb, .user_data = &data },
0) != -1) {
fprintf (stderr, "unexpected block status success\n");
exit (EXIT_FAILURE);
}
assert (nbd_get_errno () == EPROTO && nbd_aio_is_ready (nbd));
assert (data.seen_base && data.seen_dirty);
if (nbd_pread (nbd, &c, 1, 0, 0) == -1) {
fprintf (stderr, "%s\n", nbd_get_error ());
exit (EXIT_FAILURE);
}
if (nbd_shutdown (nbd, 0) == -1) {
fprintf (stderr, "%s\n", nbd_get_error ());
exit (EXIT_FAILURE);
}
nbd_close (nbd);
exit (EXIT_SUCCESS);
}
```
3.
Additional information:
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