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qemu-nbd: Support -s and -l at the same time
qemu-nbd already has the ability to wrap a read-write layer on top of the export of a read-only image (-s) and the ability to export an internal snapshot rather than the main image in a qcow2 file (-l SNAPID). But it gave a confusing error message when trying to mix the two, because the snapshot overlay created first no longer has access to the internal snapshots of the image being wrapped. WIth a bit of finesse, this useful scenario is now supported. https://gitlab.com/qemu-project/qemu/-/work_items/159 Signed-off-by: Eric Blake <eblake@redhat.com> Message-ID: <20260408021636.794794-2-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> [eblake: improve test with Vladimir's suggestions, add lock to satisfy TSAN] Signed-off-by: Eric Blake <eblake@redhat.com>
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@@ -95,9 +95,10 @@ driver options if :option:`--image-opts` is specified.
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.. option:: -l, --load-snapshot=SNAPSHOT_PARAM
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Load an internal snapshot inside *filename* and export it
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as an read-only device, SNAPSHOT_PARAM format is
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``snapshot.id=[ID],snapshot.name=[NAME]`` or ``[ID_OR_NAME]``
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Load an internal snapshot inside *filename* and export it.
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SNAPSHOT_PARAM format is ``snapshot.id=[ID],snapshot.name=[NAME]``
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or ``[ID_OR_NAME]``. The export is read-only unless ``-s`` is also
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in use.
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.. option:: --cache=CACHE
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+19
-3
@@ -534,6 +534,7 @@ int main(int argc, char **argv)
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{
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BlockBackend *blk;
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BlockDriverState *bs;
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BlockDriverState *orig_bs;
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uint64_t dev_offset = 0;
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bool readonly = false;
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bool disconnect = false;
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@@ -716,7 +717,7 @@ int main(int argc, char **argv)
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} else {
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sn_id_or_name = optarg;
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}
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/* fall through */
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break;
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case 'r':
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readonly = true;
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flags &= ~BDRV_O_RDWR;
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@@ -1110,6 +1111,15 @@ int main(int argc, char **argv)
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bdrv_init();
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atexit(qemu_nbd_shutdown);
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/*
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* -s and -l can be used together to create read-write wrapper
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* around an internal snapshot. Otherwise, -l in isolation implies
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* read-only.
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*/
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if ((sn_opts || sn_id_or_name) && (flags & BDRV_O_SNAPSHOT) == 0) {
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readonly = true;
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flags &= ~BDRV_O_RDWR;
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}
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opts.srcpath = argv[optind];
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if (imageOpts) {
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QemuOpts *o;
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@@ -1155,13 +1165,19 @@ int main(int argc, char **argv)
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blk_set_enable_write_cache(blk, !writethrough);
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orig_bs = bs;
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if (flags & BDRV_O_SNAPSHOT) {
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WITH_GRAPH_RDLOCK_GUARD() {
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orig_bs = bs->backing->bs;
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}
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}
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if (sn_opts) {
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ret = bdrv_snapshot_load_tmp(bs,
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ret = bdrv_snapshot_load_tmp(orig_bs,
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qemu_opt_get(sn_opts, SNAPSHOT_OPT_ID),
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qemu_opt_get(sn_opts, SNAPSHOT_OPT_NAME),
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&local_err);
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} else if (sn_id_or_name) {
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ret = bdrv_snapshot_load_tmp_by_id_or_name(bs, sn_id_or_name,
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ret = bdrv_snapshot_load_tmp_by_id_or_name(orig_bs, sn_id_or_name,
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&local_err);
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}
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if (ret < 0) {
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Executable
+81
@@ -0,0 +1,81 @@
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#!/usr/bin/env bash
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# group: rw quick
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#
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# Test qemu-nbd snapshot handling
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#
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# Copyright (C) Red Hat, Inc.
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#
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# SPDX-License-Identifier: GPL-2.0-or-later
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seq="$(basename $0)"
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echo "QA output created by $seq"
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status=1 # failure is the default!
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_cleanup()
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{
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_cleanup_test_img
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nbd_server_stop
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}
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trap "_cleanup; exit \$status" 0 1 2 3 15
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# get standard environment, filters and checks
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cd ..
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. ./common.rc
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. ./common.filter
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. ./common.nbd
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_supported_fmt qcow2
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_supported_proto file
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_supported_os Linux
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_require_command QEMU_NBD
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echo
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echo "=== Initial image setup ==="
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echo
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# Create qcow2 image with one internal snapshot
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_make_test_img 10M
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$QEMU_IO -c 'w -P 1 3M 1M' -f $IMGFMT "$TEST_IMG" | _filter_qemu_io
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$QEMU_IMG snapshot -c snap -f $IMGFMT "$TEST_IMG" | _filter_qemu_img
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$QEMU_IO -c 'w -P 2 3M 1M' -f $IMGFMT "$TEST_IMG" | _filter_qemu_io
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echo
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echo "=== Main image in snapshot mode ==="
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echo
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nbd_server_start_unix_socket -s -f qcow2 "$TEST_IMG"
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$QEMU_IO -c 'r -P 2 3M 1M' -c 'w -P 3 3M 1M' -c 'r -P 3 3M 1M' -f raw \
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nbd:unix:$nbd_unix_socket | _filter_qemu_io
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nbd_server_stop
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echo
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echo "=== Read-only snapshot access ==="
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echo
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nbd_server_start_unix_socket -l snap -f qcow2 "$TEST_IMG"
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$QEMU_IO -r -c 'r -P 1 3M 1M' -f raw \
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nbd:unix:$nbd_unix_socket | _filter_qemu_io
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$QEMU_IO -c 'r -P 1 3M 1M' -f raw \
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nbd:unix:$nbd_unix_socket 2>&1 | _filter_qemu_io | _filter_testdir
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nbd_server_stop
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echo
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echo "=== Combine -s and -l for read-write internal snapshot access ==="
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echo
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nbd_server_start_unix_socket -s -l snap -f qcow2 "$TEST_IMG"
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$QEMU_IO -c 'r -P 1 3M 1M' -c 'w -P 3 3M 1M' -c 'r -P 3 3M 1M' -f raw \
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nbd:unix:$nbd_unix_socket | _filter_qemu_io
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nbd_server_stop
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echo
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echo "=== Check that original image is unchanged ==="
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echo
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$QEMU_IO -c 'r -P 2 3M 1M' -f $IMGFMT "$TEST_IMG" | _filter_qemu_io
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# success, all done
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echo '*** done'
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rm -f $seq.full
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status=0
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@@ -0,0 +1,39 @@
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QA output created by qemu-nbd-snapshots
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=== Initial image setup ===
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Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=10485760
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wrote 1048576/1048576 bytes at offset 3145728
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1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
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wrote 1048576/1048576 bytes at offset 3145728
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1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
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=== Main image in snapshot mode ===
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read 1048576/1048576 bytes at offset 3145728
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1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
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wrote 1048576/1048576 bytes at offset 3145728
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1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
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read 1048576/1048576 bytes at offset 3145728
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1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
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=== Read-only snapshot access ===
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read 1048576/1048576 bytes at offset 3145728
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1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
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qemu-io: can't open device nbd:unix:SOCK_DIR/qemu-nbd.sock: Could not open image: Permission denied
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=== Combine -s and -l for read-write internal snapshot access ===
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read 1048576/1048576 bytes at offset 3145728
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1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
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wrote 1048576/1048576 bytes at offset 3145728
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1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
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read 1048576/1048576 bytes at offset 3145728
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1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
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=== Check that original image is unchanged ===
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read 1048576/1048576 bytes at offset 3145728
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1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
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*** done
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