Currently, behavior on copy-before-write operation failure is simple:
report error to the guest.
Let's implement alternative behavior: break the whole copy-before-write
process (and corresponding backup job or NBD client) but keep guest
working. It's needed if we consider guest stability as more important.
The realisation is simple: on copy-before-write failure we set
s->snapshot_ret and continue guest operations. s->snapshot_ret being
set will lead to all further snapshot API requests. Note that all
in-flight snapshot-API requests may still success: we do wait for them
on BREAK_SNAPSHOT-failure path in cbw_do_copy_before_write().
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
---
qapi/block-core.json | 25 ++++++++++++++++++++++++-
block/copy-before-write.c | 32 ++++++++++++++++++++++++++++++--
2 files changed, 54 insertions(+), 3 deletions(-)
diff --git a/qapi/block-core.json b/qapi/block-core.json
index beeb91952a..085f1666af 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -4163,6 +4163,25 @@
'base': 'BlockdevOptionsGenericFormat',
'data': { '*bottom': 'str' } }
+##
+# @OnCbwError:
+#
+# An enumeration of possible behaviors for copy-before-write operation
+# failures.
+#
+# @break-guest-write: report the error to the guest. This way, the guest
+# will not be able to overwrite areas that cannot be
+# backed up, so the backup process remains valid.
+#
+# @break-snapshot: continue guest write. Doing so will make the provided
+# snapshot state invalid and any backup or export
+# process based on it will finally fail.
+#
+# Since: 7.1
+##
+{ 'enum': 'OnCbwError',
+ 'data': [ 'break-guest-write', 'break-snapshot' ] }
+
##
# @BlockdevOptionsCbw:
#
@@ -4184,11 +4203,15 @@
# modifications (or removing) of specified bitmap doesn't
# influence the filter. (Since 7.0)
#
+# @on-cbw-error: Behavior on failure of copy-before-write operation.
+# Default is @break-guest-write. (Since 7.0)
+#
# Since: 6.2
##
{ 'struct': 'BlockdevOptionsCbw',
'base': 'BlockdevOptionsGenericFormat',
- 'data': { 'target': 'BlockdevRef', '*bitmap': 'BlockDirtyBitmap' } }
+ 'data': { 'target': 'BlockdevRef', '*bitmap': 'BlockDirtyBitmap',
+ '*on-cbw-error': 'OnCbwError' } }
##
# @BlockdevOptions:
diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index 6877ff893a..ffb05d22f8 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -41,6 +41,7 @@
typedef struct BDRVCopyBeforeWriteState {
BlockCopyState *bcs;
BdrvChild *target;
+ OnCbwError on_cbw_error;
/*
* @lock: protects access to @access_bitmap, @done_bitmap and
@@ -65,6 +66,14 @@ typedef struct BDRVCopyBeforeWriteState {
* node. These areas must not be rewritten by guest.
*/
BlockReqList frozen_read_reqs;
+
+ /*
+ * @snapshot_error is normally zero. But on first copy-before-write failure
+ * when @on_cbw_error == ON_CBW_ERROR_BREAK_SNAPSHOT, @snapshot_error takes
+ * value of this error (<0). After that all in-flight and further
+ * snapshot-API requests will fail with that error.
+ */
+ int snapshot_error;
} BDRVCopyBeforeWriteState;
static coroutine_fn int cbw_co_preadv(
@@ -95,16 +104,27 @@ static coroutine_fn int cbw_do_copy_before_write(BlockDriverState *bs,
return 0;
}
+ if (s->snapshot_error) {
+ return 0;
+ }
+
off = QEMU_ALIGN_DOWN(offset, cluster_size);
end = QEMU_ALIGN_UP(offset + bytes, cluster_size);
ret = block_copy(s->bcs, off, end - off, true);
- if (ret < 0) {
+ if (ret < 0 && s->on_cbw_error == ON_CBW_ERROR_BREAK_GUEST_WRITE) {
return ret;
}
WITH_QEMU_LOCK_GUARD(&s->lock) {
- bdrv_set_dirty_bitmap(s->done_bitmap, off, end - off);
+ if (ret < 0) {
+ assert(s->on_cbw_error == ON_CBW_ERROR_BREAK_SNAPSHOT);
+ if (!s->snapshot_error) {
+ s->snapshot_error = ret;
+ }
+ } else {
+ bdrv_set_dirty_bitmap(s->done_bitmap, off, end - off);
+ }
reqlist_wait_all(&s->frozen_read_reqs, off, end - off, &s->lock);
}
@@ -176,6 +196,11 @@ static BlockReq *cbw_snapshot_read_lock(BlockDriverState *bs,
QEMU_LOCK_GUARD(&s->lock);
+ if (s->snapshot_error) {
+ g_free(req);
+ return NULL;
+ }
+
if (bdrv_dirty_bitmap_next_zero(s->access_bitmap, offset, bytes) != -1) {
g_free(req);
return NULL;
@@ -351,6 +376,7 @@ static BlockdevOptions *cbw_parse_options(QDict *options, Error **errp)
* object for original options.
*/
qdict_extract_subqdict(options, NULL, "bitmap");
+ qdict_del(options, "on-cbw-error");
out:
visit_free(v);
@@ -394,6 +420,8 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
return -EINVAL;
}
}
+ s->on_cbw_error = opts->has_on_cbw_error ? opts->on_cbw_error :
+ ON_CBW_ERROR_BREAK_GUEST_WRITE;
bs->total_sectors = bs->file->bs->total_sectors;
bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
--
2.35.1