block: Reject writethrough mode except at the root

Writethrough mode is going to become a BlockBackend feature rather than
a BDS one, so forbid it in places where we won't be able to support it
when the code finally matches the envisioned design.

We only allowed setting the cache mode of non-root nodes after the 2.5
release, so we're still free to make this change.

The target of block jobs is now always opened in a writeback mode
because it doesn't have a BlockBackend attached. This makes more sense
anyway because block jobs know when to flush. If the graph is modified
on job completion, the original cache mode moves to the new root, so
for the guest device writethough always stays enabled if it was
configured this way.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
This commit is contained in:
Kevin Wolf 2016-03-14 15:46:03 +01:00
parent b8816a4386
commit 73ac451f34
4 changed files with 68 additions and 106 deletions

View file

@ -975,6 +975,13 @@ static int bdrv_open_common(BlockDriverState *bs, BdrvChild *file,
/* Apply cache mode options */
update_flags_from_options(&bs->open_flags, opts);
if (!bs->blk && (bs->open_flags & BDRV_O_CACHE_WB) == 0) {
error_setg(errp, "Can't set writethrough mode except for the root");
ret = -EINVAL;
goto free_and_fail;
}
bdrv_set_enable_write_cache(bs, bs->open_flags & BDRV_O_CACHE_WB);
/* Open the image, either directly or using a protocol */

View file

@ -1773,6 +1773,12 @@ static void external_snapshot_prepare(BlkActionState *common,
flags |= BDRV_O_NO_BACKING;
}
/* There is no BB attached during bdrv_open(), so we can't set a
* writethrough mode. bdrv_append() will swap the WCE setting so that the
* backing file becomes unconditionally writeback (which is what backing
* files should always be) and the new overlay gets the original setting. */
flags |= BDRV_O_CACHE_WB;
assert(state->new_bs == NULL);
ret = bdrv_open(&state->new_bs, new_image_file, snapshot_ref, options,
flags, errp);
@ -2517,6 +2523,7 @@ void qmp_blockdev_change_medium(const char *device, const char *filename,
BlockBackend *blk;
BlockDriverState *medium_bs = NULL;
int bdrv_flags, ret;
bool writethrough;
QDict *options = NULL;
Error *err = NULL;
@ -2535,6 +2542,12 @@ void qmp_blockdev_change_medium(const char *device, const char *filename,
bdrv_flags &= ~(BDRV_O_TEMPORARY | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING |
BDRV_O_PROTOCOL);
/* Must open the image in writeback mode as long as no BlockBackend is
* attached. The right mode can be set as the final step after changing the
* medium. */
writethrough = !(bdrv_flags & BDRV_O_CACHE_WB);
bdrv_flags |= BDRV_O_CACHE_WB;
if (!has_read_only) {
read_only = BLOCKDEV_CHANGE_READ_ONLY_MODE_RETAIN;
}
@ -2592,6 +2605,8 @@ void qmp_blockdev_change_medium(const char *device, const char *filename,
goto fail;
}
bdrv_set_enable_write_cache(medium_bs, !writethrough);
qmp_blockdev_close_tray(device, errp);
fail:
@ -3217,7 +3232,7 @@ static void do_drive_backup(const char *device, const char *target,
goto out;
}
flags = bs->open_flags | BDRV_O_RDWR;
flags = bs->open_flags | BDRV_O_CACHE_WB | BDRV_O_RDWR;
/* See if we have a backing HD we can use to create our new image
* on top of. */
@ -3512,7 +3527,7 @@ void qmp_drive_mirror(const char *device, const char *target,
format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : bs->drv->format_name;
}
flags = bs->open_flags | BDRV_O_RDWR;
flags = bs->open_flags | BDRV_O_CACHE_WB | BDRV_O_RDWR;
source = backing_bs(bs);
if (!source && sync == MIRROR_SYNC_MODE_TOP) {
sync = MIRROR_SYNC_MODE_FULL;

View file

@ -96,36 +96,36 @@ function check_cache_all()
# bs->backing
echo -e "cache.direct=on on none0"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.direct=on | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.direct=on | grep -e "Cache" -e "[Cc]annot|[Cc]ould not|[Cc]an't"
echo -e "\ncache.direct=on on file"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.direct=on | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.direct=on | grep -e "Cache" -e "[Cc]annot|[Cc]ould not|[Cc]an't"
echo -e "\ncache.direct=on on backing"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.direct=on | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.direct=on | grep -e "Cache" -e "[Cc]annot|[Cc]ould not|[Cc]an't"
echo -e "\ncache.direct=on on backing-file"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.direct=on | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.direct=on | grep -e "Cache" -e "[Cc]annot|[Cc]ould not|[Cc]an't"
# cache.writeback is supposed to be inherited by bs->backing; bs->file
# always gets cache.writeback=on
echo -e "\n\ncache.writeback=off on none0"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.writeback=off | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.writeback=off | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.writeback=off on file"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.writeback=off | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.writeback=off | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.writeback=off on backing"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.writeback=off | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.writeback=off | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.writeback=off on backing-file"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.writeback=off | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.writeback=off | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
# cache.no-flush is supposed to be inherited by both bs->file and bs->backing
echo -e "\n\ncache.no-flush=on on none0"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.no-flush=on | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",cache.no-flush=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.no-flush=on on file"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.no-flush=on | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",file.cache.no-flush=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.no-flush=on on backing"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.no-flush=on | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.cache.no-flush=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.no-flush=on on backing-file"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.no-flush=on | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$files","$ids",backing.file.cache.no-flush=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
}
echo
@ -218,7 +218,7 @@ info block image
info block blkdebug
info block file"
echo "$hmp_cmds" | run_qemu -drive if=none,file="blkdebug::json:{\"filename\":\"$TEST_IMG\",,\"cache\":{\"writeback\":false,,\"direct\":false}}",node-name=image,file.node-name=blkdebug,file.image.node-name=file | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive if=none,file="blkdebug::json:{\"filename\":\"$TEST_IMG\",,\"cache\":{\"direct\":false}}",node-name=image,file.node-name=blkdebug,file.image.node-name=file | grep "Cache"
echo
echo "=== Check that referenced BDSes don't inherit ==="
@ -234,35 +234,35 @@ function check_cache_all_separate()
# Check cache.direct
echo -e "cache.direct=on on blk"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.direct=on | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.direct=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.direct=on on file"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.direct=on -drive "$drv_img" | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.direct=on -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.direct=on on backing"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.direct=on -drive "$drv_file" -drive "$drv_img" | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.direct=on -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.direct=on on backing-file"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.direct=on -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.direct=on -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
# Check cache.writeback
echo -e "\n\ncache.writeback=off on blk"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.writeback=off | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.writeback=off | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.writeback=off on file"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.writeback=off -drive "$drv_img" | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.writeback=off -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.writeback=off on backing"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.writeback=off -drive "$drv_file" -drive "$drv_img" | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.writeback=off -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.writeback=off on backing-file"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.writeback=off -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.writeback=off -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
# Check cache.no-flush
echo -e "\n\ncache.no-flush=on on blk"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.no-flush=on | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img",cache.no-flush=on | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.no-flush=on on file"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.no-flush=on -drive "$drv_img" | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk" -drive "$drv_file",cache.no-flush=on -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.no-flush=on on backing"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.no-flush=on -drive "$drv_file" -drive "$drv_img" | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile" -drive "$drv_bk",cache.no-flush=on -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
echo -e "\ncache.no-flush=on on backing-file"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.no-flush=on -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep "Cache"
echo "$hmp_cmds" | run_qemu -drive "$drv_bkfile",cache.no-flush=on -drive "$drv_bk" -drive "$drv_file" -drive "$drv_img" | grep -e "Cache" -e "[Cc]annot\|[Cc]ould not\|[Cc]an't"
}
echo

View file

@ -66,22 +66,13 @@ cache.writeback=off on none0
Cache mode: writeback
cache.writeback=off on file
Cache mode: writeback
Cache mode: writethrough
Cache mode: writeback
Cache mode: writeback
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
cache.writeback=off on backing
Cache mode: writeback
Cache mode: writeback
Cache mode: writethrough
Cache mode: writeback
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.writeback=off on backing-file
Cache mode: writeback
Cache mode: writeback
Cache mode: writeback
Cache mode: writethrough
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.no-flush=on on none0
@ -147,25 +138,13 @@ cache.writeback=off on none0
Cache mode: writeback
cache.writeback=off on file
Cache mode: writeback
Cache mode: writeback
Cache mode: writethrough
Cache mode: writeback
Cache mode: writeback
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
cache.writeback=off on backing
Cache mode: writeback
Cache mode: writeback
Cache mode: writeback
Cache mode: writethrough
Cache mode: writeback
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.writeback=off on backing-file
Cache mode: writeback
Cache mode: writeback
Cache mode: writeback
Cache mode: writeback
Cache mode: writethrough
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.no-flush=on on none0
@ -230,22 +209,13 @@ cache.writeback=off on none0
Cache mode: writeback, direct
cache.writeback=off on file
Cache mode: writeback, direct
Cache mode: writethrough, direct
Cache mode: writeback, direct
Cache mode: writeback, direct
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
cache.writeback=off on backing
Cache mode: writeback, direct
Cache mode: writeback, direct
Cache mode: writethrough, direct
Cache mode: writeback, direct
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.writeback=off on backing-file
Cache mode: writeback, direct
Cache mode: writeback, direct
Cache mode: writeback, direct
Cache mode: writethrough, direct
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.no-flush=on on none0
@ -306,22 +276,13 @@ cache.writeback=off on none0
Cache mode: writeback, direct
cache.writeback=off on file
Cache mode: writeback, direct
Cache mode: writethrough, direct
Cache mode: writeback, direct
Cache mode: writeback, direct
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
cache.writeback=off on backing
Cache mode: writeback, direct
Cache mode: writeback, direct
Cache mode: writethrough, direct
Cache mode: writeback, direct
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.writeback=off on backing-file
Cache mode: writeback, direct
Cache mode: writeback, direct
Cache mode: writeback, direct
Cache mode: writethrough, direct
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.no-flush=on on none0
@ -387,25 +348,13 @@ cache.writeback=off on none0
Cache mode: writeback
cache.writeback=off on file
Cache mode: writeback, direct
Cache mode: writeback
Cache mode: writethrough
Cache mode: writeback
Cache mode: writeback
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
cache.writeback=off on backing
Cache mode: writeback, direct
Cache mode: writeback
Cache mode: writeback
Cache mode: writethrough
Cache mode: writeback
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.writeback=off on backing-file
Cache mode: writeback, direct
Cache mode: writeback
Cache mode: writeback
Cache mode: writeback
Cache mode: writethrough
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.no-flush=on on none0
@ -440,7 +389,7 @@ cache.no-flush=on on backing-file
Cache mode: writethrough, direct, ignore flushes
Cache mode: writeback, direct, ignore flushes
Cache mode: writethrough, ignore flushes
Cache mode: writeback, ignore flushes
=== Check that referenced BDSes don't inherit ===
@ -722,22 +671,13 @@ cache.writeback=off on none0
Cache mode: writeback, direct
cache.writeback=off on file
Cache mode: writeback
Cache mode: writethrough
Cache mode: writeback, direct
Cache mode: writeback, direct
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,file.cache.writeback=off: Can't set writethrough mode except for the root
cache.writeback=off on backing
Cache mode: writeback
Cache mode: writeback
Cache mode: writethrough, direct
Cache mode: writeback, direct
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.writeback=off on backing-file
Cache mode: writeback
Cache mode: writeback
Cache mode: writeback, direct
Cache mode: writethrough, direct
QEMU_PROG: -drive if=none,file=TEST_DIR/t.qcow2,backing.file.filename=TEST_DIR/t.qcow2.base,node-name=image,backing.node-name=backing,backing.file.node-name=backing-file,file.node-name=file,backing.file.cache.writeback=off: Could not open backing file: Can't set writethrough mode except for the root
cache.no-flush=on on none0