qemu-img create: add 'nocow' option

Add 'nocow' option so that users could have a chance to set NOCOW flag to
newly created files. It's useful on btrfs file system to enhance performance.

Btrfs has low performance when hosting VM images, even more when the guest
in those VM are also using btrfs as file system. One way to mitigate this bad
performance is to turn off COW attributes on VM files. Generally, there are
two ways to turn off NOCOW on btrfs: a) by mounting fs with nodatacow, then
all newly created files will be NOCOW. b) per file. Add the NOCOW file
attribute. It could only be done to empty or new files.

This patch tries the second way, according to the option, it could add NOCOW
per file.

For most block drivers, since the create file step is in raw-posix.c, so we
can do setting NOCOW flag ioctl in raw-posix.c only.

But there are some exceptions, like block/vpc.c and block/vdi.c, they are
creating file by calling qemu_open directly. For them, do the same setting
NOCOW flag ioctl work in them separately.

[Fixed up 082.out due to the new 'nocow' creation option
--Stefan]

Signed-off-by: Chunyan Liu <cyliu@suse.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/tests/qemu-iotests/082.out b/tests/qemu-iotests/082.out
index 28309a0..413e7ef 100644
--- a/tests/qemu-iotests/082.out
+++ b/tests/qemu-iotests/082.out
@@ -66,6 +66,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: create -f qcow2 -o ? TEST_DIR/t.qcow2 128M
 Supported options:
@@ -77,6 +78,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: create -f qcow2 -o cluster_size=4k,help TEST_DIR/t.qcow2 128M
 Supported options:
@@ -88,6 +90,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: create -f qcow2 -o cluster_size=4k,? TEST_DIR/t.qcow2 128M
 Supported options:
@@ -99,6 +102,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: create -f qcow2 -o help,cluster_size=4k TEST_DIR/t.qcow2 128M
 Supported options:
@@ -110,6 +114,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: create -f qcow2 -o ?,cluster_size=4k TEST_DIR/t.qcow2 128M
 Supported options:
@@ -121,6 +126,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: create -f qcow2 -o cluster_size=4k -o help TEST_DIR/t.qcow2 128M
 Supported options:
@@ -132,6 +138,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: create -f qcow2 -o cluster_size=4k -o ? TEST_DIR/t.qcow2 128M
 Supported options:
@@ -143,6 +150,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: create -f qcow2 -o backing_file=TEST_DIR/t.qcow2,,help TEST_DIR/t.qcow2 128M
 Formatting 'TEST_DIR/t.qcow2', fmt=qcow2 size=134217728 backing_file='TEST_DIR/t.qcow2,help' encryption=off cluster_size=65536 lazy_refcounts=off 
@@ -247,6 +255,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: convert -O qcow2 -o ? TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
 Supported options:
@@ -258,6 +267,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: convert -O qcow2 -o cluster_size=4k,help TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
 Supported options:
@@ -269,6 +279,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: convert -O qcow2 -o cluster_size=4k,? TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
 Supported options:
@@ -280,6 +291,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: convert -O qcow2 -o help,cluster_size=4k TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
 Supported options:
@@ -291,6 +303,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: convert -O qcow2 -o ?,cluster_size=4k TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
 Supported options:
@@ -302,6 +315,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: convert -O qcow2 -o cluster_size=4k -o help TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
 Supported options:
@@ -313,6 +327,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: convert -O qcow2 -o cluster_size=4k -o ? TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
 Supported options:
@@ -324,6 +339,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: convert -O qcow2 -o backing_file=TEST_DIR/t.qcow2,,help TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
 qemu-img: Could not open 'TEST_DIR/t.qcow2.base': Could not open backing file: Could not open 'TEST_DIR/t.qcow2,help': No such file or directory
@@ -417,6 +433,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: amend -f qcow2 -o ? TEST_DIR/t.qcow2
 Supported options:
@@ -428,6 +445,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: amend -f qcow2 -o cluster_size=4k,help TEST_DIR/t.qcow2
 Supported options:
@@ -439,6 +457,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: amend -f qcow2 -o cluster_size=4k,? TEST_DIR/t.qcow2
 Supported options:
@@ -450,6 +469,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: amend -f qcow2 -o help,cluster_size=4k TEST_DIR/t.qcow2
 Supported options:
@@ -461,6 +481,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: amend -f qcow2 -o ?,cluster_size=4k TEST_DIR/t.qcow2
 Supported options:
@@ -472,6 +493,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: amend -f qcow2 -o cluster_size=4k -o help TEST_DIR/t.qcow2
 Supported options:
@@ -483,6 +505,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: amend -f qcow2 -o cluster_size=4k -o ? TEST_DIR/t.qcow2
 Supported options:
@@ -494,6 +517,7 @@ encryption       Encrypt the image
 cluster_size     qcow2 cluster size
 preallocation    Preallocation mode (allowed values: off, metadata)
 lazy_refcounts   Postpone refcount updates
+nocow            Turn off copy-on-write (valid only on btrfs)
 
 Testing: amend -f qcow2 -o backing_file=TEST_DIR/t.qcow2,,help TEST_DIR/t.qcow2