[Xen-devel] [PATCH] vhd-util create: add -C|nocow option

[Chunyan Liu <cyliu@xxxxxxxx>]

Add '-C' (nocow) option to vhd-util create.

Btrfs has terrible 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 (since having copy on
write for this kind of data is not useful). According to 'chattr' manpage, NOCOW
could be set to new or empty file only on btrfs, so add a option here so that
users could have a chance to set NOCOW to a vhd image.

Signed-off-by: Chunyan Liu <cyliu@xxxxxxxx>
---
 tools/blktap2/include/libvhd.h          |    1 +
 tools/blktap2/vhd/lib/libvhd.c          |   24 ++++++++++++++++++++++++
 tools/blktap2/vhd/lib/vhd-util-create.c |    8 ++++++--
 3 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/tools/blktap2/include/libvhd.h b/tools/blktap2/include/libvhd.h
index 8e854e4..fd0ca75 100644
--- a/tools/blktap2/include/libvhd.h
+++ b/tools/blktap2/include/libvhd.h
@@ -87,6 +87,7 @@
 #define VHD_OPEN_IGNORE_DISABLED   0x00010
 
 #define VHD_FLAG_CREAT_PARENT_RAW        0x00001
+#define VHD_FLAG_CREAT_NOCOW             0x00002
 
 #define vhd_flag_set(word, flag)         ((word) |= (flag))
 #define vhd_flag_clear(word, flag)       ((word) &= ~(flag))
diff --git a/tools/blktap2/vhd/lib/libvhd.c b/tools/blktap2/vhd/lib/libvhd.c
index 95eb5d6..12a9667 100644
--- a/tools/blktap2/vhd/lib/libvhd.c
+++ b/tools/blktap2/vhd/lib/libvhd.c
@@ -41,6 +41,14 @@
 #include "libvhd.h"
 #include "relative-path.h"
 
+#ifdef __linux__
+#include <linux/fs.h>
+#include <sys/ioctl.h>
+#ifndef FS_NOCOW_FL
+#define FS_NOCOW_FL                     0x00800000 /* Do not cow file */
+#endif
+#endif
+
 /* VHD uses an epoch of 12:00AM, Jan 1, 2000. This is the Unix timestamp for
  * the start of the VHD epoch. */
 #define VHD_EPOCH_START 946684800
@@ -2829,6 +2837,7 @@ __vhd_create(const char *name, const char *parent, 
uint64_t bytes, int type,
        vhd_footer_t *footer;
        vhd_header_t *header;
        uint64_t size, blks;
+        int attr;
 
        switch (type) {
        case HD_TYPE_DIFF:
@@ -2855,6 +2864,21 @@ __vhd_create(const char *name, const char *parent, 
uint64_t bytes, int type,
        if (ctx.fd == -1)
                return -errno;
 
+        if (flags & VHD_FLAG_CREAT_NOCOW) {
+            flags &= ~VHD_FLAG_CREAT_NOCOW;
+#ifdef __linux__
+            /* Set NOCOW flag to solve performance issue on fs like btrfs.
+             * This is an optimisation. The FS_IOC_SETFLAGS ioctl return value 
will
+             * be ignored since any failure of this operation should not block 
the
+             * left work.
+             */
+            if (ioctl(ctx.fd, FS_IOC_GETFLAGS, &attr) == 0) {
+                attr |= FS_NOCOW_FL;
+                ioctl(ctx.fd, FS_IOC_SETFLAGS, &attr);
+            }
+#endif
+        }
+
        ctx.file = strdup(name);
        if (!ctx.file) {
                err = -ENOMEM;
diff --git a/tools/blktap2/vhd/lib/vhd-util-create.c 
b/tools/blktap2/vhd/lib/vhd-util-create.c
index a9bdf05..be632f8 100644
--- a/tools/blktap2/vhd/lib/vhd-util-create.c
+++ b/tools/blktap2/vhd/lib/vhd-util-create.c
@@ -49,7 +49,7 @@ vhd_util_create(int argc, char **argv)
                goto usage;
 
        optind = 0;
-       while ((c = getopt(argc, argv, "n:s:rh")) != -1) {
+       while ((c = getopt(argc, argv, "n:s:rCh")) != -1) {
                switch (c) {
                case 'n':
                        name = optarg;
@@ -61,6 +61,10 @@ vhd_util_create(int argc, char **argv)
                case 'r':
                        sparse = 0;
                        break;
+                case 'C':
+                        /* NOCOW: this will remove COW for file on btrfs */ 
+                        flags |= VHD_FLAG_CREAT_NOCOW;
+                        break;
                case 'h':
                default:
                        goto usage;
@@ -75,6 +79,6 @@ vhd_util_create(int argc, char **argv)
                                  flags);
 
 usage:
-       printf("options: <-n name> <-s size (MB)> [-r reserve] [-h help]\n");
+       printf("options: <-n name> <-s size (MB)> [-r reserve] [-C nocow] [-h 
help]\n");
        return -EINVAL;
 }
-- 
1.6.0.2


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