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authorChunyan Liu <cyliu@suse.com>2014-06-30 14:29:58 +0800
committerStefan Hajnoczi <stefanha@redhat.com>2014-07-01 10:15:12 +0200
commit4ab1559085d688dddf4de77f1ead3102e243e668 (patch)
treec02aaa834a94aace2ef773462cea3b4829cc48a9 /block/vpc.c
parent8698e110f8aa1cdf8ba1afdda8f2658333a4ab01 (diff)
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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>
Diffstat (limited to 'block/vpc.c')
-rw-r--r--block/vpc.c29
1 files changed, 29 insertions, 0 deletions
diff --git a/block/vpc.c b/block/vpc.c
index 798d854..8b376a4 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -29,6 +29,13 @@
#if defined(CONFIG_UUID)
#include <uuid/uuid.h>
#endif
+#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
/**************************************************************/
@@ -751,6 +758,7 @@ static int vpc_create(const char *filename, QemuOpts *opts, Error **errp)
int64_t total_size;
int disk_type;
int ret = -EIO;
+ bool nocow = false;
/* Read out options */
total_size = qemu_opt_get_size_del(opts, BLOCK_OPT_SIZE, 0);
@@ -767,6 +775,7 @@ static int vpc_create(const char *filename, QemuOpts *opts, Error **errp)
} else {
disk_type = VHD_DYNAMIC;
}
+ nocow = qemu_opt_get_bool_del(opts, BLOCK_OPT_NOCOW, false);
/* Create the file */
fd = qemu_open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY, 0644);
@@ -775,6 +784,21 @@ static int vpc_create(const char *filename, QemuOpts *opts, Error **errp)
goto out;
}
+ if (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.
+ */
+ int attr;
+ if (ioctl(fd, FS_IOC_GETFLAGS, &attr) == 0) {
+ attr |= FS_NOCOW_FL;
+ ioctl(fd, FS_IOC_SETFLAGS, &attr);
+ }
+#endif
+ }
+
/*
* Calculate matching total_size and geometry. Increase the number of
* sectors requested until we get enough (or fail). This ensures that
@@ -884,6 +908,11 @@ static QemuOptsList vpc_create_opts = {
"Type of virtual hard disk format. Supported formats are "
"{dynamic (default) | fixed} "
},
+ {
+ .name = BLOCK_OPT_NOCOW,
+ .type = QEMU_OPT_BOOL,
+ .help = "Turn off copy-on-write (valid only on btrfs)"
+ },
{ /* end of list */ }
}
};