block: add cache=directsync parameter to -drive
This patch adds -drive cache=directsync for O_DIRECT | O_SYNC host file
I/O with no disk write cache presented to the guest.
This mode is useful when guests may not be sending flushes when
appropriate and therefore leave data at risk in case of power failure.
When cache=directsync is used, write operations are only completed to
the guest when data is safely on disk.
This new mode is like cache=writethrough but it bypasses the host page
cache.
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/block.c b/block.c
index dbef3ae..4186a2f 100644
--- a/block.c
+++ b/block.c
@@ -448,6 +448,8 @@
if (!strcmp(mode, "off") || !strcmp(mode, "none")) {
*flags |= BDRV_O_NOCACHE | BDRV_O_CACHE_WB;
+ } else if (!strcmp(mode, "directsync")) {
+ *flags |= BDRV_O_NOCACHE;
} else if (!strcmp(mode, "writeback")) {
*flags |= BDRV_O_CACHE_WB;
} else if (!strcmp(mode, "unsafe")) {
@@ -1188,8 +1190,8 @@
return ret;
}
- /* No flush needed for cache=writethrough, it uses O_DSYNC */
- if ((bs->open_flags & BDRV_O_CACHE_MASK) != 0) {
+ /* No flush needed for cache modes that use O_DSYNC */
+ if ((bs->open_flags & BDRV_O_CACHE_WB) != 0) {
bdrv_flush(bs);
}
diff --git a/qemu-config.c b/qemu-config.c
index 1eb6b9a..139e077 100644
--- a/qemu-config.c
+++ b/qemu-config.c
@@ -55,7 +55,8 @@
},{
.name = "cache",
.type = QEMU_OPT_STRING,
- .help = "host cache usage (none, writeback, writethrough, unsafe)",
+ .help = "host cache usage (none, writeback, writethrough, "
+ "directsync, unsafe)",
},{
.name = "aio",
.type = QEMU_OPT_STRING,
diff --git a/qemu-img.c b/qemu-img.c
index 5e203c2..10a3a8b 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -66,7 +66,8 @@
" 'filename' is a disk image filename\n"
" 'fmt' is the disk image format. It is guessed automatically in most cases\n"
" 'cache' is the cache mode used to write the output disk image, the valid\n"
- " options are: 'none', 'writeback' (default), 'writethrough' and 'unsafe'\n"
+ " options are: 'none', 'writeback' (default), 'writethrough', 'directsync'\n"
+ " and 'unsafe'\n"
" 'size' is the disk image size in bytes. Optional suffixes\n"
" 'k' or 'K' (kilobyte, 1024), 'M' (megabyte, 1024k), 'G' (gigabyte, 1024M)\n"
" and T (terabyte, 1024G) are supported. 'b' is ignored.\n"
diff --git a/qemu-options.hx b/qemu-options.hx
index d86815d..35d95d1 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -133,7 +133,7 @@
DEF("drive", HAS_ARG, QEMU_OPTION_drive,
"-drive [file=file][,if=type][,bus=n][,unit=m][,media=d][,index=i]\n"
" [,cyls=c,heads=h,secs=s[,trans=t]][,snapshot=on|off]\n"
- " [,cache=writethrough|writeback|none|unsafe][,format=f]\n"
+ " [,cache=writethrough|writeback|none|directsync|unsafe][,format=f]\n"
" [,serial=s][,addr=A][,id=name][,aio=threads|native]\n"
" [,readonly=on|off]\n"
" use 'file' as a drive image\n", QEMU_ARCH_ALL)
@@ -164,7 +164,7 @@
@item snapshot=@var{snapshot}
@var{snapshot} is "on" or "off" and allows to enable snapshot for given drive (see @option{-snapshot}).
@item cache=@var{cache}
-@var{cache} is "none", "writeback", "unsafe", or "writethrough" and controls how the host cache is used to access block data.
+@var{cache} is "none", "writeback", "unsafe", "directsync" or "writethrough" and controls how the host cache is used to access block data.
@item aio=@var{aio}
@var{aio} is "threads", or "native" and selects between pthread based disk I/O and native Linux AIO.
@item format=@var{format}
@@ -199,6 +199,10 @@
attempt to do disk IO directly to the guests memory. QEMU may still perform
an internal copy of the data.
+The host page cache can be avoided while only sending write notifications to
+the guest when the data has been reported as written by the storage subsystem
+using @option{cache=directsync}.
+
Some block drivers perform badly with @option{cache=writethrough}, most notably,
qcow2. If performance is more important than correctness,
@option{cache=writeback} should be used with qcow2.
