aboutsummaryrefslogtreecommitdiff
diff options
context:
space:
mode:
-rw-r--r--block.c6
-rw-r--r--qemu-config.c3
-rw-r--r--qemu-img.c3
-rw-r--r--qemu-options.hx8
4 files changed, 14 insertions, 6 deletions
diff --git a/block.c b/block.c
index dbef3ae..4186a2f 100644
--- a/block.c
+++ b/block.c
@@ -448,6 +448,8 @@ int bdrv_parse_cache_flags(const char *mode, int *flags)
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 @@ int bdrv_pwrite_sync(BlockDriverState *bs, int64_t offset,
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 @@ static QemuOptsList qemu_drive_opts = {
},{
.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 @@ static void help(void)
" '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 @@ ETEXI
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 @@ These options have the same definition as they have in @option{-hdachs}.
@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 @@ The host page cache can be avoided entirely with @option{cache=none}. This will
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.