* [Qemu-devel] [5485] Expand cache= option and use write-through caching by default
@ 2008-10-14 14:42 Anthony Liguori
[not found] ` <6AE33345-3374-44F0-8528-B26FF81860EA@hotmail.com>
0 siblings, 1 reply; 3+ messages in thread
From: Anthony Liguori @ 2008-10-14 14:42 UTC (permalink / raw)
To: qemu-devel
Revision: 5485
http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=5485
Author: aliguori
Date: 2008-10-14 14:42:54 +0000 (Tue, 14 Oct 2008)
Log Message:
-----------
Expand cache= option and use write-through caching by default
This patch changes the cache= option to accept none, writeback, or writethough
to control the host page cache behavior. By default, writethrough caching is
now used which internally is implemented by using O_DSYNC to open the disk
images. When using -snapshot, writeback is used by default since data integrity
it not at all an issue.
cache=none has the same behavior as cache=off previously. The later syntax is
still supported by now deprecated. I also cleaned up the O_DIRECT
implementation to avoid many of the #ifdefs.
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Modified Paths:
--------------
trunk/block-raw-posix.c
trunk/block-raw-win32.c
trunk/block.c
trunk/block.h
trunk/qemu-doc.texi
trunk/qemu-nbd.c
trunk/vl.c
Modified: trunk/block-raw-posix.c
===================================================================
--- trunk/block-raw-posix.c 2008-10-13 21:08:55 UTC (rev 5484)
+++ trunk/block-raw-posix.c 2008-10-14 14:42:54 UTC (rev 5485)
@@ -73,6 +73,11 @@
#define DEBUG_BLOCK_PRINT(formatCstr, args...)
#endif
+/* Approximate O_DIRECT with O_DSYNC if O_DIRECT isn't available */
+#ifndef O_DIRECT
+#define O_DIRECT O_DSYNC
+#endif
+
#define FTYPE_FILE 0
#define FTYPE_CD 1
#define FTYPE_FD 2
@@ -101,9 +106,7 @@
int fd_got_error;
int fd_media_changed;
#endif
-#if defined(O_DIRECT)
uint8_t* aligned_buf;
-#endif
} BDRVRawState;
static int posix_aio_init(void);
@@ -129,10 +132,13 @@
}
if (flags & BDRV_O_CREAT)
open_flags |= O_CREAT | O_TRUNC;
-#ifdef O_DIRECT
- if (flags & BDRV_O_DIRECT)
+
+ /* Use O_DSYNC for write-through caching, no flags for write-back caching,
+ * and O_DIRECT for no caching. */
+ if ((flags & BDRV_O_NOCACHE))
open_flags |= O_DIRECT;
-#endif
+ else if (!(flags & BDRV_O_CACHE_WB))
+ open_flags |= O_DSYNC;
s->type = FTYPE_FILE;
@@ -146,9 +152,8 @@
s->fd = fd;
for (i = 0; i < RAW_FD_POOL_SIZE; i++)
s->fd_pool[i] = -1;
-#if defined(O_DIRECT)
s->aligned_buf = NULL;
- if (flags & BDRV_O_DIRECT) {
+ if ((flags & BDRV_O_NOCACHE)) {
s->aligned_buf = qemu_memalign(512, ALIGNED_BUFFER_SIZE);
if (s->aligned_buf == NULL) {
ret = -errno;
@@ -156,7 +161,6 @@
return ret;
}
}
-#endif
return 0;
}
@@ -281,7 +285,6 @@
}
-#if defined(O_DIRECT)
/*
* offset and count are in bytes and possibly not aligned. For files opened
* with O_DIRECT, necessary alignments are ensured before calling
@@ -432,12 +435,6 @@
return raw_pwrite_aligned(bs, offset, buf, count) + sum;
}
-#else
-#define raw_pread raw_pread_aligned
-#define raw_pwrite raw_pwrite_aligned
-#endif
-
-
#ifdef CONFIG_AIO
/***********************************************************/
/* Unix AIO using POSIX AIO */
@@ -661,7 +658,6 @@
* If O_DIRECT is used and the buffer is not aligned fall back
* to synchronous IO.
*/
-#if defined(O_DIRECT)
BDRVRawState *s = bs->opaque;
if (unlikely(s->aligned_buf != NULL && ((uintptr_t) buf % 512))) {
@@ -672,7 +668,6 @@
qemu_bh_schedule(bh);
return &acb->common;
}
-#endif
acb = raw_aio_setup(bs, sector_num, buf, nb_sectors, cb, opaque);
if (!acb)
@@ -694,7 +689,6 @@
* If O_DIRECT is used and the buffer is not aligned fall back
* to synchronous IO.
*/
-#if defined(O_DIRECT)
BDRVRawState *s = bs->opaque;
if (unlikely(s->aligned_buf != NULL && ((uintptr_t) buf % 512))) {
@@ -705,7 +699,6 @@
qemu_bh_schedule(bh);
return &acb->common;
}
-#endif
acb = raw_aio_setup(bs, sector_num, (uint8_t*)buf, nb_sectors, cb, opaque);
if (!acb)
@@ -770,10 +763,8 @@
if (s->fd >= 0) {
close(s->fd);
s->fd = -1;
-#if defined(O_DIRECT)
if (s->aligned_buf != NULL)
qemu_free(s->aligned_buf);
-#endif
}
raw_close_fd_pool(s);
}
@@ -1003,10 +994,12 @@
open_flags |= O_RDONLY;
bs->read_only = 1;
}
-#ifdef O_DIRECT
- if (flags & BDRV_O_DIRECT)
+ /* Use O_DSYNC for write-through caching, no flags for write-back caching,
+ * and O_DIRECT for no caching. */
+ if ((flags & BDRV_O_NOCACHE))
open_flags |= O_DIRECT;
-#endif
+ else if (!(flags & BDRV_O_CACHE_WB))
+ open_flags |= O_DSYNC;
s->type = FTYPE_FILE;
#if defined(__linux__)
Modified: trunk/block-raw-win32.c
===================================================================
--- trunk/block-raw-win32.c 2008-10-13 21:08:55 UTC (rev 5484)
+++ trunk/block-raw-win32.c 2008-10-14 14:42:54 UTC (rev 5485)
@@ -104,8 +104,10 @@
#else
overlapped = FILE_ATTRIBUTE_NORMAL;
#endif
- if (flags & BDRV_O_DIRECT)
+ if ((flags & BDRV_O_NOCACHE))
overlapped |= FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH;
+ else if (!(flags & BDRV_O_CACHE_WB))
+ overlapped |= FILE_FLAG_WRITE_THROUGH;
s->hfile = CreateFile(filename, access_flags,
FILE_SHARE_READ, NULL,
create_flags, overlapped, NULL);
@@ -440,8 +442,10 @@
#else
overlapped = FILE_ATTRIBUTE_NORMAL;
#endif
- if (flags & BDRV_O_DIRECT)
+ if ((flags & BDRV_O_NOCACHE))
overlapped |= FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH;
+ else if (!(flags & BDRV_O_CACHE_WB))
+ overlapped |= FILE_FLAG_WRITE_THROUGH;
s->hfile = CreateFile(filename, access_flags,
FILE_SHARE_READ, NULL,
create_flags, overlapped, NULL);
Modified: trunk/block.c
===================================================================
--- trunk/block.c 2008-10-13 21:08:55 UTC (rev 5484)
+++ trunk/block.c 2008-10-14 14:42:54 UTC (rev 5485)
@@ -395,12 +395,12 @@
/* Note: for compatibility, we open disk image files as RDWR, and
RDONLY as fallback */
if (!(flags & BDRV_O_FILE))
- open_flags = BDRV_O_RDWR | (flags & BDRV_O_DIRECT);
+ open_flags = BDRV_O_RDWR | (flags & BDRV_O_CACHE_MASK);
else
open_flags = flags & ~(BDRV_O_FILE | BDRV_O_SNAPSHOT);
ret = drv->bdrv_open(bs, filename, open_flags);
if ((ret == -EACCES || ret == -EPERM) && !(flags & BDRV_O_FILE)) {
- ret = drv->bdrv_open(bs, filename, BDRV_O_RDONLY);
+ ret = drv->bdrv_open(bs, filename, open_flags & ~BDRV_O_RDWR);
bs->read_only = 1;
}
if (ret < 0) {
@@ -427,7 +427,7 @@
}
path_combine(backing_filename, sizeof(backing_filename),
filename, bs->backing_file);
- if (bdrv_open(bs->backing_hd, backing_filename, 0) < 0)
+ if (bdrv_open(bs->backing_hd, backing_filename, open_flags) < 0)
goto fail;
}
Modified: trunk/block.h
===================================================================
--- trunk/block.h 2008-10-13 21:08:55 UTC (rev 5484)
+++ trunk/block.h 2008-10-14 14:42:54 UTC (rev 5485)
@@ -47,8 +47,11 @@
use a disk image format on top of
it (default for
bdrv_file_open()) */
-#define BDRV_O_DIRECT 0x0020
+#define BDRV_O_NOCACHE 0x0020 /* do not use the host page cache */
+#define BDRV_O_CACHE_WB 0x0040 /* use write-back caching */
+#define BDRV_O_CACHE_MASK (BDRV_O_NOCACHE | BDRV_O_CACHE_WB)
+
void bdrv_info(void);
void bdrv_info_stats(void);
Modified: trunk/qemu-doc.texi
===================================================================
--- trunk/qemu-doc.texi 2008-10-13 21:08:55 UTC (rev 5484)
+++ trunk/qemu-doc.texi 2008-10-14 14:42:54 UTC (rev 5485)
@@ -267,13 +267,28 @@
@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 "on" or "off" and allows to disable host cache to access data.
+@var{cache} is "none", "writeback", or "writethrough" and controls how the host cache is used to access block data.
@item format=@var{format}
Specify which disk @var{format} will be used rather than detecting
the format. Can be used to specifiy format=raw to avoid interpreting
an untrusted format header.
@end table
+By default, writethrough caching is used for all block device. This means that
+the host page cache will be used to read and write data but write notification
+will be sent to the guest only when the data has been reported as written by
+the storage subsystem.
+
+Writeback caching will report data writes as completed as soon as the data is
+present in the host page cache. This is safe as long as you trust your host.
+If your host crashes or loses power, then the guest may experience data
+corruption. When using the @option{-snapshot} option, writeback caching is
+used by default.
+
+The host page 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.
+
Instead of @option{-cdrom} you can use:
@example
qemu -drive file=file,index=2,media=cdrom
Modified: trunk/qemu-nbd.c
===================================================================
--- trunk/qemu-nbd.c 2008-10-13 21:08:55 UTC (rev 5484)
+++ trunk/qemu-nbd.c 2008-10-14 14:42:54 UTC (rev 5485)
@@ -232,7 +232,7 @@
flags |= BDRV_O_SNAPSHOT;
break;
case 'n':
- flags |= BDRV_O_DIRECT;
+ flags |= BDRV_O_NOCACHE;
break;
case 'b':
bindto = optarg;
Modified: trunk/vl.c
===================================================================
--- trunk/vl.c 2008-10-13 21:08:55 UTC (rev 5484)
+++ trunk/vl.c 2008-10-14 14:42:54 UTC (rev 5485)
@@ -5648,10 +5648,12 @@
}
if (get_param_value(buf, sizeof(buf), "cache", str)) {
- if (!strcmp(buf, "off"))
+ if (!strcmp(buf, "off") || !strcmp(buf, "none"))
cache = 0;
- else if (!strcmp(buf, "on"))
+ else if (!strcmp(buf, "writethrough"))
cache = 1;
+ else if (!strcmp(buf, "writeback"))
+ cache = 2;
else {
fprintf(stderr, "qemu: invalid cache option\n");
return -1;
@@ -5770,10 +5772,14 @@
if (!file[0])
return 0;
bdrv_flags = 0;
- if (snapshot)
+ if (snapshot) {
bdrv_flags |= BDRV_O_SNAPSHOT;
- if (!cache)
- bdrv_flags |= BDRV_O_DIRECT;
+ cache = 2; /* always use write-back with snapshot */
+ }
+ if (cache == 0) /* no caching */
+ bdrv_flags |= BDRV_O_NOCACHE;
+ else if (cache == 2) /* write-back */
+ bdrv_flags |= BDRV_O_CACHE_WB;
if (bdrv_open2(bdrv, file, bdrv_flags, drv) < 0 || qemu_key_check(bdrv, file)) {
fprintf(stderr, "qemu: could not open disk image %s\n",
file);
@@ -8145,7 +8151,7 @@
"-cdrom file use 'file' as IDE cdrom image (cdrom is ide1 master)\n"
"-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=on|off][,format=f]\n"
+ " [,cache=writethrough|writeback|none][,format=f]\n"
" use 'file' as a drive image\n"
"-mtdblock file use 'file' as on-board Flash memory image\n"
"-sd file use 'file' as SecureDigital card image\n"
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [Qemu-devel] [5485] Expand cache= option and use write-through caching by default
[not found] ` <6AE33345-3374-44F0-8528-B26FF81860EA@hotmail.com>
@ 2008-10-14 16:19 ` C.W. Betts
2008-10-14 18:01 ` Anthony Liguori
0 siblings, 1 reply; 3+ messages in thread
From: C.W. Betts @ 2008-10-14 16:19 UTC (permalink / raw)
To: qemu-devel
[-- Attachment #1: Type: text/plain, Size: 13920 bytes --]
Mac OS X doesn't seem to have O_DSYNC in it:
gcc-4.2 -O2 -g -fno-strict-aliasing -Wall -Wundef -Wendif-labels -
Wwrite-strings -mdynamic-no-pic -m32 -I. -I/Users/cwbetts/makestuff/
qemu-allmac/src -MMD -MP -MT block-raw-posix.o -D_GNU_SOURCE -
D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -I/Users/cwbetts/makestuff/
qemu-allmac/src/slirp -c -o block-raw-posix.o /Users/cwbetts/makestuff/
qemu-allmac/src/block-raw-posix.c
/Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c: In
function ‘raw_open’:
/Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c:139: error:
‘O_DSYNC’ undeclared (first use in this function)
/Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c:139: error:
(Each undeclared identifier is reported only once
/Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c:139: error:
for each function it appears in.)
/Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c: In
function ‘hdev_open’:
/Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c:1000:
error: ‘O_DSYNC’ undeclared (first use in this function)
On Oct 14, 2008, at 8:42 AM, Anthony Liguori wrote:
> Revision: 5485
> http://svn.sv.gnu.org/viewvc/?
> view=rev&root=qemu&revision=5485
> Author: aliguori
> Date: 2008-10-14 14:42:54 +0000 (Tue, 14 Oct 2008)
>
> Log Message:
> -----------
> Expand cache= option and use write-through caching by default
>
> This patch changes the cache= option to accept none, writeback, or
> writethough
> to control the host page cache behavior. By default, writethrough
> caching is
> now used which internally is implemented by using O_DSYNC to open
> the disk
> images. When using -snapshot, writeback is used by default since
> data integrity
> it not at all an issue.
>
> cache=none has the same behavior as cache=off previously. The later
> syntax is
> still supported by now deprecated. I also cleaned up the O_DIRECT
> implementation to avoid many of the #ifdefs.
>
> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
>
> Modified Paths:
> --------------
> trunk/block-raw-posix.c
> trunk/block-raw-win32.c
> trunk/block.c
> trunk/block.h
> trunk/qemu-doc.texi
> trunk/qemu-nbd.c
> trunk/vl.c
>
> Modified: trunk/block-raw-posix.c
> ===================================================================
> --- trunk/block-raw-posix.c 2008-10-13 21:08:55 UTC (rev 5484)
> +++ trunk/block-raw-posix.c 2008-10-14 14:42:54 UTC (rev 5485)
> @@ -73,6 +73,11 @@
> #define DEBUG_BLOCK_PRINT(formatCstr, args...)
> #endif
>
> +/* Approximate O_DIRECT with O_DSYNC if O_DIRECT isn't available */
> +#ifndef O_DIRECT
> +#define O_DIRECT O_DSYNC
> +#endif
> +
> #define FTYPE_FILE 0
> #define FTYPE_CD 1
> #define FTYPE_FD 2
> @@ -101,9 +106,7 @@
> int fd_got_error;
> int fd_media_changed;
> #endif
> -#if defined(O_DIRECT)
> uint8_t* aligned_buf;
> -#endif
> } BDRVRawState;
>
> static int posix_aio_init(void);
> @@ -129,10 +132,13 @@
> }
> if (flags & BDRV_O_CREAT)
> open_flags |= O_CREAT | O_TRUNC;
> -#ifdef O_DIRECT
> - if (flags & BDRV_O_DIRECT)
> +
> + /* Use O_DSYNC for write-through caching, no flags for write-
> back caching,
> + * and O_DIRECT for no caching. */
> + if ((flags & BDRV_O_NOCACHE))
> open_flags |= O_DIRECT;
> -#endif
> + else if (!(flags & BDRV_O_CACHE_WB))
> + open_flags |= O_DSYNC;
>
> s->type = FTYPE_FILE;
>
> @@ -146,9 +152,8 @@
> s->fd = fd;
> for (i = 0; i < RAW_FD_POOL_SIZE; i++)
> s->fd_pool[i] = -1;
> -#if defined(O_DIRECT)
> s->aligned_buf = NULL;
> - if (flags & BDRV_O_DIRECT) {
> + if ((flags & BDRV_O_NOCACHE)) {
> s->aligned_buf = qemu_memalign(512, ALIGNED_BUFFER_SIZE);
> if (s->aligned_buf == NULL) {
> ret = -errno;
> @@ -156,7 +161,6 @@
> return ret;
> }
> }
> -#endif
> return 0;
> }
>
> @@ -281,7 +285,6 @@
> }
>
>
> -#if defined(O_DIRECT)
> /*
> * offset and count are in bytes and possibly not aligned. For files
> opened
> * with O_DIRECT, necessary alignments are ensured before calling
> @@ -432,12 +435,6 @@
> return raw_pwrite_aligned(bs, offset, buf, count) + sum;
> }
>
> -#else
> -#define raw_pread raw_pread_aligned
> -#define raw_pwrite raw_pwrite_aligned
> -#endif
> -
> -
> #ifdef CONFIG_AIO
> /***********************************************************/
> /* Unix AIO using POSIX AIO */
> @@ -661,7 +658,6 @@
> * If O_DIRECT is used and the buffer is not aligned fall back
> * to synchronous IO.
> */
> -#if defined(O_DIRECT)
> BDRVRawState *s = bs->opaque;
>
> if (unlikely(s->aligned_buf != NULL && ((uintptr_t) buf % 512))) {
> @@ -672,7 +668,6 @@
> qemu_bh_schedule(bh);
> return &acb->common;
> }
> -#endif
>
> acb = raw_aio_setup(bs, sector_num, buf, nb_sectors, cb, opaque);
> if (!acb)
> @@ -694,7 +689,6 @@
> * If O_DIRECT is used and the buffer is not aligned fall back
> * to synchronous IO.
> */
> -#if defined(O_DIRECT)
> BDRVRawState *s = bs->opaque;
>
> if (unlikely(s->aligned_buf != NULL && ((uintptr_t) buf % 512))) {
> @@ -705,7 +699,6 @@
> qemu_bh_schedule(bh);
> return &acb->common;
> }
> -#endif
>
> acb = raw_aio_setup(bs, sector_num, (uint8_t*)buf, nb_sectors,
> cb, opaque);
> if (!acb)
> @@ -770,10 +763,8 @@
> if (s->fd >= 0) {
> close(s->fd);
> s->fd = -1;
> -#if defined(O_DIRECT)
> if (s->aligned_buf != NULL)
> qemu_free(s->aligned_buf);
> -#endif
> }
> raw_close_fd_pool(s);
> }
> @@ -1003,10 +994,12 @@
> open_flags |= O_RDONLY;
> bs->read_only = 1;
> }
> -#ifdef O_DIRECT
> - if (flags & BDRV_O_DIRECT)
> + /* Use O_DSYNC for write-through caching, no flags for write-
> back caching,
> + * and O_DIRECT for no caching. */
> + if ((flags & BDRV_O_NOCACHE))
> open_flags |= O_DIRECT;
> -#endif
> + else if (!(flags & BDRV_O_CACHE_WB))
> + open_flags |= O_DSYNC;
>
> s->type = FTYPE_FILE;
> #if defined(__linux__)
>
> Modified: trunk/block-raw-win32.c
> ===================================================================
> --- trunk/block-raw-win32.c 2008-10-13 21:08:55 UTC (rev 5484)
> +++ trunk/block-raw-win32.c 2008-10-14 14:42:54 UTC (rev 5485)
> @@ -104,8 +104,10 @@
> #else
> overlapped = FILE_ATTRIBUTE_NORMAL;
> #endif
> - if (flags & BDRV_O_DIRECT)
> + if ((flags & BDRV_O_NOCACHE))
> overlapped |= FILE_FLAG_NO_BUFFERING |
> FILE_FLAG_WRITE_THROUGH;
> + else if (!(flags & BDRV_O_CACHE_WB))
> + overlapped |= FILE_FLAG_WRITE_THROUGH;
> s->hfile = CreateFile(filename, access_flags,
> FILE_SHARE_READ, NULL,
> create_flags, overlapped, NULL);
> @@ -440,8 +442,10 @@
> #else
> overlapped = FILE_ATTRIBUTE_NORMAL;
> #endif
> - if (flags & BDRV_O_DIRECT)
> + if ((flags & BDRV_O_NOCACHE))
> overlapped |= FILE_FLAG_NO_BUFFERING |
> FILE_FLAG_WRITE_THROUGH;
> + else if (!(flags & BDRV_O_CACHE_WB))
> + overlapped |= FILE_FLAG_WRITE_THROUGH;
> s->hfile = CreateFile(filename, access_flags,
> FILE_SHARE_READ, NULL,
> create_flags, overlapped, NULL);
>
> Modified: trunk/block.c
> ===================================================================
> --- trunk/block.c 2008-10-13 21:08:55 UTC (rev 5484)
> +++ trunk/block.c 2008-10-14 14:42:54 UTC (rev 5485)
> @@ -395,12 +395,12 @@
> /* Note: for compatibility, we open disk image files as RDWR, and
> RDONLY as fallback */
> if (!(flags & BDRV_O_FILE))
> - open_flags = BDRV_O_RDWR | (flags & BDRV_O_DIRECT);
> + open_flags = BDRV_O_RDWR | (flags & BDRV_O_CACHE_MASK);
> else
> open_flags = flags & ~(BDRV_O_FILE | BDRV_O_SNAPSHOT);
> ret = drv->bdrv_open(bs, filename, open_flags);
> if ((ret == -EACCES || ret == -EPERM) && !(flags & BDRV_O_FILE)) {
> - ret = drv->bdrv_open(bs, filename, BDRV_O_RDONLY);
> + ret = drv->bdrv_open(bs, filename, open_flags &
> ~BDRV_O_RDWR);
> bs->read_only = 1;
> }
> if (ret < 0) {
> @@ -427,7 +427,7 @@
> }
> path_combine(backing_filename, sizeof(backing_filename),
> filename, bs->backing_file);
> - if (bdrv_open(bs->backing_hd, backing_filename, 0) < 0)
> + if (bdrv_open(bs->backing_hd, backing_filename, open_flags)
> < 0)
> goto fail;
> }
>
>
> Modified: trunk/block.h
> ===================================================================
> --- trunk/block.h 2008-10-13 21:08:55 UTC (rev 5484)
> +++ trunk/block.h 2008-10-14 14:42:54 UTC (rev 5485)
> @@ -47,8 +47,11 @@
> use a disk image format on top of
> it (default for
> bdrv_file_open()) */
> -#define BDRV_O_DIRECT 0x0020
> +#define BDRV_O_NOCACHE 0x0020 /* do not use the host page cache
> */
> +#define BDRV_O_CACHE_WB 0x0040 /* use write-back caching */
>
> +#define BDRV_O_CACHE_MASK (BDRV_O_NOCACHE | BDRV_O_CACHE_WB)
> +
> void bdrv_info(void);
> void bdrv_info_stats(void);
>
>
> Modified: trunk/qemu-doc.texi
> ===================================================================
> --- trunk/qemu-doc.texi 2008-10-13 21:08:55 UTC (rev 5484)
> +++ trunk/qemu-doc.texi 2008-10-14 14:42:54 UTC (rev 5485)
> @@ -267,13 +267,28 @@
> @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 "on" or "off" and allows to disable host cache to
> access data.
> +@var{cache} is "none", "writeback", or "writethrough" and controls
> how the host cache is used to access block data.
> @item format=@var{format}
> Specify which disk @var{format} will be used rather than detecting
> the format. Can be used to specifiy format=raw to avoid interpreting
> an untrusted format header.
> @end table
>
> +By default, writethrough caching is used for all block device.
> This means that
> +the host page cache will be used to read and write data but write
> notification
> +will be sent to the guest only when the data has been reported as
> written by
> +the storage subsystem.
> +
> +Writeback caching will report data writes as completed as soon as
> the data is
> +present in the host page cache. This is safe as long as you trust
> your host.
> +If your host crashes or loses power, then the guest may experience
> data
> +corruption. When using the @option{-snapshot} option, writeback
> caching is
> +used by default.
> +
> +The host page 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.
> +
> Instead of @option{-cdrom} you can use:
> @example
> qemu -drive file=file,index=2,media=cdrom
>
> Modified: trunk/qemu-nbd.c
> ===================================================================
> --- trunk/qemu-nbd.c 2008-10-13 21:08:55 UTC (rev 5484)
> +++ trunk/qemu-nbd.c 2008-10-14 14:42:54 UTC (rev 5485)
> @@ -232,7 +232,7 @@
> flags |= BDRV_O_SNAPSHOT;
> break;
> case 'n':
> - flags |= BDRV_O_DIRECT;
> + flags |= BDRV_O_NOCACHE;
> break;
> case 'b':
> bindto = optarg;
>
> Modified: trunk/vl.c
> ===================================================================
> --- trunk/vl.c 2008-10-13 21:08:55 UTC (rev 5484)
> +++ trunk/vl.c 2008-10-14 14:42:54 UTC (rev 5485)
> @@ -5648,10 +5648,12 @@
> }
>
> if (get_param_value(buf, sizeof(buf), "cache", str)) {
> - if (!strcmp(buf, "off"))
> + if (!strcmp(buf, "off") || !strcmp(buf, "none"))
> cache = 0;
> - else if (!strcmp(buf, "on"))
> + else if (!strcmp(buf, "writethrough"))
> cache = 1;
> + else if (!strcmp(buf, "writeback"))
> + cache = 2;
> else {
> fprintf(stderr, "qemu: invalid cache option\n");
> return -1;
> @@ -5770,10 +5772,14 @@
> if (!file[0])
> return 0;
> bdrv_flags = 0;
> - if (snapshot)
> + if (snapshot) {
> bdrv_flags |= BDRV_O_SNAPSHOT;
> - if (!cache)
> - bdrv_flags |= BDRV_O_DIRECT;
> + cache = 2; /* always use write-back with snapshot */
> + }
> + if (cache == 0) /* no caching */
> + bdrv_flags |= BDRV_O_NOCACHE;
> + else if (cache == 2) /* write-back */
> + bdrv_flags |= BDRV_O_CACHE_WB;
> if (bdrv_open2(bdrv, file, bdrv_flags, drv) < 0 ||
> qemu_key_check(bdrv, file)) {
> fprintf(stderr, "qemu: could not open disk image %s\n",
> file);
> @@ -8145,7 +8151,7 @@
> "-cdrom file use 'file' as IDE cdrom image (cdrom is
> ide1 master)\n"
> "-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=on|off][,format=f]\n"
> + " [,cache=writethrough|writeback|none][,format=f]\n"
> " use 'file' as a drive image\n"
> "-mtdblock file use 'file' as on-board Flash memory image
> \n"
> "-sd file use 'file' as SecureDigital card image\n"
>
>
>
>
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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [Qemu-devel] [5485] Expand cache= option and use write-through caching by default
2008-10-14 16:19 ` C.W. Betts
@ 2008-10-14 18:01 ` Anthony Liguori
0 siblings, 0 replies; 3+ messages in thread
From: Anthony Liguori @ 2008-10-14 18:01 UTC (permalink / raw)
To: qemu-devel
C.W. Betts wrote:
> Mac OS X doesn't seem to have O_DSYNC in it:
> gcc-4.2 -O2 -g -fno-strict-aliasing -Wall -Wundef -Wendif-labels
> -Wwrite-strings -mdynamic-no-pic -m32 -I.
> -I/Users/cwbetts/makestuff/qemu-allmac/src -MMD -MP -MT
> block-raw-posix.o -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64
> -D_LARGEFILE_SOURCE -I/Users/cwbetts/makestuff/qemu-allmac/src/slirp
> -c -o block-raw-posix.o
> /Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c
> /Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c: In
> function ‘raw_open’:
> /Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c:139: error:
> ‘O_DSYNC’ undeclared (first use in this function)
> /Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c:139: error:
> (Each undeclared identifier is reported only once
> /Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c:139: error:
> for each function it appears in.)
> /Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c: In
> function ‘hdev_open’:
> /Users/cwbetts/makestuff/qemu-allmac/src/block-raw-posix.c:1000:
> error: ‘O_DSYNC’ undeclared (first use in this function)
Thanks, should be fixed in SVN now.
Regards,
Anthony Liguori
>>
>
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2008-10-14 18:07 UTC | newest]
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2008-10-14 14:42 [Qemu-devel] [5485] Expand cache= option and use write-through caching by default Anthony Liguori
[not found] ` <6AE33345-3374-44F0-8528-B26FF81860EA@hotmail.com>
2008-10-14 16:19 ` C.W. Betts
2008-10-14 18:01 ` Anthony Liguori
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