Revision: 5485
http://svn.sv.gnu.org/viewvc/?view=rev&root=qemu&revision=5485Author: 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 <
address@hidden>
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 address@hidden
@var{snapshot} is "on" or "off" and allows to enable snapshot for given drive (see @option{-snapshot}).
@item address@hidden
address@hidden is "on" or "off" and allows to disable host cache to access data.
address@hidden is "none", "writeback", or "writethrough" and controls how the host cache is used to access block data.
@item address@hidden
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"