diff --git a/block.c b/block.c
index 34fedf0..b4b17fd 100644
--- a/block.c
+++ b/block.c
@@ -789,6 +789,7 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,
 
     bs->open_flags = flags;
     bs->guest_block_size = 512;
+    bs->request_alignment = 512;
     bs->zero_beyond_eof = true;
     open_flags = bdrv_open_flags(bs, flags);
     bs->read_only = !(open_flags & BDRV_O_RDWR);
@@ -856,6 +857,9 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,
         goto free_and_fail;
     }
 
+    assert(bdrv_opt_mem_align(bs) != 0);
+    assert(bs->request_alignment != 0);
+
 #ifndef _WIN32
     if (bs->is_temporary) {
         assert(bs->filename[0] != '\0');
diff --git a/block/raw-posix.c b/block/raw-posix.c
index f836c8e..732aff5 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -127,6 +127,8 @@ typedef struct BDRVRawState {
     int fd;
     int type;
     int open_flags;
+    size_t buf_align;
+
 #if defined(__linux__)
     /* linux floppy specific */
     int64_t fd_open_time;
@@ -211,6 +213,76 @@ static int raw_normalize_devicepath(const char **filename)
 }
 #endif
 
+static void raw_probe_alignment(BlockDriverState *bs)
+{
+    BDRVRawState *s = bs->opaque;
+    char *buf;
+    unsigned int sector_size;
+
+    /* For /dev/sg devices the alignment is not really used.
+       With buffered I/O, we don't have any restrictions. */
+    if (bs->sg || !(s->open_flags & O_DIRECT)) {
+        bs->request_alignment = 1;
+        s->buf_align = 1;
+        return;
+    }
+
+    /* Try a few ioctls to get the right size */
+    bs->request_alignment = 0;
+    s->buf_align = 0;
+
+#ifdef BLKSSZGET
+    if (ioctl(s->fd, BLKSSZGET, &sector_size) >= 0) {
+        bs->request_alignment = sector_size;
+    }
+#endif
+#ifdef DKIOCGETBLOCKSIZE
+    if (ioctl(s->fd, DKIOCGETBLOCKSIZE, &sector_size) >= 0) {
+        bs->request_alignment = sector_size;
+    }
+#endif
+#ifdef DIOCGSECTORSIZE
+    if (ioctl(s->fd, DIOCGSECTORSIZE, &sector_size) >= 0) {
+        bs->request_alignment = sector_size;
+    }
+#endif
+#ifdef CONFIG_XFS
+    if (s->is_xfs) {
+        struct dioattr da;
+        if (xfsctl(NULL, s->fd, XFS_IOC_DIOINFO, &da) >= 0) {
+            bs->request_alignment = da.d_miniosz;
+            /* The kernel returns wrong information for d_mem */
+            /* s->buf_align = da.d_mem; */
+        }
+    }
+#endif
+
+    /* If we could not get the sizes so far, we can only guess them */
+    if (!s->buf_align) {
+        size_t align;
+        buf = qemu_memalign(MAX_BLOCKSIZE, 2 * MAX_BLOCKSIZE);
+        for (align = 512; align <= MAX_BLOCKSIZE; align <<= 1) {
+            if (pread(s->fd, buf + align, MAX_BLOCKSIZE, 0) >= 0) {
+                s->buf_align = align;
+                break;
+            }
+        }
+        qemu_vfree(buf);
+    }
+
+    if (!bs->request_alignment) {
+        size_t align;
+        buf = qemu_memalign(s->buf_align, MAX_BLOCKSIZE);
+        for (align = 512; align <= MAX_BLOCKSIZE; align <<= 1) {
+            if (pread(s->fd, buf, align, 0) >= 0) {
+                bs->request_alignment = align;
+                break;
+            }
+        }
+        qemu_vfree(buf);
+    }
+}
+
 static void raw_parse_flags(int bdrv_flags, int *open_flags)
 {
     assert(open_flags != NULL);
@@ -330,6 +402,8 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
     }
 #endif
 
+    raw_probe_alignment(bs);
+
     ret = 0;
 fail:
     qemu_opts_del(opts);
@@ -432,7 +506,6 @@ static int raw_reopen_prepare(BDRVReopenState *state,
     return ret;
 }
 
-
 static void raw_reopen_commit(BDRVReopenState *state)
 {
     BDRVRawReopenState *raw_s = state->opaque;
@@ -468,23 +541,11 @@ static void raw_reopen_abort(BDRVReopenState *state)
     state->opaque = NULL;
 }
 
-
-/* XXX: use host sector size if necessary with:
-#ifdef DIOCGSECTORSIZE
-        {
-            unsigned int sectorsize = 512;
-            if (!ioctl(fd, DIOCGSECTORSIZE, &sectorsize) &&
-                sectorsize > bufsize)
-                bufsize = sectorsize;
-        }
-#endif
-#ifdef CONFIG_COCOA
-        uint32_t blockSize = 512;
-        if ( !ioctl( fd, DKIOCGETBLOCKSIZE, &blockSize ) && blockSize > bufsize) {
-            bufsize = blockSize;
-        }
-#endif
-*/
+static int raw_opt_mem_align(BlockDriverState *bs)
+{
+    BDRVRawState *s = bs->opaque;
+    return s->buf_align;
+}
 
 static ssize_t handle_aiocb_ioctl(RawPosixAIOData *aiocb)
 {
@@ -1227,6 +1288,7 @@ static BlockDriver bdrv_file = {
     .bdrv_aio_writev = raw_aio_writev,
     .bdrv_aio_flush = raw_aio_flush,
     .bdrv_aio_discard = raw_aio_discard,
+    .bdrv_opt_mem_align = raw_opt_mem_align,
 
     .bdrv_truncate = raw_truncate,
     .bdrv_getlength = raw_getlength,
@@ -1582,6 +1644,7 @@ static BlockDriver bdrv_host_device = {
     .bdrv_aio_writev	= raw_aio_writev,
     .bdrv_aio_flush	= raw_aio_flush,
     .bdrv_aio_discard   = hdev_aio_discard,
+    .bdrv_opt_mem_align = raw_opt_mem_align,
 
     .bdrv_truncate      = raw_truncate,
     .bdrv_getlength	= raw_getlength,
@@ -1712,6 +1775,7 @@ static BlockDriver bdrv_host_floppy = {
     .bdrv_aio_readv     = raw_aio_readv,
     .bdrv_aio_writev    = raw_aio_writev,
     .bdrv_aio_flush	= raw_aio_flush,
+    .bdrv_opt_mem_align = raw_opt_mem_align,
 
     .bdrv_truncate      = raw_truncate,
     .bdrv_getlength      = raw_getlength,
@@ -1822,6 +1886,7 @@ static BlockDriver bdrv_host_cdrom = {
     .bdrv_aio_readv     = raw_aio_readv,
     .bdrv_aio_writev    = raw_aio_writev,
     .bdrv_aio_flush	= raw_aio_flush,
+    .bdrv_opt_mem_align = raw_opt_mem_align,
 
     .bdrv_truncate      = raw_truncate,
     .bdrv_getlength      = raw_getlength,
@@ -1951,6 +2016,7 @@ static BlockDriver bdrv_host_cdrom = {
     .bdrv_aio_readv     = raw_aio_readv,
     .bdrv_aio_writev    = raw_aio_writev,
     .bdrv_aio_flush	= raw_aio_flush,
+    .bdrv_opt_mem_align = raw_opt_mem_align,
 
     .bdrv_truncate      = raw_truncate,
     .bdrv_getlength      = raw_getlength,
diff --git a/block/raw-win32.c b/block/raw-win32.c
index 2bad5a3..6fe64d2 100644
--- a/block/raw-win32.c
+++ b/block/raw-win32.c
@@ -202,6 +202,35 @@ static int set_sparse(int fd)
 				 NULL, 0, NULL, 0, &returned, NULL);
 }
 
+static void raw_probe_alignment(BlockDriverState *bs)
+{
+    BDRVRawState *s = bs->opaque;
+    DWORD sectorsPerCluster, freeClusters, totalClusters, count;
+    DISK_GEOMETRY_EX dg;
+    BOOL status;
+
+    if (s->type == FTYPE_CD) {
+        bs->request_alignment = 2048;
+        return;
+    }
+    if (s->type == FTYPE_HARDDISK) {
+        status = DeviceIoControl(s->hfile, IOCTL_DISK_GET_DRIVE_GEOMETRY_EX,
+                                 NULL, 0, &dg, sizeof(dg), &count, NULL);
+        if (status != 0) {
+            bs->request_alignment = dg.Geometry.BytesPerSector;
+            return;
+        }
+        /* try GetDiskFreeSpace too */
+    }
+
+    if (s->drive_path[0]) {
+        GetDiskFreeSpace(s->drive_path, &sectorsPerCluster,
+                         &dg.Geometry.BytesPerSector,
+                         &freeClusters, &totalClusters);
+        bs->request_alignment = dg.Geometry.BytesPerSector;
+    }
+}
+
 static void raw_parse_flags(int flags, int *access_flags, DWORD *overlapped)
 {
     assert(access_flags != NULL);
@@ -269,6 +298,17 @@ static int raw_open(BlockDriverState *bs, QDict *options, int flags,
         }
     }
 
+    if (filename[0] && filename[1] == ':') {
+        snprintf(s->drive_path, sizeof(s->drive_path), "%c:\\", filename[0]);
+    } else if (filename[0] == '\\' && filename[1] == '\\') {
+        s->drive_path[0] = 0;
+    } else {
+        /* Relative path.  */
+        char buf[MAX_PATH];
+        GetCurrentDirectory(MAX_PATH, buf);
+        snprintf(s->drive_path, sizeof(s->drive_path), "%c:\\", buf[0]);
+    }
+
     s->hfile = CreateFile(filename, access_flags,
                           FILE_SHARE_READ, NULL,
                           OPEN_EXISTING, overlapped, NULL);
@@ -293,6 +333,7 @@ static int raw_open(BlockDriverState *bs, QDict *options, int flags,
         s->aio = aio;
     }
 
+    raw_probe_alignment(bs);
     ret = 0;
 fail:
     qemu_opts_del(opts);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 760cfc5..31ce7d4 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -293,6 +293,9 @@ struct BlockDriverState {
     /* Whether produces zeros when read beyond eof */
     bool zero_beyond_eof;
 
+    /* Alignment requirement for offset/length of I/O requests */
+    unsigned int request_alignment;
+
     /* the block size for which the guest device expects atomicity */
     int guest_block_size;
 
