@@ -52,6 +52,7 @@ block-nested-y += raw.o cow.o qcow.o vdi.o vmdk.o cloop.o dmg.o bochs.o vpc.o vv
block-nested-y += qcow2.o qcow2-refcount.o qcow2-cluster.o qcow2-snapshot.o qcow2-cache.o
block-nested-y += qed.o qed-gencb.o qed-l2-cache.o qed-table.o qed-cluster.o
block-nested-y += qed-check.o
+block-nested-y += add-cow.o add-cow-cache.o
block-nested-y += parallels.o nbd.o blkdebug.o sheepdog.o blkverify.o
block-nested-y += stream.o
block-nested-$(CONFIG_WIN32) += raw-win32.o
@@ -194,7 +194,7 @@ static void bdrv_io_limits_intercept(BlockDriverState *bs,
}
/* check if the path starts with "<protocol>:" */
-static int path_has_protocol(const char *path)
+int path_has_protocol(const char *path)
{
#ifdef _WIN32
if (is_windows_drive(path) ||
@@ -319,6 +319,7 @@ char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn);
char *get_human_readable_size(char *buf, int buf_size, int64_t size);
int path_is_absolute(const char *path);
+int path_has_protocol(const char *path);
void path_combine(char *dest, int dest_size,
const char *base_path,
const char *filename);
new file mode 100644
@@ -0,0 +1,197 @@
+/*
+ * Cache For QEMU ADD-COW Disk Format
+ *
+ * Copyright IBM, Corp. 2012
+ *
+ * Authors:
+ * Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+#include "block_int.h"
+#include "qemu-common.h"
+#include "add-cow.h"
+
+AddCowCache *add_cow_cache_create(BlockDriverState *bs, int num_tables)
+{
+ BDRVAddCowState *s = bs->opaque;
+ AddCowCache *c;
+ int i;
+
+ c = g_malloc0(sizeof(*c));
+ c->size = num_tables;
+ c->entries = g_malloc0(sizeof(*c->entries) * num_tables);
+
+ for (i = 0; i < c->size; i++) {
+ c->entries[i].table = qemu_blockalign(bs, 8 * s->cluster_size);
+ c->entries[i].offset = -1;
+ }
+
+ return c;
+}
+
+void add_cow_cache_destroy(BlockDriverState *bs, AddCowCache *c)
+{
+ int i;
+
+ for (i = 0; i < c->size; i++) {
+ qemu_vfree(c->entries[i].table);
+ }
+
+ g_free(c->entries);
+ g_free(c);
+}
+
+static int add_cow_cache_find_entry_to_replace(AddCowCache *c)
+{
+ int i;
+ int min_count = INT_MAX;
+ int min_index = -1;
+
+
+ for (i = 0; i < c->size; i++) {
+ if (c->entries[i].cache_hits < min_count) {
+ min_index = i;
+ min_count = c->entries[i].cache_hits;
+ }
+
+ c->entries[i].cache_hits /= 2;
+ }
+
+ return min_index;
+}
+
+static int add_cow_cache_entry_flush(BlockDriverState *bs,
+ AddCowCache *c, int i)
+{
+ BDRVAddCowState *s = bs->opaque;
+ int ret = 0, j;
+
+ if (!c->entries[i].dirty || (-1 == c->entries[i].offset)) {
+ return 0;
+ }
+
+ for (j = 0; j < 8; j++) {
+ if (c->entries[i].bitmap & (1 << j)) {
+ ret = bdrv_pwrite(s->image_hd,
+ c->entries[i].offset * BDRV_SECTOR_SIZE + s->cluster_size * j,
+ c->entries[i].table + s->cluster_size * j,
+ s->cluster_size);
+ }
+ if (ret < 0) {
+ return ret;
+ }
+ }
+ ret = bdrv_flush(s->image_hd);
+ if (ret < 0) {
+ return ret;
+ }
+
+ ret = bdrv_pwrite(bs->file,
+ sizeof(AddCowHeader) + (c->entries[i].offset >> 10),
+ &c->entries[i].bitmap,
+ 1);
+ if (ret < 0) {
+ return ret;
+ }
+ ret = bdrv_flush(bs->file);
+ if (ret < 0) {
+ return ret;
+ }
+
+ c->entries[i].dirty = false;
+ return 0;
+}
+
+void add_cow_cache_entry_mark_dirty(AddCowCache *c, uint8_t bitmap, void *table)
+{
+ int i;
+
+ for (i = 0; i < c->size; i++) {
+ if (c->entries[i].table == table) {
+ goto found;
+ }
+ }
+ abort();
+
+found:
+ c->entries[i].dirty = true;
+ c->entries[i].bitmap = bitmap;
+}
+
+int add_cow_cache_flush(BlockDriverState *bs, AddCowCache *c)
+{
+ int result = 0;
+ int ret;
+ int i;
+
+ for (i = 0; i < c->size; i++) {
+ ret = add_cow_cache_entry_flush(bs, c, i);
+ if (ret < 0 && result != -ENOSPC) {
+ result = ret;
+ }
+ }
+ return result;
+}
+
+int add_cow_cache_get(BlockDriverState *bs, AddCowCache *c,
+ uint64_t sector_num, uint8_t *bitmap, void **table)
+{
+ BDRVAddCowState *s = bs->opaque;
+ int i, j;
+ int ret;
+ uint64_t offset = sector_num >> 10;
+
+ for (i = 0; i < c->size; i++) {
+ if (c->entries[i].offset == sector_num) {
+ goto found;
+ }
+ }
+
+ i = add_cow_cache_find_entry_to_replace(c);
+ if (i < 0) {
+ return i;
+ }
+
+ ret = add_cow_cache_entry_flush(bs, c, i);
+ if (ret < 0) {
+ return ret;
+ }
+
+ ret = bdrv_pread(bs->file, sizeof(AddCowHeader) + offset,
+ &c->entries[i].bitmap, 1);
+ if (ret < 0) {
+ return ret;
+ }
+
+ for (j = 0; j < 8; j++) {
+ if (s->image_hd->total_sectors * BDRV_SECTOR_SIZE <
+ sector_num + j * s->cluster_size) {
+ break;
+ }
+ if (c->entries[i].bitmap & (1 << j)) {
+ ret = bdrv_pread(s->image_hd,
+ sector_num * BDRV_SECTOR_SIZE + j * s->cluster_size,
+ c->entries[i].table + j * s->cluster_size,
+ s->cluster_size);
+ } else {
+ ret = bdrv_pread(bs->backing_hd,
+ sector_num * BDRV_SECTOR_SIZE + j * s->cluster_size,
+ c->entries[i].table + j * s->cluster_size,
+ s->cluster_size);
+ }
+ if (ret < 0) {
+ return ret;
+ }
+ }
+ c->entries[i].cache_hits = 32;
+ c->entries[i].offset = sector_num;
+
+found:
+ c->entries[i].cache_hits++;
+ *table = c->entries[i].table;
+ *bitmap = c->entries[i].bitmap;
+ return 0;
+}
new file mode 100644
@@ -0,0 +1,332 @@
+/*
+ * QEMU ADD-COW Disk Format
+ *
+ * Copyright IBM, Corp. 2012
+ *
+ * Authors:
+ * Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+#include "qemu-common.h"
+#include "block_int.h"
+#include "module.h"
+#include "add-cow.h"
+
+static int add_cow_probe(const uint8_t *buf, int buf_size, const char *filename)
+{
+ const AddCowHeader *header = (const AddCowHeader *)buf;
+
+ if (be64_to_cpu(header->magic) == ADD_COW_MAGIC &&
+ be32_to_cpu(header->version) == ADD_COW_VERSION) {
+ return 100;
+ } else {
+ return 0;
+ }
+}
+static int add_cow_create(const char *filename, QEMUOptionParameter *options)
+{
+ AddCowHeader header;
+ int64_t image_sectors = 0;
+ const char *backing_filename = NULL;
+ const char *image_filename = NULL;
+ int ret;
+ BlockDriverState *bs, *image_bs = NULL, *backing_bs = NULL;
+
+ while (options && options->name) {
+ if (!strcmp(options->name, BLOCK_OPT_SIZE)) {
+ image_sectors = options->value.n / BDRV_SECTOR_SIZE;
+ } else if (!strcmp(options->name, BLOCK_OPT_BACKING_FILE)) {
+ backing_filename = options->value.s;
+ } else if (!strcmp(options->name, BLOCK_OPT_IMAGE_FILE)) {
+ image_filename = options->value.s;
+ }
+ options++;
+ }
+
+ if (!backing_filename || !image_filename) {
+ error_report("Both backing_file and image_file should be given.");
+ return -EINVAL;
+ }
+
+ ret = bdrv_file_open(&image_bs, image_filename, BDRV_O_RDWR
+ | BDRV_O_CACHE_WB);
+ if (ret < 0) {
+ return ret;
+ }
+ image_sectors = image_bs->total_sectors;
+ bdrv_delete(image_bs);
+
+ ret = bdrv_file_open(&backing_bs, backing_filename, BDRV_O_RDWR
+ | BDRV_O_CACHE_WB);
+ if (ret < 0) {
+ return ret;
+ }
+ bdrv_delete(backing_bs);
+
+ ret = bdrv_create_file(filename, NULL);
+ if (ret < 0) {
+ return ret;
+ }
+
+ ret = bdrv_file_open(&bs, filename, BDRV_O_RDWR);
+ if (ret < 0) {
+ return ret;
+ }
+
+ memset(&header, 0, sizeof(header));
+ header.magic = cpu_to_be64(ADD_COW_MAGIC);
+ header.version = cpu_to_be32(ADD_COW_VERSION);
+ pstrcpy(header.backing_file, sizeof(header.backing_file), backing_filename);
+ pstrcpy(header.image_file, sizeof(header.image_file), image_filename);
+
+ ret = bdrv_pwrite(bs, 0, &header, sizeof(header));
+ if (ret < 0) {
+ bdrv_delete(bs);
+ return ret;
+ }
+
+ BlockDriver *drv = bdrv_find_format("add-cow");
+ assert(drv != NULL);
+ ret = bdrv_open(bs, filename, BDRV_O_RDWR | BDRV_O_NO_FLUSH, drv);
+ if (ret < 0) {
+ bdrv_delete(bs);
+ return ret;
+ }
+
+ ret = bdrv_truncate(bs, image_sectors * BDRV_SECTOR_SIZE);
+ bdrv_delete(bs);
+ return ret;
+}
+
+static int add_cow_open(BlockDriverState *bs, int flags)
+{
+ AddCowHeader header;
+ char image_filename[ADD_COW_FILE_LEN];
+ BlockDriver *image_drv = NULL;
+ int ret;
+ BDRVAddCowState *s = bs->opaque;
+
+ ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
+ if (ret != sizeof(header)) {
+ goto fail;
+ }
+
+ if (be64_to_cpu(header.magic) != ADD_COW_MAGIC) {
+ ret = -EINVAL;
+ goto fail;
+ }
+ if (be32_to_cpu(header.version) != ADD_COW_VERSION) {
+ char version[64];
+ snprintf(version, sizeof(version), "ADD-COW version %d",
+ be32_to_cpu(header.version));
+ qerror_report(QERR_UNKNOWN_BLOCK_FORMAT_FEATURE,
+ bs->device_name, "add-cow", version);
+ ret = -ENOTSUP;
+ goto fail;
+ }
+
+ QEMU_BUILD_BUG_ON(sizeof(bs->backing_file) != sizeof(header.backing_file));
+ pstrcpy(bs->backing_file, sizeof(bs->backing_file), header.backing_file);
+
+ if (header.image_file[0] == '\0') {
+ ret = -ENOENT;
+ goto fail;
+ }
+ header.image_file[ADD_COW_FILE_LEN - 1] = '\0';
+ s->image_hd = bdrv_new("");
+ if (path_has_protocol(header.image_file)) {
+ pstrcpy(image_filename, sizeof(image_filename), header.image_file);
+ } else {
+ path_combine(image_filename, sizeof(image_filename),
+ bs->filename, header.image_file);
+ }
+
+ image_drv = bdrv_find_format("raw");
+ ret = bdrv_open(s->image_hd, image_filename, flags, image_drv);
+ if (ret < 0) {
+ bdrv_delete(s->image_hd);
+ goto fail;
+ }
+ bs->total_sectors = s->image_hd->total_sectors;
+ s->cluster_size = ADD_COW_CLUSTER_SIZE;
+ s->bitmap_cache = add_cow_cache_create(bs, ADD_COW_CACHE_SIZE);
+ qemu_co_mutex_init(&s->lock);
+ return 0;
+ fail:
+ return ret;
+}
+
+static void add_cow_close(BlockDriverState *bs)
+{
+ BDRVAddCowState *s = bs->opaque;
+ add_cow_cache_destroy(bs, s->bitmap_cache);
+ bdrv_delete(s->image_hd);
+}
+
+static coroutine_fn int add_cow_co_readv(BlockDriverState *bs,
+ int64_t sector_num, int remaining_sectors, QEMUIOVector *qiov)
+{
+ BDRVAddCowState *s = bs->opaque;
+ int cur_nr_sectors;
+ uint64_t bytes_done = 0;
+ int ret = 0;
+ uint8_t *table;
+ uint8_t bitmap;
+ QEMUIOVector hd_qiov;
+ qemu_iovec_init(&hd_qiov, qiov->niov);
+
+ qemu_co_mutex_lock(&s->lock);
+ while (remaining_sectors != 0) {
+ cur_nr_sectors = MIN(remaining_sectors,
+ (sector_num | 1023) - sector_num + 1);
+ qemu_iovec_reset(&hd_qiov);
+ qemu_iovec_copy(&hd_qiov, qiov, bytes_done,
+ cur_nr_sectors * BDRV_SECTOR_SIZE);
+ ret = add_cow_cache_get(bs, s->bitmap_cache,
+ sector_num & ~1023, &bitmap, (void **)&table);
+ if (ret < 0) {
+ goto fail;
+ }
+ qemu_iovec_from_buffer(&hd_qiov,
+ table + (sector_num & 1023) * BDRV_SECTOR_SIZE,
+ BDRV_SECTOR_SIZE * cur_nr_sectors);
+
+ remaining_sectors -= cur_nr_sectors;
+ sector_num += cur_nr_sectors;
+ bytes_done += cur_nr_sectors * BDRV_SECTOR_SIZE;
+ }
+ ret = 0;
+fail:
+ qemu_co_mutex_unlock(&s->lock);
+ qemu_iovec_destroy(&hd_qiov);
+ return ret;
+}
+
+static coroutine_fn int add_cow_co_writev(BlockDriverState *bs,
+ int64_t sector_num, int remaining_sectors, QEMUIOVector *qiov)
+{
+ BDRVAddCowState *s = bs->opaque;
+ int ret = 0;
+ int cur_nr_sectors;
+ QEMUIOVector hd_qiov;
+ uint64_t bytes_done = 0;
+ uint8_t *table;
+ uint8_t bitmap;
+ int64_t index;
+ int i;
+ uint8_t *cluster_data = NULL;
+ qemu_co_mutex_lock(&s->lock);
+ qemu_iovec_init(&hd_qiov, qiov->niov);
+ while (remaining_sectors != 0) {
+ index = sector_num & 1023;
+ cur_nr_sectors = MIN(remaining_sectors,
+ (sector_num | 1023) - sector_num + 1);
+
+ ret = add_cow_cache_get(bs, s->bitmap_cache,
+ sector_num & ~1023, &bitmap, (void **)&table);
+ if (ret < 0) {
+ goto fail;
+ }
+
+ cluster_data = qemu_blockalign(bs, BDRV_SECTOR_SIZE * cur_nr_sectors);
+ qemu_iovec_reset(&hd_qiov);
+ qemu_iovec_copy(&hd_qiov, qiov, bytes_done,
+ cur_nr_sectors * BDRV_SECTOR_SIZE);
+ qemu_iovec_to_buffer(&hd_qiov, cluster_data);
+
+ memcpy(table + index * BDRV_SECTOR_SIZE,
+ cluster_data,
+ BDRV_SECTOR_SIZE * cur_nr_sectors);
+ for (i = index / 128;
+ i <= (index + cur_nr_sectors - 1) / 128;
+ i++) {
+ bitmap |= 1 << i;
+ }
+ add_cow_cache_entry_mark_dirty(s->bitmap_cache,
+ bitmap,
+ table);
+ remaining_sectors -= cur_nr_sectors;
+ sector_num += cur_nr_sectors;
+ bytes_done += cur_nr_sectors * BDRV_SECTOR_SIZE;
+ }
+ ret = 0;
+fail:
+ ret = add_cow_cache_flush(bs, s->bitmap_cache);
+ if (ret < 0) {
+ goto fail;
+ }
+ qemu_co_mutex_unlock(&s->lock);
+ qemu_vfree(cluster_data);
+ qemu_iovec_destroy(&hd_qiov);
+ return ret;
+}
+
+static int bdrv_add_cow_truncate(BlockDriverState *bs, int64_t offset)
+{
+ return bdrv_truncate(bs->file,
+ sizeof(AddCowHeader) + ((offset / BDRV_SECTOR_SIZE + 1023) >> 10));
+}
+
+static coroutine_fn int add_cow_co_flush(BlockDriverState *bs)
+{
+ BDRVAddCowState *s = bs->opaque;
+ int ret;
+
+ qemu_co_mutex_lock(&s->lock);
+ ret = add_cow_cache_flush(bs, s->bitmap_cache);
+ qemu_co_mutex_unlock(&s->lock);
+ if (ret < 0) {
+ return ret;
+ }
+
+ return bdrv_co_flush(bs->file);
+}
+
+static QEMUOptionParameter add_cow_create_options[] = {
+ {
+ .name = BLOCK_OPT_SIZE,
+ .type = OPT_SIZE,
+ .help = "Virtual disk size"
+ },
+ {
+ .name = BLOCK_OPT_BACKING_FILE,
+ .type = OPT_STRING,
+ .help = "File name of a base image"
+ },
+ {
+ .name = BLOCK_OPT_IMAGE_FILE,
+ .type = OPT_STRING,
+ .help = "File name of a image file"
+ },
+ {
+ .name = BLOCK_OPT_BACKING_FMT,
+ .type = OPT_STRING,
+ .help = "Image format of the base image"
+ },
+ { NULL }
+};
+
+static BlockDriver bdrv_add_cow = {
+ .format_name = "add-cow",
+ .instance_size = sizeof(BDRVAddCowState),
+ .bdrv_probe = add_cow_probe,
+ .bdrv_open = add_cow_open,
+ .bdrv_close = add_cow_close,
+ .bdrv_create = add_cow_create,
+ .bdrv_co_readv = add_cow_co_readv,
+ .bdrv_co_writev = add_cow_co_writev,
+ .bdrv_truncate = bdrv_add_cow_truncate,
+
+ .create_options = add_cow_create_options,
+ .bdrv_co_flush_to_disk = add_cow_co_flush,
+};
+
+static void bdrv_add_cow_init(void)
+{
+ bdrv_register(&bdrv_add_cow);
+}
+
+block_init(bdrv_add_cow_init);
new file mode 100644
@@ -0,0 +1,63 @@
+/*
+ * QEMU ADD-COW Disk Format
+ *
+ * Copyright IBM, Corp. 2012
+ *
+ * Authors:
+ * Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+#ifndef BLOCK_ADD_COW_H
+#define BLOCK_ADD_COW_H
+
+#define ADD_COW_MAGIC (((uint64_t)'A' << 56) | ((uint64_t)'D' << 48) | \
+ ((uint64_t)'D' << 40) | ((uint64_t)'_' << 32) | \
+ ((uint64_t)'C' << 24) | ((uint64_t)'O' << 16) | \
+ ((uint64_t)'W' << 8) | 0xFF)
+#define ADD_COW_VERSION 1
+#define ADD_COW_FILE_LEN 1024
+#define ADD_COW_CACHE_SIZE 16
+#define ADD_COW_CLUSTER_SIZE 65536
+
+typedef struct AddCowHeader {
+ uint64_t magic;
+ uint32_t version;
+ char backing_file[ADD_COW_FILE_LEN];
+ char image_file[ADD_COW_FILE_LEN];
+ char reserved[500];
+} QEMU_PACKED AddCowHeader;
+
+typedef struct AddCowCachedTable {
+ /* data for image_file */
+ void *table;
+ /* offset of bitmap */
+ int64_t offset;
+ /* bitmap indicates each cluster is dirty or not */
+ uint8_t bitmap;
+ bool dirty;
+ int cache_hits;
+} AddCowCachedTable;
+
+typedef struct AddCowCache {
+ AddCowCachedTable *entries;
+ int size;
+} AddCowCache;
+
+typedef struct BDRVAddCowState {
+ BlockDriverState *image_hd;
+ CoMutex lock;
+ int cluster_size;
+ AddCowCache *bitmap_cache;
+} BDRVAddCowState;
+
+AddCowCache *add_cow_cache_create(BlockDriverState *bs, int num_tables);
+void add_cow_cache_destroy(BlockDriverState *bs, AddCowCache *c);
+void add_cow_cache_entry_mark_dirty(AddCowCache *c,
+ uint8_t bitmap, void *table);
+int add_cow_cache_get(BlockDriverState *bs, AddCowCache *c,
+ uint64_t offset, uint8_t *bitmap, void **table);
+int add_cow_cache_flush(BlockDriverState *bs, AddCowCache *c);
+#endif
@@ -50,6 +50,7 @@
#define BLOCK_OPT_TABLE_SIZE "table_size"
#define BLOCK_OPT_PREALLOC "preallocation"
#define BLOCK_OPT_SUBFMT "subformat"
+#define BLOCK_OPT_IMAGE_FILE "image_file"
typedef struct BdrvTrackedRequest BdrvTrackedRequest;
new file mode 100644
@@ -0,0 +1,68 @@
+== General ==
+
+Raw file format does not support backing_file and copy on write feature.
+The add-cow image format makes it possible to use backing files with raw
+image by keeping a separate .add-cow metadata file. Once all sectors
+have been written to in the raw image it is safe to discard the .add-cow
+and backing files and instead use the raw image directly.
+
+When using add-cow, procedures may like this:
+(ubuntu.img is a disk image which has been installed OS.)
+ 1) Create a raw image with the same size of ubuntu.img
+ qemu-img create -f raw test.raw 8G
+ 2) Create a add-cow image which will store dirty bitmap
+ qemu-img create -f add-cow test.add-cow -o backing_file=ubuntu.img,image_file=test.raw
+ 3) Run qemu with add-cow image
+ qemu -drive if=virtio,file=test.add-cow
+
+=Specification=
+
+The file format looks like this:
+
+ +---------------+--------------------------+
+ | Header | Data |
+ +---------------+--------------------------+
+
+All numbers in add-cow are stored in Big Endian byte order.
+
+== Header ==
+
+The Header is included in the first bytes:
+
+ Byte 0 - 7: magic
+ add-cow magic string ("ADD_COW\xff")
+
+ 8 - 11: version
+ Version number (only valid value is 1 now)
+
+ 12 - 1035: backing_file
+ backing_file file name related to add-cow file. All
+ unused bytes are padded with zeros. Must not be longer
+ than 1023 bytes.
+
+ 1036 - 2059: image_file
+ image_file is a raw file. All unused bytes are padded
+ with zeros. Must not be longer than 1023 bytes.
+
+ 2060 - 2559: The Reserved field is used to make sure Data field starts
+ at the multiple of 512, not used currently. All bytes are
+ filled with 0.
+
+== Data ==
+
+The Data field starts at the 2560th byte, stores a bitmap related to backing_file
+and image_file. The bitmap will track whether the sector in backing_file is dirty
+or not.
+
+Each bit in the bitmap indicates one cluster's status. One cluster includes 128
+sectors, then each bit indicates 512 * 128 = 64k bytes, So the size of bitmap is
+calculated according to virtual size of image_file. In each byte, bit 0 to 7
+will track the 1st to 7th cluster in sequence, bit orders in one byte look like:
+ +----+----+----+----+----+----+----+----+
+ | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 |
+ +----+----+----+----+----+----+----+----+
+
+If the bit is 0, indicates the sector has not been allocated in image_file, data
+should be loaded from backing_file while reading; if the bit is 1, indicates the
+related sector has been dirty, should be loaded from image_file while reading.
+Writing to a sector causes the corresponding bit to be set to 1.