2 * Block driver for Hyper-V VHDX Images
4 * Copyright (c) 2013 Red Hat, Inc.,
7 * Jeff Cody <jcody@redhat.com>
9 * This is based on the "VHDX Format Specification v1.00", published 8/25/2012
11 * https://www.microsoft.com/en-us/download/details.aspx?id=34750
13 * This file covers the functionality of the metadata log writing, parsing, and
16 * This work is licensed under the terms of the GNU LGPL, version 2 or later.
17 * See the COPYING.LIB file in the top-level directory.
20 #include "qemu-common.h"
21 #include "block/block_int.h"
22 #include "qemu/module.h"
23 #include "block/vhdx.h"
26 typedef struct VHDXLogSequence {
30 VHDXLogEntryHeader hdr;
33 typedef struct VHDXLogDescEntries {
34 VHDXLogEntryHeader hdr;
35 VHDXLogDescriptor desc[];
38 static const MSGUID zero_guid = { 0 };
40 /* The log located on the disk is circular buffer containing
41 * sectors of 4096 bytes each.
43 * It is assumed for the read/write functions below that the
44 * circular buffer scheme uses a 'one sector open' to indicate
45 * the buffer is full. Given the validation methods used for each
46 * sector, this method should be compatible with other methods that
47 * do not waste a sector.
51 /* Allow peeking at the hdr entry at the beginning of the current
52 * read index, without advancing the read index */
53 static int vhdx_log_peek_hdr(BlockDriverState *bs, VHDXLogEntries *log,
54 VHDXLogEntryHeader *hdr)
62 /* peek is only supported on sector boundaries */
63 if (log->read % VHDX_LOG_SECTOR_SIZE) {
69 /* we are guaranteed that a) log sectors are 4096 bytes,
70 * and b) the log length is a multiple of 1MB. So, there
71 * is always a round number of sectors in the buffer */
72 if ((read + sizeof(VHDXLogEntryHeader)) > log->length) {
76 if (read == log->write) {
81 offset = log->offset + read;
83 ret = bdrv_pread(bs->file, offset, hdr, sizeof(VHDXLogEntryHeader));
87 vhdx_log_entry_hdr_le_import(hdr);
93 /* Index increment for log, based on sector boundaries */
94 static int vhdx_log_inc_idx(uint32_t idx, uint64_t length)
96 idx += VHDX_LOG_SECTOR_SIZE;
97 /* we are guaranteed that a) log sectors are 4096 bytes,
98 * and b) the log length is a multiple of 1MB. So, there
99 * is always a round number of sectors in the buffer */
100 return idx >= length ? 0 : idx;
104 /* Reset the log to empty */
105 static void vhdx_log_reset(BlockDriverState *bs, BDRVVHDXState *s)
108 s->log.read = s->log.write = 0;
109 /* a log guid of 0 indicates an empty log to any parser of v0
111 vhdx_update_headers(bs, s, false, &guid);
114 /* Reads num_sectors from the log (all log sectors are 4096 bytes),
115 * into buffer 'buffer'. Upon return, *sectors_read will contain
116 * the number of sectors successfully read.
118 * It is assumed that 'buffer' is already allocated, and of sufficient
119 * size (i.e. >= 4096*num_sectors).
121 * If 'peek' is true, then the tail (read) pointer for the circular buffer is
124 * 0 is returned on success, -errno otherwise. */
125 static int vhdx_log_read_sectors(BlockDriverState *bs, VHDXLogEntries *log,
126 uint32_t *sectors_read, void *buffer,
127 uint32_t num_sectors, bool peek)
136 while (num_sectors) {
137 if (read == log->write) {
141 offset = log->offset + read;
143 ret = bdrv_pread(bs->file, offset, buffer, VHDX_LOG_SECTOR_SIZE);
147 read = vhdx_log_inc_idx(read, log->length);
149 *sectors_read = *sectors_read + 1;
160 /* Writes num_sectors to the log (all log sectors are 4096 bytes),
161 * from buffer 'buffer'. Upon return, *sectors_written will contain
162 * the number of sectors successfully written.
164 * It is assumed that 'buffer' is at least 4096*num_sectors large.
166 * 0 is returned on success, -errno otherwise */
167 static int vhdx_log_write_sectors(BlockDriverState *bs, VHDXLogEntries *log,
168 uint32_t *sectors_written, void *buffer,
169 uint32_t num_sectors)
175 BDRVVHDXState *s = bs->opaque;
177 ret = vhdx_user_visible_write(bs, s);
185 while (num_sectors) {
187 offset = log->offset + write;
188 write = vhdx_log_inc_idx(write, log->length);
189 if (write == log->read) {
193 ret = bdrv_pwrite(bs->file, offset, buffer_tmp, VHDX_LOG_SECTOR_SIZE);
197 buffer_tmp += VHDX_LOG_SECTOR_SIZE;
200 *sectors_written = *sectors_written + 1;
209 /* Validates a log entry header */
210 static bool vhdx_log_hdr_is_valid(VHDXLogEntries *log, VHDXLogEntryHeader *hdr,
215 if (hdr->signature != VHDX_LOG_SIGNATURE) {
219 /* if the individual entry length is larger than the whole log
220 * buffer, that is obviously invalid */
221 if (log->length < hdr->entry_length) {
225 /* length of entire entry must be in units of 4KB (log sector size) */
226 if (hdr->entry_length % (VHDX_LOG_SECTOR_SIZE)) {
230 /* per spec, sequence # must be > 0 */
231 if (hdr->sequence_number == 0) {
235 /* log entries are only valid if they match the file-wide log guid
236 * found in the active header */
237 if (!guid_eq(hdr->log_guid, s->headers[s->curr_header]->log_guid)) {
241 if (hdr->descriptor_count * sizeof(VHDXLogDescriptor) > hdr->entry_length) {
252 * Given a log header, this will validate that the descriptors and the
253 * corresponding data sectors (if applicable)
255 * Validation consists of:
256 * 1. Making sure the sequence numbers matches the entry header
257 * 2. Verifying a valid signature ('zero' or 'desc' for descriptors)
258 * 3. File offset field is a multiple of 4KB
259 * 4. If a data descriptor, the corresponding data sector
260 * has its signature ('data') and matching sequence number
262 * @desc: the data buffer containing the descriptor
263 * @hdr: the log entry header
265 * Returns true if valid
267 static bool vhdx_log_desc_is_valid(VHDXLogDescriptor *desc,
268 VHDXLogEntryHeader *hdr)
272 if (desc->sequence_number != hdr->sequence_number) {
275 if (desc->file_offset % VHDX_LOG_SECTOR_SIZE) {
279 if (desc->signature == VHDX_LOG_ZERO_SIGNATURE) {
280 if (desc->zero_length % VHDX_LOG_SECTOR_SIZE == 0) {
284 } else if (desc->signature == VHDX_LOG_DESC_SIGNATURE) {
294 /* Prior to sector data for a log entry, there is the header
295 * and the descriptors referenced in the header:
299 * [ hdr, desc ][ desc ][ ... ][ data ][ ... ]
301 * The first sector in a log entry has a 64 byte header, and
302 * up to 126 32-byte descriptors. If more descriptors than
303 * 126 are required, then subsequent sectors can have up to 128
304 * descriptors. Each sector is 4KB. Data follows the descriptor
307 * This will return the number of sectors needed to encompass
308 * the passed number of descriptors in desc_cnt.
310 * This will never return 0, even if desc_cnt is 0.
312 static int vhdx_compute_desc_sectors(uint32_t desc_cnt)
314 uint32_t desc_sectors;
316 desc_cnt += 2; /* account for header in first sector */
317 desc_sectors = desc_cnt / 128;
318 if (desc_cnt % 128) {
326 /* Reads the log header, and subsequent descriptors (if any). This
327 * will allocate all the space for buffer, which must be NULL when
328 * passed into this function. Each descriptor will also be validated,
329 * and error returned if any are invalid. */
330 static int vhdx_log_read_desc(BlockDriverState *bs, BDRVVHDXState *s,
331 VHDXLogEntries *log, VHDXLogDescEntries **buffer,
335 uint32_t desc_sectors;
336 uint32_t sectors_read;
337 VHDXLogEntryHeader hdr;
338 VHDXLogDescEntries *desc_entries = NULL;
339 VHDXLogDescriptor desc;
342 assert(*buffer == NULL);
344 ret = vhdx_log_peek_hdr(bs, log, &hdr);
349 if (vhdx_log_hdr_is_valid(log, &hdr, s) == false) {
354 desc_sectors = vhdx_compute_desc_sectors(hdr.descriptor_count);
355 desc_entries = qemu_try_blockalign(bs->file,
356 desc_sectors * VHDX_LOG_SECTOR_SIZE);
357 if (desc_entries == NULL) {
362 ret = vhdx_log_read_sectors(bs, log, §ors_read, desc_entries,
363 desc_sectors, false);
367 if (sectors_read != desc_sectors) {
372 /* put in proper endianness, and validate each desc */
373 for (i = 0; i < hdr.descriptor_count; i++) {
374 desc = desc_entries->desc[i];
375 vhdx_log_desc_le_import(&desc);
376 if (convert_endian) {
377 desc_entries->desc[i] = desc;
379 if (vhdx_log_desc_is_valid(&desc, &hdr) == false) {
384 if (convert_endian) {
385 desc_entries->hdr = hdr;
388 *buffer = desc_entries;
392 qemu_vfree(desc_entries);
398 /* Flushes the descriptor described by desc to the VHDX image file.
399 * If the descriptor is a data descriptor, than 'data' must be non-NULL,
400 * and >= 4096 bytes (VHDX_LOG_SECTOR_SIZE), containing the data to be
403 * Verification is performed to make sure the sequence numbers of a data
404 * descriptor match the sequence number in the desc.
406 * For a zero descriptor, it may describe multiple sectors to fill with zeroes.
407 * In this case, it should be noted that zeroes are written to disk, and the
408 * image file is not extended as a sparse file. */
409 static int vhdx_log_flush_desc(BlockDriverState *bs, VHDXLogDescriptor *desc,
410 VHDXLogDataSector *data)
413 uint64_t seq, file_offset;
419 buffer = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);
421 if (desc->signature == VHDX_LOG_DESC_SIGNATURE) {
428 /* The sequence number of the data sector must match that
429 * in the descriptor */
430 seq = data->sequence_high;
432 seq |= data->sequence_low & 0xffffffff;
434 if (seq != desc->sequence_number) {
439 /* Each data sector is in total 4096 bytes, however the first
440 * 8 bytes, and last 4 bytes, are located in the descriptor */
441 memcpy(buffer, &desc->leading_bytes, 8);
444 memcpy(buffer+offset, data->data, 4084);
447 memcpy(buffer+offset, &desc->trailing_bytes, 4);
449 } else if (desc->signature == VHDX_LOG_ZERO_SIGNATURE) {
450 /* write 'count' sectors of sector */
451 memset(buffer, 0, VHDX_LOG_SECTOR_SIZE);
452 count = desc->zero_length / VHDX_LOG_SECTOR_SIZE;
454 error_report("Invalid VHDX log descriptor entry signature 0x%" PRIx32,
460 file_offset = desc->file_offset;
462 /* count is only > 1 if we are writing zeroes */
463 for (i = 0; i < count; i++) {
464 ret = bdrv_pwrite_sync(bs->file, file_offset, buffer,
465 VHDX_LOG_SECTOR_SIZE);
469 file_offset += VHDX_LOG_SECTOR_SIZE;
477 /* Flush the entire log (as described by 'logs') to the VHDX image
478 * file, and then set the log to 'empty' status once complete.
480 * The log entries should be validate prior to flushing */
481 static int vhdx_log_flush(BlockDriverState *bs, BDRVVHDXState *s,
482 VHDXLogSequence *logs)
486 uint32_t cnt, sectors_read;
487 uint64_t new_file_size;
489 VHDXLogDescEntries *desc_entries = NULL;
490 VHDXLogEntryHeader hdr_tmp = { 0 };
494 data = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);
496 ret = vhdx_user_visible_write(bs, s);
501 /* each iteration represents one log sequence, which may span multiple
504 ret = vhdx_log_peek_hdr(bs, &logs->log, &hdr_tmp);
508 /* if the log shows a FlushedFileOffset larger than our current file
509 * size, then that means the file has been truncated / corrupted, and
510 * we must refused to open it / use it */
511 if (hdr_tmp.flushed_file_offset > bdrv_getlength(bs->file)) {
516 ret = vhdx_log_read_desc(bs, s, &logs->log, &desc_entries, true);
521 for (i = 0; i < desc_entries->hdr.descriptor_count; i++) {
522 if (desc_entries->desc[i].signature == VHDX_LOG_DESC_SIGNATURE) {
523 /* data sector, so read a sector to flush */
524 ret = vhdx_log_read_sectors(bs, &logs->log, §ors_read,
529 if (sectors_read != 1) {
533 vhdx_log_data_le_import(data);
536 ret = vhdx_log_flush_desc(bs, &desc_entries->desc[i], data);
541 if (bdrv_getlength(bs->file) < desc_entries->hdr.last_file_offset) {
542 new_file_size = desc_entries->hdr.last_file_offset;
543 if (new_file_size % (1024*1024)) {
544 /* round up to nearest 1MB boundary */
545 new_file_size = ((new_file_size >> 20) + 1) << 20;
546 bdrv_truncate(bs->file, new_file_size);
549 qemu_vfree(desc_entries);
554 /* once the log is fully flushed, indicate that we have an empty log
555 * now. This also sets the log guid to 0, to indicate an empty log */
556 vhdx_log_reset(bs, s);
560 qemu_vfree(desc_entries);
564 static int vhdx_validate_log_entry(BlockDriverState *bs, BDRVVHDXState *s,
565 VHDXLogEntries *log, uint64_t seq,
566 bool *valid, VHDXLogEntryHeader *entry)
569 VHDXLogEntryHeader hdr;
571 uint32_t i, desc_sectors, total_sectors, crc;
572 uint32_t sectors_read = 0;
573 VHDXLogDescEntries *desc_buffer = NULL;
577 ret = vhdx_log_peek_hdr(bs, log, &hdr);
582 if (vhdx_log_hdr_is_valid(log, &hdr, s) == false) {
587 if (hdr.sequence_number != seq + 1) {
592 desc_sectors = vhdx_compute_desc_sectors(hdr.descriptor_count);
594 /* Read all log sectors, and calculate log checksum */
596 total_sectors = hdr.entry_length / VHDX_LOG_SECTOR_SIZE;
599 /* read_desc() will increment the read idx */
600 ret = vhdx_log_read_desc(bs, s, log, &desc_buffer, false);
605 crc = vhdx_checksum_calc(0xffffffff, (void *)desc_buffer,
606 desc_sectors * VHDX_LOG_SECTOR_SIZE, 4);
609 buffer = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);
610 if (total_sectors > desc_sectors) {
611 for (i = 0; i < total_sectors - desc_sectors; i++) {
613 ret = vhdx_log_read_sectors(bs, log, §ors_read, buffer,
615 if (ret < 0 || sectors_read != 1) {
618 crc = vhdx_checksum_calc(crc, buffer, VHDX_LOG_SECTOR_SIZE, -1);
623 if (crc != hdr.checksum) {
632 log->read = vhdx_log_inc_idx(log->read, log->length);
636 qemu_vfree(desc_buffer);
640 /* Search through the log circular buffer, and find the valid, active
641 * log sequence, if any exists
643 static int vhdx_log_search(BlockDriverState *bs, BDRVVHDXState *s,
644 VHDXLogSequence *logs)
648 bool seq_valid = false;
649 VHDXLogSequence candidate = { 0 };
650 VHDXLogEntryHeader hdr = { 0 };
651 VHDXLogEntries curr_log;
653 memcpy(&curr_log, &s->log, sizeof(VHDXLogEntries));
654 curr_log.write = curr_log.length; /* assume log is full */
658 /* now we will go through the whole log sector by sector, until
659 * we find a valid, active log sequence, or reach the end of the
662 uint64_t curr_seq = 0;
663 VHDXLogSequence current = { 0 };
665 tail = curr_log.read;
667 ret = vhdx_validate_log_entry(bs, s, &curr_log, curr_seq,
674 current.valid = true;
675 current.log = curr_log;
676 current.log.read = tail;
677 current.log.write = curr_log.read;
683 ret = vhdx_validate_log_entry(bs, s, &curr_log, curr_seq,
688 if (seq_valid == false) {
691 current.log.write = curr_log.read;
694 curr_seq = hdr.sequence_number;
699 if (candidate.valid == false ||
700 current.hdr.sequence_number > candidate.hdr.sequence_number) {
705 if (curr_log.read < tail) {
712 if (candidate.valid) {
713 /* this is the next sequence number, for writes */
714 s->log.sequence = candidate.hdr.sequence_number + 1;
722 /* Parse the replay log. Per the VHDX spec, if the log is present
723 * it must be replayed prior to opening the file, even read-only.
725 * If read-only, we must replay the log in RAM (or refuse to open
726 * a dirty VHDX file read-only) */
727 int vhdx_parse_log(BlockDriverState *bs, BDRVVHDXState *s, bool *flushed,
732 VHDXLogSequence logs = { 0 };
734 hdr = s->headers[s->curr_header];
738 /* s->log.hdr is freed in vhdx_close() */
739 if (s->log.hdr == NULL) {
740 s->log.hdr = qemu_blockalign(bs, sizeof(VHDXLogEntryHeader));
743 s->log.offset = hdr->log_offset;
744 s->log.length = hdr->log_length;
746 if (s->log.offset < VHDX_LOG_MIN_SIZE ||
747 s->log.offset % VHDX_LOG_MIN_SIZE) {
752 /* per spec, only log version of 0 is supported */
753 if (hdr->log_version != 0) {
758 /* If either the log guid, or log length is zero,
759 * then a replay log is not present */
760 if (guid_eq(hdr->log_guid, zero_guid)) {
764 if (hdr->log_length == 0) {
768 if (hdr->log_length % VHDX_LOG_MIN_SIZE) {
774 /* The log is present, we need to find if and where there is an active
775 * sequence of valid entries present in the log. */
777 ret = vhdx_log_search(bs, s, &logs);
785 error_setg_errno(errp, EPERM,
786 "VHDX image file '%s' opened read-only, but "
787 "contains a log that needs to be replayed. To "
788 "replay the log, execute:\n qemu-img check -r "
790 bs->filename, bs->filename);
793 /* now flush the log */
794 ret = vhdx_log_flush(bs, s, &logs);
808 static void vhdx_log_raw_to_le_sector(VHDXLogDescriptor *desc,
809 VHDXLogDataSector *sector, void *data,
812 /* 8 + 4084 + 4 = 4096, 1 log sector */
813 memcpy(&desc->leading_bytes, data, 8);
815 cpu_to_le64s(&desc->leading_bytes);
816 memcpy(sector->data, data, 4084);
818 memcpy(&desc->trailing_bytes, data, 4);
819 cpu_to_le32s(&desc->trailing_bytes);
822 sector->sequence_high = (uint32_t) (seq >> 32);
823 sector->sequence_low = (uint32_t) (seq & 0xffffffff);
824 sector->data_signature = VHDX_LOG_DATA_SIGNATURE;
826 vhdx_log_desc_le_export(desc);
827 vhdx_log_data_le_export(sector);
831 static int vhdx_log_write(BlockDriverState *bs, BDRVVHDXState *s,
832 void *data, uint32_t length, uint64_t offset)
836 void *merged_sector = NULL;
837 void *data_tmp, *sector_write;
840 uint32_t desc_sectors, sectors, total_length;
841 uint32_t sectors_written = 0;
842 uint32_t aligned_length;
843 uint32_t leading_length = 0;
844 uint32_t trailing_length = 0;
845 uint32_t partial_sectors = 0;
846 uint32_t bytes_written = 0;
847 uint64_t file_offset;
849 VHDXLogEntryHeader new_hdr;
850 VHDXLogDescriptor *new_desc = NULL;
851 VHDXLogDataSector *data_sector = NULL;
852 MSGUID new_guid = { 0 };
854 header = s->headers[s->curr_header];
856 /* need to have offset read data, and be on 4096 byte boundary */
858 if (length > header->log_length) {
859 /* no log present. we could create a log here instead of failing */
864 if (guid_eq(header->log_guid, zero_guid)) {
865 vhdx_guid_generate(&new_guid);
866 vhdx_update_headers(bs, s, false, &new_guid);
868 /* currently, we require that the log be flushed after
874 /* 0 is an invalid sequence number, but may also represent the first
875 * log write (or a wrapped seq) */
876 if (s->log.sequence == 0) {
880 sector_offset = offset % VHDX_LOG_SECTOR_SIZE;
881 file_offset = (offset / VHDX_LOG_SECTOR_SIZE) * VHDX_LOG_SECTOR_SIZE;
883 aligned_length = length;
885 /* add in the unaligned head and tail bytes */
887 leading_length = (VHDX_LOG_SECTOR_SIZE - sector_offset);
888 leading_length = leading_length > length ? length : leading_length;
889 aligned_length -= leading_length;
893 sectors = aligned_length / VHDX_LOG_SECTOR_SIZE;
894 trailing_length = aligned_length - (sectors * VHDX_LOG_SECTOR_SIZE);
895 if (trailing_length) {
899 sectors += partial_sectors;
901 /* sectors is now how many sectors the data itself takes, not
902 * including the header and descriptor metadata */
904 new_hdr = (VHDXLogEntryHeader) {
905 .signature = VHDX_LOG_SIGNATURE,
907 .sequence_number = s->log.sequence,
908 .descriptor_count = sectors,
910 .flushed_file_offset = bdrv_getlength(bs->file),
911 .last_file_offset = bdrv_getlength(bs->file),
914 new_hdr.log_guid = header->log_guid;
916 desc_sectors = vhdx_compute_desc_sectors(new_hdr.descriptor_count);
918 total_length = (desc_sectors + sectors) * VHDX_LOG_SECTOR_SIZE;
919 new_hdr.entry_length = total_length;
921 vhdx_log_entry_hdr_le_export(&new_hdr);
923 buffer = qemu_blockalign(bs, total_length);
924 memcpy(buffer, &new_hdr, sizeof(new_hdr));
926 new_desc = buffer + sizeof(new_hdr);
927 data_sector = buffer + (desc_sectors * VHDX_LOG_SECTOR_SIZE);
930 /* All log sectors are 4KB, so for any partial sectors we must
931 * merge the data with preexisting data from the final file
933 merged_sector = qemu_blockalign(bs, VHDX_LOG_SECTOR_SIZE);
935 for (i = 0; i < sectors; i++) {
936 new_desc->signature = VHDX_LOG_DESC_SIGNATURE;
937 new_desc->sequence_number = s->log.sequence;
938 new_desc->file_offset = file_offset;
940 if (i == 0 && leading_length) {
941 /* partial sector at the front of the buffer */
942 ret = bdrv_pread(bs->file, file_offset, merged_sector,
943 VHDX_LOG_SECTOR_SIZE);
947 memcpy(merged_sector + sector_offset, data_tmp, leading_length);
948 bytes_written = leading_length;
949 sector_write = merged_sector;
950 } else if (i == sectors - 1 && trailing_length) {
951 /* partial sector at the end of the buffer */
952 ret = bdrv_pread(bs->file,
954 merged_sector + trailing_length,
955 VHDX_LOG_SECTOR_SIZE - trailing_length);
959 memcpy(merged_sector, data_tmp, trailing_length);
960 bytes_written = trailing_length;
961 sector_write = merged_sector;
963 bytes_written = VHDX_LOG_SECTOR_SIZE;
964 sector_write = data_tmp;
967 /* populate the raw sector data into the proper structures,
968 * as well as update the descriptor, and convert to proper
970 vhdx_log_raw_to_le_sector(new_desc, data_sector, sector_write,
973 data_tmp += bytes_written;
976 file_offset += VHDX_LOG_SECTOR_SIZE;
979 /* checksum covers entire entry, from the log header through the
980 * last data sector */
981 vhdx_update_checksum(buffer, total_length,
982 offsetof(VHDXLogEntryHeader, checksum));
984 /* now write to the log */
985 ret = vhdx_log_write_sectors(bs, &s->log, §ors_written, buffer,
986 desc_sectors + sectors);
991 if (sectors_written != desc_sectors + sectors) {
992 /* instead of failing, we could flush the log here */
999 s->log.tail = s->log.write;
1003 qemu_vfree(merged_sector);
1007 /* Perform a log write, and then immediately flush the entire log */
1008 int vhdx_log_write_and_flush(BlockDriverState *bs, BDRVVHDXState *s,
1009 void *data, uint32_t length, uint64_t offset)
1012 VHDXLogSequence logs = { .valid = true,
1017 /* Make sure data written (new and/or changed blocks) is stable
1018 * on disk, before creating log entry */
1020 ret = vhdx_log_write(bs, s, data, length, offset);
1026 /* Make sure log is stable on disk */
1028 ret = vhdx_log_flush(bs, s, &logs);