5 Appendix A: Product Behavior
The information in this specification is applicable to the following Microsoft products or supplemental software. References to product versions include updates to those products.
Windows 8 operating system
Windows Server 2012 operating system
Windows 8.1 operating system
Windows Server 2012 R2 operating system
Windows 10 operating system
Windows Server 2016 operating system
Windows Server operating system
Windows Server 2019 operating system
Windows Server 2022 operating system
Windows 11 operating system
Windows Server 2025 operating system
Exceptions, if any, are noted in this section. If an update version, service pack or Knowledge Base (KB) number appears with a product name, the behavior changed in that update. The new behavior also applies to subsequent updates unless otherwise specified. If a product edition appears with the product version, behavior is different in that product edition.
Unless otherwise specified, any statement of optional behavior in this specification that is prescribed using the terms "SHOULD" or "SHOULD NOT" implies product behavior in accordance with the SHOULD or SHOULD NOT prescription. Unless otherwise specified, the term "MAY" implies that the product does not follow the prescription.
<1> Section 2.2.2: This ensures that isolated updates to the headers can be performed on storage systems with up to a 64-KB sector size. The resiliency breaks down if the hosting volume's physical sector size is greater than 64 KB.
<2> Section 2.2.2: The DataWriteGuid field is used in the integrity validation of a differential VHDX chain. Implementations have to ensure that they are updated as specified.
<3> Section 2.3.1: Each log entry has at least a 4-KB alignment, but it can be greater, if necessary, to ensure that writes are isolated on the host-disk storage disk. All implementations have to be prepared to replay from a log that has 4-KB alignment or smaller, and no implementation is obligated to maintain an alignment greater than 4 KB. An implementation running on a large-sector-size disk might need to make a copy of the log, expanding the writes to the larger sector size, before replaying.
<4> Section 2.3.2: Having a matching LogGuid in the log entry is necessary to ensure that a non-current but fully written entry is not incorrectly used in log replay, which would be possible with the sequence number check alone.
<5> Section 2.3.3: If the storage media on which the file is stored is read-only or the file is opened in read-only mode, the implementation cannot allow reads from the virtual disk without replaying the log in memory.
<6> Section 2.5.1.1: Implementations can accept the VHDX files with these block states, but an implementation cannot transition blocks to these states in fixed VHDX files.
<7> Section 2.5.1.1: If an implementation chooses to preserve sector stability, it has to continue to return the same data on subsequent block reads while the block remains in this state. To preserve sector stability across different implementations, it is recommended to transition the block to a state that has a tightly defined read behavior.
<8> Section 2.5.1.1: If an implementation chooses to preserve sector stability, it has to continue to return the same data on subsequent block reads while the block remains in this state. To preserve sector stability across different implementations, the recommendation is to transition the block to a state that has a tightly defined read behavior.
<9> Section 2.5.1.1: If an implementation chooses to preserve sector stability, it has to continue to return the same data on subsequent block reads while the block remains in this state. To preserve sector stability across different implementations, the recommendation is to transition the block to a state that has a tightly defined read behavior.
<10> Section 2.5.1.1: If an implementation chooses to preserve sector stability guarantees, it has to continue to return the same data on subsequent block reads while the block remains in this state. To preserve sector stability across different implementations, it has to transition the block to a state that has a tightly defined read behavior.
<11> Section 2.5.1.1: If the implementation chooses to return data that was previously present elsewhere on the disk, it could result in private data being leaked.
<12> Section 2.6.2.6.3: There are two fields so that parent-child links can be safely maintained while merging child data into the parent. In that case, the parent's write GUID has to be changed. When this occurs, the implementation has to first store the new GUID in the parent identifier, and then update the parent's write GUID.