Sparse files are generally referred to as files, often large, that contain only a small amount of nonzero data relative to their size. In this blog, I would like to chat about sparse files on Windows operating systems and the related SMB commands. From an inter-operability perspective, I intend to discuss:
- implications of a SMB file server reporting sparse files support;
- considerations about a client manipulating sparse files;
- zero filling NTFS files;
- related Windows APIs and SMB / SMB2 commands.
A sparse file optimizes disk space consumption in the extent that large ranges of zeroes do not require allocation on the volume. Once data is written to a sparse file, the needed space for nonzero portions, i.e. sparse data sets, will be allocated whereas portions that an application designates as empty will not necessitate space allocation. The information about the empty ranges is kept in the file metadata and suffices to rebuild those zeros ranges.
From SMB perspective, the logic of file sparseness resides in the object store. Related SMB commands translate directly into down-level calls to the file system, namely NTFS for Windows-based sparse files.
In the rest of this blog, I will introduce FILE_SUPPORTS_SPARSE_FILES and FILE_ATTRIBUTE_SPARSE_FILE. Then, I will look at the file system from the user perspective, and touch on zero filling NTFS files. Finally, I will provide some relevant Windows APIs and SMB commands on this topic.
The FILE_SUPPORTS_SPARSE_FILES bit flag [MS-SMB] in the file system attributes indicates whether a file system supports sparse files. If the server sets the FILE_SUPPORTS_SPARSE_FILES flag in the SmbTrans2QueryFsInformation response (or the equivalent SMB2, see table below), the client will expect the support of sparse files. The client will then expect to issue requests such as FSCTL_SET_SPARSE, FSCTL_SET_ZERO_DATA (See [MS-FSCC]).
One question that arises here is whether a given SMB server could advertise sparse file support even though the underlying file system does not support sparse files. Technically, the server could, but there is at least one major issue in doing so, which is excess disk space consumption; nonetheless in general, file operations should work correctly.
It is important to note that all Windows-based file systems properly report file sparseness support.
The FSCTL_SET_SPARSE IOCTL (see [MS-FSCC]) is used to mark a given file as sparse or not sparse. When a file system that supports sparse files, i.e. NTFS, receives an FSCTL_SET_SPARSE IOCTL request on a file, it should set or unset the FILE_ATTRIBUTE_SPARSE_FILE attribute of the specified file.
If you attempt to clear the FILE_ATTRIBUTE_SPARSE_FILE attribute, the operation is valid only if there is no sparse region in the file. The FSCTL_QUERY_ALLOCATED_RANGES (see [MS-FSCC]) control code is used to determine whether there are sparse regions in a file.
In Windows NTFS file systems, files are not made sparse by default. The application or user needs to explicitly mark the file sparse via the FSCTL_SET_SPARSE control code. Unlike Unix or Linux file systems, when a file is created on Windows and some data is written at an offset, let assume at 1 MB offset, NTFS will allocate clusters and fill in all space from 0 to 1 MB. This means, the file has to be explicitly marked sparse before it becomes sparse.
Users and applications change the sparseness of a file by setting or clearing the FILE_ATTRIBUTE_SPARSE_FILE flag through the FSCTL_SET_SPARSE control code. From a file system API point of view, this is done by a DeviceIoControl(FSCTL_SET_SPARSE) function call.
File system APIs can be used to copy sparse stream ranges, or query allocated ranges. Applications then need only to read allocated ranges.
If a file system user copies or moves a sparse file to a volume on a file system which does not support NTFS sparse file feature, the file will be built to its originally specified size. This means the required space needs to be available for the operation to succeed, and the required space depends on whether or not sparse files are supported on the destination volume.
It should also be noted that the FILE_ATTRIBUTE_SPARSE_FILE flag might influence write caching logic at the client side, but this is an implementation-specific decision; a client may choose to perform only read caching when a file is sparse. As such, there is no particular requirement in caching of sparse files.
An application can query a sparse file to identify its allocated ranges by using the FSCTL_QUERY_ALLOCATED_RANGES control code (see MS-FSCC). This control code returns zero or more FILE_ALLOCATED_RANGE_BUFFER data elements. Each data element corresponds to an allocated range and contains a file offset and a length.
This allows an application to read non-zeroed ranges in a sparse stream. When ranges are copied to a sparse file, the NTFS file system will take care of the rest. Note that if the application simply reads a sparse file without explicitly reading allocated ranges, NTFS will fill-in the zeroed ranges.
Zero filling NTFS files is a common question when it comes to sparse files and inter-operability.
Let's assume a scenario where the non-Windows client (e.g. Solaris client) opens a file on a SMB share, then sets the file size, let's say 1 GB.
If the file was created on a Solaris CIFS server, the file would have been sparse by default, a read returns zeroes.
Let's assume sparseness is not considered. If the client sets the size of a file on a Windows-based SMB share, the ranges/clusters/blocks will be allocated and reserved.
Windows will consistently return zeroes when the client reads the file. This is covered in the algorithms described in the MS-FSA document (Sections 188.8.131.52 Per Stream, and 184.108.40.206 Application Requests a Read). NTFS guarantees to never expose the previous contents of a sector that is allocated to a file.
In the context where a file is not marked as sparse, the NTFS object store maintains a persistent attribute called ValidDataLength (VDL) for the valid data in the stream. VDL is a high water mark of where valid data has been written to the file. Any read between VDL and EOF is returned as zeroes. This is similar to the concept of physical file size and logical file size variables maintained in Unix style file systems.
If the NTFS file is not marked sparse, when a user creates a file and sets EOF to 1 GB, the VDL is still at zero so any read on the file (between VDL = 0 and EOF) will return zeroes. If the user writes 1 KB data at offset 99 KB, NTFS writes zeroes from 0 to 99 KB, writes the 1 KB data and updates the VDL to 100 KB.
This is done by calling GetVolumeInformation() and then by checking if the volume flags has the FILE_SUPPORTS_SPARSE_FILES bit set.
This can be done by calling DeviceIoControl() with the FSCTL_SET_SPARSE file system control code.
One way to achieve this is to call GetFileInformationByHandle() and check the FILE_ATTRIBUTE_SPARSE_FILE of the file attributes. GetFileInformationByHandle() works for default or alternate streams of a files. If you detect one sparse stream, then the file is sparse.
This can be done by calling DeviceIoControl() with the FSCTL_QUERY_ALLOCATED_RANGES control code. This is used to scans a file or alternate stream looking for ranges that contain nonzero data.
The following table summarizes related SMB and SMB2 commands.
SMB Transact2, Query FS Info, Query FS Volume Info (NT)
SMB Transact2, Query FS Info, Query FS Attribute Info (NT)
SMB2 QUERY INFORMATION (0x10), Class=Query FS Volume Info
SMB2 QUERY INFORMATION (0x10), Class=Query FS Attribute Info
DeviceIoControl() call with the FSCTL_SET_SPARSE control code
SMB Nt Transact, NT_TRANSACT_IOCTL, FunctionCode: 0x000900C4 - FSCTL_SET_SPARSE
SMB2 IOCTL (0xb), CtlCode: 0x000900C4 - FSCTL_SET_SPARSE
SMB Transact2, Query File Info, Query File Basic Info
SMB2 QUERY INFORMATION (0x10), Class=FileNetworkOpenInformation (34)
DeviceIoControl() call with the FSCTL_QUERY_ALLOCATED_RANGES control code
SMB Nt Transact, NT_TRANSACT_IOCTL, FunctionCode: 0x000940CF - FSCTL_QUERY_ALLOCATED_RANGES
SMB2 IOCTL (0xb), CtlCode: 0x000940CF - FSCTL_QUERY_ALLOCATED_RANGES
The following network trace snippets were captured between a Windows 7 client and Windows Server 2008 R2.
- SMB2: C QUERY INFORMATION (0x10), Class=Query FS Volume Info, FID=0xFFFFFFFF0000000D(temp@#7231)
InfoType: SMB2_0_INFO_FILESYSTEM - File system information is requested.
FileInfoClass: Query FS Volume Info
- SMB2: R QUERY INFORMATION (0x10), File=temp\testme.txt@#7231
VolumeCreationTime: 04/09/2010, 22:41:53.761513 UTC
VolumeSerialNumber: 1826342482 (0x6CDBC652)
VolumeLabelLength: 0 (0x0)
Reserved: 0 (0x0)
- SMB2: C QUERY INFORMATION (0x10), Class=Query FS Attribute Info, FID=0xFFFFFFFF0000000D(temp@#7231)
FileInfoClass: Query FS Attribute Info
- SMB2: R QUERY INFORMATION (0x10), File=temp@#7231
- AttributeInformation: FileSystemName NTFS
- FSCCFileSystemAttribute: 13041919 (0xC700FF)
SUPPORTS_SPARSE_FILES: (.........................1......) supports sparse files.
- SMB2: C IOCTL (0xb), FID=0xFFFFFFFF00000011
CtlCode: 0x000900C4 - FSCTL_SET_SPARSE - Marks the indicated file as sparse or not sparse.
+ FileId: Persistent: 0x1000000155, Volatile: 0xFFFFFFFF00000011
Flags: (00000000000000000000000000000001) FSCTL request
- SMB2: C QUERY INFORMATION (0x10), Class=FileNetworkOpenInformation (34), FID=0xFFFFFFFF00000011(temp\testme.txt@#7238)
InfoType: SMB2_0_INFO_FILE - File information is requested.
FileInfoClass: FileNetworkOpenInformation (34)
- SMB2: R QUERY INFORMATION (0x10), File=temp\testme.txt@#7238
- FSCCFileAttribute: 544 (0x220)
Sparse: (......................1.........) Sparse
CtlCode: 0x000940CF - FSCTL_QUERY_ALLOCATED_RANGES - Scans a file or alternate stream looking for ranges that contain nonzero data.
- QueryAllocatedRanges: FileOffset: 0, Length: 841
- SMB2: R IOCTL (0xb), File=temp\testme.txt@#7238
InputOffset: 112 (0x70)
InputCount: 16 (0x10)
OutputOffset: 128 (0x80)
OutputCount: 16 (0x10)
Flags: 0 (0x0)
Reserved2: 0 (0x0)
InputData: Binary Large Object (16 Bytes)
padding: Binary Large Object (16 Bytes)
- Smb: C; Transact2, Query FS Info, Query FS Volume Info (NT)
Command: Transact2 50(0x32)
SubCommand: Query FS Info, 3(0x0003)
QueryInfoLevel: Query FS Volume Info (NT)
- Smb: R; Transact2, Query FS Info, Query FS Volume Info (NT)
- QueryFSInfoDataBlock: Query FS Volume Info (NT)
CreationTime: 04/09/2010, 22:41:53.761513 UTC
VolumeSerialNumber: 1826342482 (0x6CDBC652)
VolumeLabelLength: 0 (0x0)
- Smb: C; Transact2, Query FS Info, Query FS Attribute Info (NT)
QueryInfoLevel: Query FS Attribute Info (NT)
- Smb: R; Transact2, Query FS Info, Query FS Attribute Info (NT), FS = NTFS
- QueryFSInfoDataBlock: Query FS Attribute Info (NT)
- FileSystemAttributes: 13041919 (0xC700FF)
SupportSparseFile: (.........................1......) File supports sparse files (FILE_SUPPORTS_SPARSE_FILES)
FileSystemNameLength: 8 (0x8)
+ FSName: NTFS
- Smb: C; Nt Transact, NT_TRANSACT_IOCTL, FID = 0x4000 (\temp\testme.txt@#7362)
Command: Nt Transact 160(0xA0)
FunctionCode: 0x000900C4 - FSCTL_SET_SPARSE - Marks the indicated file as sparse or not sparse., 590020 (0x900C4)
FileID: 16384 (0x4000)
IsFsctl: File system control command
+ IsFlag: 0 (0x0)
ByteCount: 3 (0x3)
Pad1: Binary Large Object (3 Bytes)
- Smb: R; Nt Transact, NT_TRANSACT_IOCTL, FID = 0x4000 (\temp\testme.txt@#41)
SetupWords: 0 (0x0)
- Smb: C; Transact2, Query File Info, Query File Basic Info, FID = 0x4000 (\temp\testme.txt@#7362)
SubCommand: Query File Info, 7(0x0007)
ByteCount: 7 (0x7)
FileInfoLevel: Query File Basic Info
- Smb: R; Transact2, Query File Info, Query File Basic Info, FID = 0x4000 (\temp\testme.txt@#7362)
CreationTime: 10/29/2010, 20:34:24.683222 UTC
AccessTime: 10/29/2010, 20:34:24.683222 UTC
LastWriteTime: 08/30/2005, 18:42:50.000000 UTC
LastChangeTime: 10/29/2010, 20:34:24.683222 UTC
- Attributes: 0x0220
SparseFile: (......................1.........) Sparse file (FILE_ATTRIBUTE_SPARSE_FILE)
FunctionCode: 0x000940CF - FSCTL_QUERY_ALLOCATED_RANGES - Scans a file or alternate stream looking for ranges that contain nonzero data., 606415 (0x940CF)
ByteCount: 19 (0x13)
FileOffset: 0 (0x0)
Length: 841 (0x349)
WordCount: 19 (0x13)
Reserved: 0 (0x0)
TotalParameterCount: 0 (0x0)
TotalDataCount: 16 (0x10)
ParameterCount: 0 (0x0)
ParameterOffset: 76 (0x4C)
ParamDisplacement: 0 (0x0)
DataCount: 16 (0x10)
DataOffset: 76 (0x4C)
DataDisplacement: 0 (0x0)
SetupCount: 1 (0x1)
SetupWords: 16 (0x10)
Pad: Binary Large Object (16 Bytes)
[MS-FSCC]: File System Control Codes
[MS-SMB]: Server Message Block (SMB) Protocol Specification
[MS-SMB2]: Server Message Block (SMB) Version 2 Protocol Specification
[MS-FSA]: File System Algorithms
Sparse File Operations
A File System for the 21st Century