Manage file locks

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Azure Files provides access to cloud file shares through the following protocols:

  • Server Message Block (SMB)
  • Network File System (NFS)
  • FileREST (HTTPS)

This topic describes how to manage file locking interactions between SMB and FileREST. NFS file shares have different locking semantics, and support a subset of the FileREST APIs. This topic is not applicable for NFS file shares.

SMB file locking

SMB clients that mount file shares can use file system locking mechanisms to manage access to shared files. These include:

  • Whole file access sharing for read, write, and delete.
  • Byte range locks to manage read and write access to regions within a single file.

When an SMB client opens a file, it specifies both the file access and the share mode. The following file access options are typically used by SMB clients, though all combinations are allowed:

  • None: Opens a file for query attribute access only.
  • Read: Opens a file for read access only.
  • Write: Opens a file for write access only.
  • Read/Write: Opens a file with read/write permissions.
  • Delete: Opens a file for delete access only.

The following file share modes are typically used by SMB clients:

  • None: Declines sharing of the current file. Any request to open the file with read, write, or delete access will fail, until the file is closed.
  • Shared Read: Allows subsequent opening of the file for reading. If this flag isn't specified, any request to open the file for reading will fail, until the file is closed.
  • Shared Write: Allows subsequent opening of the file for writing. If this flag isn't specified, any request to open the file for writing will fail, until the file is closed.
  • Shared Read/Write: Allows subsequent opening of the file for reading or writing. If this flag isn't specified, any request to open the file for reading or writing will fail, until the file is closed.
  • Shared Delete: Allows subsequent deleting of a file. If this flag isn't specified, any request to delete the file will fail, until the file is closed.

SMB client open file examples

Consider the following open file examples:

  • File opens without sharing violation

    • Client A opens the file with FileAccess.Read and FileShare.Write (denies subsequent Read/Delete while open).
    • Client B then opens the file with FileAccess.Write with FileShare.Read (denies subsequent Write/Delete while open).
    • Result: This is allowed, because there's no conflict between file access and file share modes.

  • Sharing violation due to file access

    • Client A opens the file with FileAccess.Write and FileShare.Read (denies subsequent Write/Delete while open).
    • Client B then opens the file with FileAccess.Write with FileShare.Write (denies subsequent Read/Delete while open).
    • Result: Client B encounters a sharing violation. The client specified a file access that is denied by the share mode specified previously by Client A.

  • Sharing violation due to share mode

    • Client A opens the file with FileAccess.Write and FileShare.Write (denies subsequent Read/Delete while open).
    • Client B then opens the file with FileAccess.Write with FileShare.Read (denies subsequent Write/Delete while open).
    • Result: Client B encounters a sharing violation. The client specified a share mode that denies write access to a file that is still open for write access.

FileREST file access

When you perform a FileREST operation, this operation must respect the share mode specified for any file open on an SMB client. Use the following file access mode to determine whether the operation can be completed:

FileREST operation File access equivalent
List Directories and Files N/A.
Create File Write, Delete.
Get File Read.
Set File Properties Write.
Get File Properties N/A.
Set File Metadata Write.
Get File Metadata N/A.
Delete File Delete.
Put Range Write.
List Ranges Read.
Lease File Write, Delete, and Shared Read for the duration of the lease.

List Directories and Files, Get File Properties, and Get File Metadata don't operate on file content. These operations don't require read access to the file (that is, these operations succeed even if an SMB client has the file open for exclusive read access).

The following are examples of FileREST requests interacting with the SMB share modes:

  • FileREST Get File sharing violation

    • The SMB client opens the file with FileAccess.Read and FileShare.Write (denies subsequent Read/Delete while open).
    • The REST client then performs a Get File operation on the file (thereby using FileAccess.Read as specified in the preceding table).
    • Result: The REST client's request fails with status code 409 (Conflict), and error code SharingViolation. The SMB client still has the file open, and denies Read/Delete access.

  • FileREST Put Range sharing violation

    • The SMB client opens the file with FileAccess.Write and FileShare.Read (denies subsequent Write/Delete while open).
    • The REST client then performs a Put Range operation on the file (thereby using FileAccess.Write as specified in the preceding table).
    • Result: The REST client's request fails with status code 409 (Conflict), and error code SharingViolation. The SMB client still has the file open, and denies Write/Delete access.

The next section includes a comprehensive table of FileREST API sharing violation scenarios.

SMB client sharing mode implications on FileREST

Depending on the share mode you specify when an SMB client opens a file, it's possible for FileREST to return status code 409 (Conflict) with error code SharingViolation. The following table lists a number of scenarios.

SMB client file sharing mode FileREST operations failing with a sharing violation
None

(Deny Read, Write, Delete)		 The following read, write, lease, and delete operations on the file will fail:

  • Create File
  • Get File
  • Set File Properties
  • Set File Metadata
  • Delete File
  • Put Range
  • List Ranges
  • Lease File
Shared Read

Deny Write, Delete)		 The following write, lease, and delete operations on the file will fail:

  • Create File
  • Set File Properties
  • Set File Metadata
  • Delete File
  • Put Range
  • Lease File
Shared Write

(Deny Read, Delete)		 The following read, lease, and delete operations on the file will fail:

  • Create File
  • Get File
  • Delete File
  • List Ranges
  • Lease File
Shared Delete

(Deny Read, Write)		 The following read, write, and lease operations on the file will fail:

  • Create File
  • Get File
  • Set File Properties
  • Set File Metadata
  • Put Range
  • List Ranges
  • Delete File
  • Lease File
Shared Read/Write

(Deny Delete)		 The following lease and delete operations on the file will fail:

  • Create File
  • Delete File
  • Lease File
Shared Read/Delete

(Deny Write)		 The following write, lease, and delete operations on the file will fail:

  • Create File
  • Set File Properties
  • Set File Metadata
  • Put Range
  • Delete File
  • Lease File
Shared Write/Delete

(Deny Read)		 The following read and lease operations on the file will fail:

  • Get File
  • List Ranges
  • Delete File
  • Lease File
Shared Read/Write/Delete

(Deny Nothing)		 Delete File

Azure Files returns sharing violations only when files are open on SMB clients. For a FileREST Delete File operation to succeed, there can be no SMB clients with handles open against that file. For more information, see the Delete File operation and Interaction between FileREST and SMB opportunistic locks.

SMB file locking implications on FileREST Lease File API

Depending on the file access options you specify when an SMB client opens a file, it's possible for the FileREST Lease File API to return status code 409 (Conflict), with error code SharingViolation. The following table provides further information:

SMB client file access option Acquire lease on file without an active lease with Lease File API
None Succeeds
Read Succeeds
Write Fails due to SharingViolation
Delete Fails due to SharingViolation
Read|Write Fails due to SharingViolation
Read|Delete Fails due to SharingViolation
Write|Delete Fails due to SharingViolation
Read|Write|Delete Fails due to SharingViolation

Azure Files returns sharing violations only when files are open on SMB clients. Note that for a FileREST Lease File operation to succeed, there can be no SMB clients with Write or Delete handles open against that file. For more information, see the Lease File operation and Interaction between FileREST and SMB opportunistic locks.

FileREST Lease File implications for SMB file locking

A lease on a file provides exclusive write and delete access to the file. When an SMB client opens a file, it must respect the lock for any file leased by the FileREST Lease File operation. You can use the following table to determine whether the SMB open file operation can be completed:

FileREST file lease state SMB operations failing with a sharing violation
Leased SMB clients opening the file with the following file access will fail:

  • FileAccess.Write
  • FileAccess.Delete
  • FileAccess.Read|FileAccess.Write
  • FileAccess.Write|FileAccess.Delete
  • FileAccess.Read|FileAccess.Write|FileAccess.Delete
Available None
Broken None

SMB delete implications on FileREST

When an SMB client opens a file for delete, it marks the file as pending delete, until all other SMB client open handles on that file are closed. While a file is marked as pending delete, any FileREST operation on that file will return status code 409 (Conflict), with error code SMBDeletePending. Status code 404 (Not Found) isn't returned, because it's possible for the SMB client to remove the pending deletion flag prior to closing the file. In other words, status code 404 (Not Found) is only expected when the file has been removed.

While a file is in an SMB pending delete state, it won't be included in the List Files results.

Also note that the FileREST Delete File and Delete Directory operations are committed atomically, and don't result in pending delete state.

File attribute implications on FileREST

It is possible for SMB clients to read and set file attributes, including:

  • Archive
  • Read-only
  • Hidden
  • System

If a file or directory is marked read-only, then any FileREST operation that attempts to write to the file will fail with status code 412 (Precondition Failed), and error code ReadOnlyAttribute. These operations include:

  • Create File
  • Set File Properties
  • Set File Metadata
  • Put Range

These file attributes can't be set or read from REST clients. After a file is made read-only, REST clients are unable to write to the file until the SMB client removes the read-only attribute.

Interaction between FileREST and SMB opportunistic locks

SMB opportunistic lock (oplock) is a caching mechanism that SMB clients request in order to improve performance and reduce network transfers. An SMB client can cache the latest state of a particular file or directory. There are multiple opportunistic lock types, referred to as SMB lease types:

  • Read (R): The SMB client can read from local cache.
  • Write (W): The SMB client can write locally, without the need to flush the data back to the Azure file share.
  • Handle (H): The SMB client isn't required to immediately notify the Azure file share when a handle is closed. This lock type is useful when an application continues opening and closing files with the same access and sharing mode.

These lease types are independent of the access and sharing mode specified. Typically, an SMB client attempts to acquire all lease types whenever it opens a new handle against a file, regardless of access and sharing mode.

Depending on the FileREST operation called, you might need to request to break an existing opportunistic lock. In the case of a write oplock, the SMB client must flush cached changes to the Azure file share. Here are some cases where each type of oplock needs to be broken:

  • A Read (R) oplock needs to be broken whenever a write operation is issued, such as Put Range.

  • A Write (W) oplock needs to be broken whenever a read operation is issued, such as Get File.

  • A Handle (H) oplock needs to be broken whenever a client issues a delete operation. Azure Files requires that no handles can be open if a delete operation is to succeed.

    Handle oplocks are also broken when a FileREST operation faces a sharing violation with an existing SMB handle. This occurs to verify that the handles are still opened by an application running on the clients.

Breaking the oplock might require flushing cached SMB client changes, which can cause delays in operation response time. Flushing might also cause the operation to fail with status code 408 (Request Timeout), and error code ClientCacheFlushDelay.

The following sections discuss scenarios where oplocks are broken.

An oplock break and SMB client flush are required, and the REST client experiences a delay

Consider the following example:

  1. The SMB client opens a file, acquires an RWH oplock, and writes data locally.

  2. The REST client sends a Get File request.

    • The Azure file share breaks the write (W) oplock, leaving the client with an RH oplock.
    • The SMB client flushes its cached data against the Azure file share, and acknowledges the oplock break.
    • The Azure file share processes the Get File request and responds back with the requested data.

In this example, the REST client experiences delays. This situation is caused by the oplock break, and the time taken by the SMB client to flush its data against the Azure file share.

Subsequent calls to Get File don't experience any additional delays, because the write (W) oplock has already been broken.

An oplock break is required, but the REST client won't experience a delay

Consider the following example:

  1. The SMB client has acquired an RH oplock.

  2. The REST client sends a Put Range request.

    • The Azure file share sends an oplock break request to the SMB client, and doesn't wait for a response.
    • The Azure file share processes the Put Range request.

In this example, the oplock break is required, but the Put Range request doesn't experience any additional delays. A response isn't needed when breaking the read oplock.

Azure Files behavior

The following table summarizes the behavior of Azure Files for each FileREST operation. This behavior is based on the oplock state of the SMB client that has already acquired a handle on the same file. Additionally, the behavior assumes that the SMB handle access and sharing don't conflict with the FileREST operation.

If there is a conflict, the handle oplock is also broken to ensure that the handle is still open on the client. In the case of a blocking oplock break, Azure Files must wait for an acknowledgment that the break was successful. In the case of a non-blocking oplock break, Azure Files doesn't have to wait.

FileREST operation Current oplock types Oplock break performed Resulting oplock
Get File RWH Yes (Blocking) RH
Get File RH No RH
Get File RW Yes (Blocking) R
Get File Properties RWH Yes (Blocking) RH
Get File Properties RH No RH
Get File Properties RW Yes (Blocking) R
List Ranges RWH Yes (Blocking) RH
List Ranges RH No RH
List Ranges RW Yes (Blocking) R
Get File Metadata RWH Yes (Blocking) RH
Get File Metadata RH No RH
Get File Metadata RW Yes (Blocking) R
List Files RWH No RWH
List Files RH No RH
List Files RW No RW
Put Range RWH Yes (Blocking) None
Put Range RH Yes (Non-Blocking) None
Put Range RW Yes (Blocking) None
Set File Properties RWH Yes (Blocking) None
Set File Properties RH Yes (Non-Blocking) None
Set File Properties RW Yes (Blocking) None
Set File Metadata RWH Yes (Blocking) None
Set File Metadata RH Yes (Non-Blocking) None
Set File Metadata RW Yes (Blocking) None
Delete File RWH Yes (Blocking) RW
Delete File RH Yes (Blocking) R
Delete File RW No RW

In the case where a blocking oplock break is required, under certain conditions, the FileREST operation fails. If the break doesn't succeed within the specified request timeout, or within 30 seconds, whichever completes first, the service returns status code 408 (Request Timeout), and error code ClientCacheFlushDelay.

The Delete File request also requires breaking the oplock handle (H) lease. Breaking the handle ensures that there are no file handles still opened by an SMB client application when a REST client calls Delete File. If there is a sharing violation, the request fails with status code 409 (Conflict), and error code SharingViolation.

See also

Azure Files concepts