Error: -2 The system cannot find the file specified.

Article Number: 
289

Problem:

The ShadowProtect backup reports that a successful snapshot has been taken and later reports that it cannot open the STC_SnapShot_Volume, which causes the backup to fail.

 12-Feb-2013 21:08:42 service 103 snapshot was created by  VSS API by STC provider. It took 59 seconds

12-Feb-2013 21:08:42 sbvol 404 Cannot open file \\?\STC_SnapShot_Volume_21_1 (-2 The system cannot find the file specified.) 

 

Cause:

There is insufficient space in shadow storage to use the VSS snapshot during the ShadowProtect backup process.

Exceptions:

If the sbvol 404 "cannot open file" error is preceded by an sbvol 504 error:

It is likely a problem with corrupt registry settings from a previous installation of ShadowProtect that was not completely removed. This may require opening a technical support request for problem resolution.

Explanation:

ShadowProtect backup uses Microsoft’s VSS to create snapshots of the partition that is being backed up. VSS creates the snapshot by the copy-on-write method. This means that no data is actually written into the VSS snapshot until a modification is made to the data in the live partition. http://technet.microsoft.com/en-us/library/cc785914(v=ws.10).aspx describes this process:

Copy-on-Write (Differential Copy)

The copy-on-write method creates shadow copies that are differential rather than full copies of the original data. … This method makes a copy of the original data before it is overwritten with new changes… When a change to the original volume occurs, but before it is written to disk, the block about to be modified is read and then written to a “differences area”, which preserves a copy of the data block before it is overwritten with the change. Using the blocks in the differences area and unchanged blocks in the original volume, a shadow copy can be logically constructed that represents the shadow copy at the point in time in which it was created. (Emphasis added.)

The snapshot, also refered to as a shadow copy, is created in unallocated space, on the volume, that has been reserved for VSS shadow copies. Shadow copies that have not been deleted can be seen by issuing the following command in a command prompt. (Figure 1)

vssadmin list shadows

You can also check for orphaned shadow copies by checking the Previous Versions tab in the Properties window of the volume. (Figure 2) 

Figure 1 - vssadmin list shadows output

 

Figure 2 - Volume, Properties, Previous Versions

After the writers of VSS-aware applications (vssadmin list writers) have been asked to flush-and-freeze writes to the volume, a map is made of the disk sectors on the live partition. Subsequent writes to the live partition will be preceded by VSS copying the to-be-modified sectors into the VSS snapshot shadow storage. Because the snapshot expands as more sectors are modified on the active data partition, VSS cannot predict how much storage will be required when the initial request is made to perform the snapshot.

This means that the size of the snapshot is very small when it is initially created and that it expands as more and more sectors are modified on the live partition, causing the point-in-time original sectors to be written to the snapshot storage.

The error “Cannot open file \\?\STC_SnapShot_Volume_xxx (-2 The system cannot find the file specified.)” is caused when the snapshot attempts to grow beyond the amount of available snapshot storage.  The snapshot is then deleted by VSS to prevent a ShadowProtect from assuming the snapshot is valid.  The failed snapshot is invalid because there is no longer space available to add modified sectors.

The size of available snapshot storage can be seen by issuing the following command in a command prompt. (Figure 3):

vssadmin list shadowstorage

 

Figure 3 - vssadmin list shadowstorage output

Resolution

Ensure that the VSS shadow storage is sufficiently large to accommodate the type of backup being performed. A general rule of thumb is 20% free space should be available on each partition, although the actual size required for shadow storage is dependent upon how quickly applications are modifying data after the backup has begun and how quickly ShadowProtect is able to copy the data into the backup.

Shadow storage can be managed with the following command in a command prompt.:

vssadmin resize shadowstorage

Factors that may affect ShadowProtect’s ability to complete the backup before shadow storage is exhausted include:

      • Whether the backup is being written over the network. Network speeds, congestion and the performance of the remote share, NAS, etc. will affect how quickly ShadowProtect is able to complete the backup before shadow storage is exhausted.
      • Whether the file system is being modified by many applications.
      • The volume of modifications on the live partition.
      • The overall performance of the system, including CPU queues, disk queues, etc.

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