Glossary

BLOCKSIZE

This is the size, in bytes, that is used as the device BLOCKSIZE. All data transfers are in integral multiples of this value. Values must be a power of 2 between 512 bytes and 64 kilobytes (KB) inclusive. If this option is not specified with the command, then a default of 512 bytes is used.

The BLOCKSIZE parameter is specified in the WITH clause of the BACKUP and RESTORE statements. For example:

BACKUP DATABASE pubs to VIRTUAL_DEVICE='...' WITH BLOCKSIZE=65536

BUFFERCOUNT

This is the total number of buffers (of size MAXTRANSFERSIZE) used by the BACKUP or RESTORE operation.

The BUFFERCOUNT parameter is specified in the WITH clause of the BACKUP and RESTORE statements. For example:

BACKUP DATABASE pubs to VIRTUAL_DEVICE='...' WITH BUFFERCOUNT=20

buffers

Shared buffers are provided from the virtual device set to Microsoft® SQL Server™ on demand. These buffers are referenced by commands issued to the virtual devices. While the commands are being processed by the device, the buffers are in client control. The server will not read or write to the buffer while the command is outstanding to the client. The client may only read or write to the buffer, or remember the buffer's address, from the time it receives a command until the time it completes the command. When the client indicates that the command is completed, then the buffer is implicitly returned to SQL Server control.

data stream

A data stream is an ordered sequence of bytes and filemarks.

MAXTRANSFERSIZE

This is the size, in bytes, of the maximum input or output request that is issued by SQL Server to the device. This is the size of the data portion of the buffer. It does not include the prefix zone, if any. The MAXTRANSFERSIZE must be a multiple of 64KB. The range is from 64KB through 4 megabytes (MB). If this option is not specified with the command, then a default of 64KB is used.

The MAXTRANSFERSIZE parameter is specified in the WITH clause of the BACKUP and RESTORE statements. For example:

BACKUP DATABASE pubs to VIRTUAL_DEVICE= '...' WITH MAXTRANSFERSIZE=524288

virtual device

The virtual device is implemented by the client. It is used by SQL Server as a storage device, like any other device. During BACKUP, a data stream is written to the device. During RESTORE, the data stream is read from the device.

The number of devices used during a RESTORE will typically be the same as that used for the BACKUP. It is possible to use fewer devices during RESTORE operations if the media is removable. However, for removable media, it is possible to use fewer devices. In that case, once each data stream has been read, SQL Server requests a new, unprocessed data stream by issuing a mount request. For more information about media sets, backup sets, and media families, see Microsoft SQL Server Books Online.

Virtual Device Interface (VDI)

The VDI is a set of Component Object Model (COM) interfaces. Behind the interfaces are COM objects that implement the behavior of virtual devices.

virtual device set

The virtual device set is the top-level object to be manipulated by the client and server sides of the interface. The client is responsible for creating the virtual device set. The server opens and configures the virtual device set.