Advantages and disadvantages of VSS and snapshot

From: http://www.storworld.com/wlcc/520055.html

Volume Shadow Copy Service (VSS) is a service introduced by Microsoft in Windows Server 2003. It allows you to use it without the assistance of professional IT personnel, it is easier to restore lost files. In addition, VSS provides more flexible backup solutions. Combined with good planning and the latest backup and recovery technologies, the Shadow Copy Service will be very helpful for the enhancement of the disaster recovery plan. VSS even enables small enterprises to restore data loss. In fact, VSS also provides more recovery options for large enterprises than basic tools, and helps them reduce the number of IT professionals in data recovery tasks.

Snapshot, that is, Snapshot technology, is widely used in backup. It has been applied to arrays and hosts for a long time, mainly using the copy on write algorithm. For detailed technical details, go to snia.org. Generally, they are all volume-based and block-level processing. All the large arrays are supported. Volume managers on the host OS, such as the volume manager on the ESX Server, can take snapshots.

Snia defines snapshot as a fully available copy of a specified data set. This copy includes an image of the corresponding data at a certain time point (the start time of the copy. A snapshot can be a copy of the data it represents or a copy of the data.

A snapshot actually points to the reference mark or pointer of the data stored in the storage device. To put it bluntly, it is the status of the data at a certain time point. The core of its working principle is to create a pointer list, indicates the address of the data to be read, providing an image of the instantaneous data. When the data changes, the data is copied.

There are roughly two types of snapshots. One is a copy-on-write snapshot, which is also known as a pointer snapshot, and the other is VSS, another type is image snapshot, which is often called image snapshot. Pointer-type snapshots occupy a small amount of space and have little impact on system performance. However, if the original data disk is not backed up and the data disk is broken, the data cannot be recovered; the image snapshot is actually the full image of the data at that time, which will impose a certain amount of load on the system performance, but even if the original data is damaged, it will not have a big impact, but it will take up equal space.

There are two implementation methods for pointer-type snapshots. One is that before the original data is modified, first, copy the original data to a specific snapshot area (a separate disk partition or a region occupied by the original file system), and then modify the pointer in the snapshot, in this way, the pointer structure of the original file system can be changed to reduce disk fragments. Most manufacturers' snapshots do this, including VSS; another way is to write the modified data to the free space of the same file system, and modify the original pointer of the file system, instead of modifying the snapshot pointer. There are not many vendors using this method, the best thing is netapp wafl, which depends on its special file system structure and good compatibility with NVRAM.

Image snapshots make it possible to access data offline, such as EMC timefiner, IBM flashcopy, HDS shadowimage, and Veritas Volume Manager snapshot. However, it cannot be generalized. For example, IBM flashcopy can be divided into two methods: foreground copy and background copy. Foreground copy is an image snapshot, and background copy is a pointer snapshot.

In practical applications, snapshots can be implemented by the software in the storage device controller, or by third-party software such as volume manager on the OS or the technology of the file system itself.

Therefore, VSS is a type in snapshot!

VSS is a snapshot technology in windows. It consists of requestor, write, and provider. It mainly consists of the driver volsnap. sys implementation is added between the volume management driver and the file system driver, and the COM technology is integrated. Therefore, it is no longer simply dealing with block at the volume level, but associated with various system applications, such as SQL, exchange, AD, and so on. This allows you to take snapshots without shutting down or stopping applications. VSS is widely used in Windows Backup processing. In addition to its built-in backup applications, VSS is also used by VERITAS, CA, commvault, and other manufacturers.

VSS is very different from the snapshot of the disk array. It requires VSS requestor sending for backup. Instead of using the backup software of Veritas and Ca, VSS regards veritas and Ca as requestor, issued by the backup software. The disk array is not.

VSS has the advantage of backing up the NTFS shared folder on the host, which cannot be done by disk array snapshots. It is not intended for host downtime, it protects data and files in the storage.

Why can't disk arrays work? That's because it export out is a Lun, which is based on the volume level and processes blocks. It processes the details of the file system, which is completed by the OS.

The biggest difficulty in VSS is how to lock various applications and perform hot backup to ensure data integrity. Because during operating the OS, each module in the kernel is highly correlated, it's just a trigger!