Then the VSS informs the VSS writers that a snapshot has been taken and that they can release the database.ħ. Once a snapshot is taken, it informs VSS that its job is done.Ħ. A snapshot is taken which takes a maximum of 10 secs. The VSS then informs the providers to take a snapshot. Once the VSS writers complete their job ( have flushed and freezed the database ), it informs Volume Shadow Copy Service ( VSS ), that they are ready now.ĥ. The maximum time VSS writers can freeze their databases is for 60 seconds.Ĥ. The VSS informs the respective writers that a snapshot creation is going to happen, so they must flush everything and freeze their databases for a a few seconds. The request is sent to Volume Shadow Copy Service ( VSS ) or volsnap.sysģ. We initiate a backup using a backup application.Ģ. WHAT HAPPENS WHEN WE INITIATE A BACKUP using VSSġ. If you want to learn about file systems, please GOOGLE. Further explanation not required for this. Without a file system, nothing is possible.
It may cause the writers to fail too and generate event errors like Event id 8, 12298 in the event Viewer.Īlso known as NTFS.sys is the most important part of an OS. If the providers fail to do their job, it also affects the writers. Sometimes, the providers may conflict or may malfunction. Note: When we run the command "vssadmin list providers" - The one that comes up on the top of the list, is the one installed last, and is the one used by default. If we add a backup program like StorageCraft, then a provider from StorageCraft named "StorageCraft Shadow Copy Provider" will be installed on top of the default provider and will be used as the default provider.įurther if we add Symantec Backup Exec, then a provider from Symantec will also be installed on top of the other two and will be used as the default provider.
It is the most important part of a shadow copy creation.īy default, we always have the "Microsoft Software Shadow Copy Provider" also known as "swprv.dll" installed on the system. Now, these are the ones responsible for taking a snapshot. The writers will not interfere with each other as they are only responsible for their own database. This will also flush existing transactions ( read/write operations ) in its own Exchange database before a snapshot can be taken. So, the registry writer will flush everything in its own database and stop further transactions ( read/write operations in registry ) during a snapshot creation process. It comes with the package of the individual component.
The moment we install Exchange, SharePoint,IIS or SQL, their writers are also installed. Writers like Exchange, SQL, IIS are not preinstalled on the system. The OS comes with some default writers like Registry Writer, System Writer, COM+ Writer etc. The data in memory has to be flushed and further transaction have to stopped For a perfect snapshot, these open transactions have to be closed first. If we take a snapshot without preparing them first, the snapshot might be corrupt due to open transactions. They just prepare the database for a snapshot creation.įor example, there could be open transactions for Exchange, SQL, Registry, DCOM etc. Again, VSS Writers do not take snapshots.
VSS Writers are the ones who prepare the database for a snapshot ( shadow copy creation ). If we further add Exchange, database for Exchange is created. If we add SQL to the server, then a database for SQL is created. For example, on a clean installed system, we have a database for the Registry, System, DCOM etc. On a Server, we have a database for everything. Any transaction has to go through this service. ( Writers, Providers & OS ) communicate with each other effectively. The point to note is that VSS ( Volume Shadow Copy Service ) DOES NOT TAKE SNAPSHOTS as we all assume it does. This is the service which helps in co-ordination ( communication between the OS, the VSS writers, the VSS providers ). Volume Shadow Copy Service - (volsnap.sys)
There are 3 important components involved in a snapshot creation. Snapshot:- This is a point in time copy (shadow copy) of a volume. Itįirst let us understand some background of VSS.
I have been analyzing some tasks and have found that in cases where after updating the drivers for VSS, deleting the Subscriptions key and re-registering the DLL's, if we still find that the VSS writers fail, then it is not the writers which are at fault. It may be a single VSS Writer or many of them from the list we get on running "vssadmin list writers". Very often we come across issues, in which the VSS Writers keep failing after two or three backups.