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vss cache file

Level 6

I have a NBU issues with error 156.


·         In Windows, right-click My Computer and select Manage

·         In the console tree, right-click Shared Folders, select All Tasks, and then Configure Shadow Copies

·         Select the volume on which to make changes, and then click the Settings button

·         In the Settings dialog box, change the Maximum Size setting to either No Limit or a size large enough to suit the requirements of the installation and usage of VSS.


How copy-on-write works

A copy-on-write snapshot is a detailed account of data as it existed at a certain moment. Unlike a mirror, a copy-on-write is not a copy of the data, but a specialized account of it.

The copy-on-write process works as follows: when a snapshot is required, any unfinished transactions or changes to the source data are allowed to complete, but new changes are temporarily stalled. The source is momentarily idled (made quiescent). Once the copy-on-write is activated, new transactions or changes (writes) to the source data are allowed to take place. However, the copy-on-write process briefly intercepts or holds the first write request that is issued for any particular block of data. While it holds those requests, it copies to cache the blocks that are affected by those writes, and keeps a record of the cached blocks. In other words, it reads each source block that is about to change for the first time. Then it copies the block's current data to cache, and records the location and identity of the cached blocks. Then the intercepted writes are allowed to take place in the source blocks.



This size is for the snapshot that is reserved for the  OS  under configure shadow copies ?or this is the size of the open file backup .(files that are active during the backup)?



Level 6

I guess You are referring to COW technology.

That space which u r refering too is the shadow copy space.That is use when OS creates the snapshot.Snapshot is the metadata of the drive which is being backed up.10 % free space of actual size of drive. 

Level 6

then what is this cache reserved pool or cache LUN for active files?