SQL Server VSS allows ISVs to develop backup solutions using APIs exposed by VSS framework. One such API method which is commonly used while performing differential backup using SQL VSS is IVssComponent::GetPartialFile. While performing differential backup, IVssComponent::GetPartialFile is used iteratively to obtain the range of changed differential bytes for backup which changed since last full backup. The output for GetPartialFile provides the file path, file name and pbstrRange which is a pointer to 64KB string buffer containing the comma-separated list of the form offset1:length1, offset2:length2, where each offset and length is a 64-bit integer specifying a file offset in bytes and length of the range in bytes, respectively. If the changed differential bytes within a partial file is large enough to fall outside the tracking range limit of the 64 bit string buffer, pbstrRange returns the partial filename containing those ranges instead of the range and offset within the current partial file.
The ISV Backup code should be able to handle both the output for pbstrRange viz offset/length range string for backup range within the current file or filename containing the backup range if the range falls outside the current file.
Following T-SQL sample code can help the backup vendors to simulate this behavior consistently to test and handle this scenario where pbstrRange generates filename. The T-SQL sample code below updates alternate extents to create enough non-contiguous extent such that the changed differential extent range falls outside 64KB buffer so pbstrRange outputs filename containing the ranges to be backed up.
The script is uploaded and made in SQL Server Sample repository in GitHub for the community.
-- create database create database test; use test; -- create a table such that each row is on a page create table tab(ID INT IDENTITY NOT NULL PRIMARY KEY, BIGSTRING VARCHAR(7996)); declare @dml NVARCHAR(300); --for query declare @RowCount int; -- RowCount iterator set @RowCount = 0; set @dml = 'INSERT INTO tab(BIGSTRING) VALUES(REPLICATE(''a'', 7996));' --Max row count we need to insert, 150k rows gives us close to 65kb buffer --using below updates, I have kept it 200k which gives 98kb buffer, anything above --64 kb will give us filename instead of range values. declare @MaxRowCount int; set @MaxRowCount = 200000; --Insert the rows while @RowCount < @MaxRowCount begin exec(@dml); set @RowCount = @RowCount + 1; end -- TAKE THE FULL BACKUP HERE, I executed the betest.exe here. -- BETEST.EXE /B /E /T FULL /S backupdoc_full.txt /D c:\tools\sqlbetest\backup\full /C comp1_file.txt -- here we update the rows I re-declared the variables as I executed it as separate script -- I am updating every 17th row so, we update alternate extent, which will make range buffer big. declare @dml NVARCHAR(300); declare @RowCount int; declare @MaxRowCount int; set @MaxRowCount = 200000; set @RowCount = 1; while @RowCount < @MaxRowCount begin set @dml = 'UPDATE tab set BIGSTRING = REPLICATE(''b'', 7996) where ID = ' + cast(@RowCount as varchar(10)) + ';'; exec(@dml); set @RowCount = @RowCount + 16; end -- Take differential backup here. -- BETEST.exe /B /E /T DIFFERENTIAL /Pre backupdoc_full.txt /S backupdoc_diff.txt /D c:\tools\sqlbetest\backup\diff /C comp1_file.txt
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