Stuart Moore

Musings of a Data professional

Stuart Moore

Month: November 2017

Complex SQL Server restore scenarios with the dbatools Restore-DbaDatabase pipeline

dbatools logoNo matter how hard the dbatools team try, there’s always someone who wants to do things we’d never thought. This is one of the great things with getting feedback direct from a great community. Unfortunately a lot of these ideas are either too niche to implement, or would be a lot of complex code for a single use case

As part of the Restore-DbaDatabase stack rewrite, I wanted to do make things easier for users to be able to get their hands dirty within the Restore stack. Not necessarily needing to dive into the core code and the world of Github Pull Requests, but by manipulating the data flowing through the pipeline using standard PowerShell techniques All the while being able to do the heavy listing with out code.

So, below the fold we’ll be looking at some examples of how you can start going to town with your restores

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Deep copy arrays in PowerShell

PowerShellJust a quick post for something I had to deal with for another post I’m writing. I wanted to do some work where I’d reuse a base array over a number of passes in a loop, and didn’t want any change made impacting on later iterations. If you’ve tried copying an array containing arrays or other objects in PowerShell in the usual manner then you’ll have come across this problem:

Copying array of arrays fails in powerShell

As is common in .Net, the copy is ‘Copy by Reference’ so you don’t get a nice shiny new independent array to play with. All that’s copied is the references to the original’s place in memory. Therefore any changes to either object affects both, as both variable names are looking at the same piece of memory. This is nice and efficient in terms of storage and speed of copying, but not great for my purposes.

There are various workarounds kicking around if you’re using simple arrays, but they tend to breakdown when you’ve got arrays that contain arrays or other PowerShell objects. My method for copying them is a little down and dirty, but it works 95% of the time for what I want. The trick is to Serialize the object, and then DeSerialize it into the new one:

Deep Copy Powershell array using serialization

And, voila we have the outcome I wanted. Just to make the line of code easier to read, here it is:

$arr3 = [Management.Automation.PSSerializer]::DeSerialize([Management.Automation.PSSerializer]::Serialize($arr2))

Now, I mentioned up front that this works ~95% of the time. The times it doesn’t work for me are when the underlying object type doesn’t serialize nicely. The most common one I come across is BigInt. This ‘deserializes’ back in as a non integer type and then won’t play nice when compared to other ‘real’ BigInt value, so make sure to check you have the values you think you should do

Debugging the new dbatools Restore-DbaDatabase pipeline

A new version of dbatools Restore-DbaDatabase command was released into the wild this week. One of the main aims of this release was to make it easier to debug failures in the restore process, and to drag information out of the pipeline easily (and anonymously) so we can increase our Pestering of the module with Unit and Integration tests.

So I’d like to share some of the features I’ve put in so you can take part.

The biggest change is that Restore-DbaDatabase is now a wrapper around 5 public functions. The 5 functions are:

  • Get-DbabackupInformation
  • Select-DbabackupInformation
  • Format–DbabackupInformation
  • Test–DbabackupInformation
  • Invoke-DbaAdvancedRestore

These can be used individually for advanced restore scenarios, I’ll go through some examples in a later post.

For now it’s enough to know that Restore-DbaDatabase is a wrapper around this pipeline:

Get-DbabackupInformation |Select-DbabackupInformation | Format-DbabackupInformation | Test-DbabackupInformation | Invoke-DbaAdvancedRestore

and it’s other function is passing parameters into these sub functions as needed.

With version of Restore-DbaDatabase you were restricted to throwing data into one end, and seeing what came out of the other end, with some insight produced by Verbose messages. Now things can be stepped through, data extracted as need, and in a format that plugs straight into out testing functions.

Get-DbaBackupInformation

This is the function that gets all of the information about backup files. It scans the given paths, and uses Read-DbaBackupHeader to extract the information from them. This is stored in a dbatools BackupHistory object (this is the same as the output from Get-DbaBackupHistory, so we are standardising on a format for Backup information to be passed between functions).

So this would be a good place to check that you’ve gotten the files you think you should have, and is also the first place we’d be looking if you had a report of a break in the LSN chain

To get the output from the pipeline at this point we use the GetBackupInformation parameter:

Restore-DbaDatabase - -GetBackupInformation gbi

This will create a globally scoped variable $gbi containing the ouput from Get-DbaBackupHistory. Note, that when passing the name to Restore-DbaDatabase you do not need to specify the $.

If you want to stop execution at this point, then use the -StopAfterGetBackupInformation switch. This will stop Restore-DbaDatabase from going any further.

This is also a good way of saving time on future runs, as the BackupHistory object can be passed straight in, saving the overhead of reading all the file heasers again:

$gbi | Restore-DbaDatabase [Usual Parameters] -TrustDbBackupHistory

Select-DbaBackupInformation

Here we filter down the output from Get-DbaBackupInformation to restore to the point in time requested, or the latest point we can. This means we find :
– the last full backup before the point in time
– the latest differential between the full backup and the point in time
– and then all transaction log backups to get us to the requested time
This is done for every database found in the BackupHistory object

Here is where we’d begin looking for issues if you had a ‘good’ LSN chain from Get-DbaBackupInformation and then it broke.

To get this data you use the SelectBackupInformation parameter, passing in the name of the variable you want to store the data in (without the $ as per GetBackupInformation above)

There is also a corresponsing StopAfterSelectBackupInformation switch to halt processing at this point. We stop processing at the first stop in the pipeline, so specifying multiple StopAfter* switches won’t have an effect

Format-DbaBackupInformation

This function performs the transforms on the BackupHistory object per the parameters pushed in. This includes renaming databases, and file moves and rename. For everything we touch we add an extra property of Orignal to the BackupHistory object. For example the original name of the database will be in OriginalDatabase, and the target name will be in Database

So this is a good spot to test why transforms aren’t working as expected.

To get data out at this pipeline stage use the FormatBackupInformation paramter with a variable name. And as normal it has an accompanying StopAfterFormatBackupInformation switch to halt things there

Test-DbaBackupInformation

Before passing the BackupHistory object off to be restored we do some checks to make sure everything is OK. The following checks are made:

  • LSN chain complete
  • Does a destination file exist, if owned by a different database then fail
  • Does a destination file exist, if owned by the database being restored is WithReplace specfied
  • Can SQL Server see and write to all the destination folders
  • Can SQL Server create any destination folders missing
  • Can SQL Server see all the backup files

If a database passes all these checks then it’s backup history is marked as restorable by the IsVerified property being set $True.

To get the data stream out at this point use the TestBackupInformation parameter.

General Errors with restores

Once we’re past these stages, then our error reporting is at the mercy of the SMO Restore class. This doesn’t always provide an obvious cause straight away. Usually the main error can be found with:

$error[1] | Select-Object *

We like to think we capture most restore failure scenarios nicely, but if you find something we don’t then please let you know, either on Slack or by raising a Github issue

As usually the dbatools terminating error will be in $error[0].

Providing the information for testing or debugging.

If you’re running in to problems then the dbatools team may ask you to provide the output from one of these stages so we can debug it, or incorporate the information into our tests.

Of course you won’t want to share confidential information with us, so we would recommend anonymising your data. My normal way of doing this is to use these 2 stubbing functions:

So if we’ve asked for the Select-DbaBackupInformation the process would be:

Restore-DbaDatabase -[Normal parameters] -SelectBackupInfomation sbi -StopAfterSelectBackupInformation
Filter-DbaToolsHelpRequest $sbi
$sbi | Export-CliXml -Depth -Path c:\some\path\file.xml

And then upload the resulting xml file.

This method will anonymise the values in ComputerName, InstanceName, SqlInstance, Database, UserName, Path, FullName, FileList, OriginalDatabase, OriginalFileList, OriginalFullName and ReplaceDatabaseName. But will produce the same output for the same input, so we can work with multiple database sets at once.

I hope that’s been of some help. As always if you’ve a question then drop a comment below, ping me on twitter (@napalmgram) or raise an issue with dbatools on Slack or Github

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