Thursday, June 19, 2008

Powershell Function: Get-LogicalDisk

In this post, I will discuss Get-LogicalDisk. Get-LogicalDisk is a function that retrieves disk information from remote servers via WMI. This allows for quick and flexible reporting on storage usage and availibility.

This is a bit of a departure from "SQL Server"-oriented functionality that I have been describing in recent posts. In some shops, free disk space or, more likely, the lack of it is a daily battle. Every morning, I run a script that checks for many different kinds of "negative events" that might have happened over night. A "negative event" might be a failed job, database growth or a server running low on disk space. The latter check calls Get-SQLServerList and Get-LogicalDisk to do all of the heavy work.

The script is straightforward, so I'll just continue with my tradition of a brief overview with some examples. (This is a blog, after all, and not really suited to voluminous detail. FWIW, it is my opinion that detail on what a confusing part of a script does should be in the comments in the script, not in a blog posting. If you find a part of any script confusing, let me know and I'll update the comments in the script and possibly do a blog posting to explain how it works.)

The function takes a list of computers as pipeline input, so feeding a list of SQL Servers using Get-SQLServerList (which I normally alias as "ssl") or any other method is easy. Get-SQLServerList uses WMI to retrieve the following information from each "fixed disk" on the remote system:
  • Computer
  • Letter
  • Label
  • Space, which gives the total size of the disk
  • FreeSpace, which gives the size of the free space on the drive
  • UsedSpace, which gives the size of the space which is being used on the drive
  • Free-%
  • Used-%
  • Unit
The computer name is returned to allow discrimination between servers in reports that cover more than one server. The function also reports back what units (MB, GB, TB, etc.) the three space values are measured in. this gives a visual cue to a user who is manually running reports from a command line.

I would like to point out that one could calculate UsedSpace by subtracting FreeSpace from Space and that one could calculate Free-% given Used-%, yet they are returned by Get-SQLServerList. These calculations are done by Get-SQLServerList so a calling program (or something that you've hacked together on a command line) do not have to. This makes for a more flexible function and for less typing.


Here is a simple example, including some output, which finds information on the local computer:
PS> get-logicaldisk . | ft -a

Computer Letter Label Space FreeSpace UsedSpace Free-% Used-% Unit
-------- ------ ----- ----- --------- --------- ------ ------ ----
MYLAPTOP C: local.boot 33.17 7.94 25.23 24 76 GB

If you are using a naming scheme (or some other method) to discriminate between development, quality assurance and production servers, the following query might provide a list of disks where you might be able to store that 'temporary' backup:

PS> ssl de | Get-LogicalDisk | where {$_.Letter -match "c"} | sort FreeSpace | ft -a


Here is a command that looks for disks on production servers with less than 5% free space:
PS> ssl pr | Get-LogicalDisk | where {$_."Free-%" -lt 5} | ft -a

I hope that you can see the power of the Get-LogicalDisk function, which can easily be leveraged by additional simple Powershell syntax to provide complicated, real-time reports on disk space issues.
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