Shrinking Database Log Files

Files in SQL Server need to grow as the database grows, and in very specific circumstances need to be shrunk as well with additional maintenance.  There is quite a bit different between log files and data files, so I created a separate post for Shrinking Database Data Files.

When should you shrink log files

First, if a large process that will never happen again caused your log file to grow to a ridiculous size then it’s reasonable to shrink the file down to about 150% of the most space you expect to use in the next year.  Cleaning this up will also cut down on restore times.

Second, if your log file grew in small increments then you’re left with a large number of VLFs like I talked about in file growths.  The only way to get rid of these is to shrink your log files down then have them grow in larger increments.  When I do this, I’m also growing them manually immediately afterwards in the same script.

What happens when you shrink log files

The log file will find a group of empty VLFs and delete them.  The end, your file is shrunk.

Of course there’s a little more to it than that.  The parts I’m most worried about are why did you need that space in the first place, why do you need the space freed up, and what happens when you need it again?

To understand why you needed it we need to talk about what a log file really is for a second.  It is every change made to your database since the last point-in-time where it didn’t need all the changes saved off.  This is typically either the oldest open transaction (so it can roll back any change if the transaction fails) or, if your database is in full or bulk-logged recovery, the last time you ran a log backup   It can also be due to other functionality such as database mirroring, but you typically have an experienced DBA on hand for that.

Why is this common to do?

There is typically at least once in every large database’s life where the log needs to be shrunk for one of a couple reasons.

Going off of the two reasons above, the first two are because a large process ran that won’t run again.  The third reason I go over here is because of small VLFs.

1. Log backups weren’t set up

The first large process is when a database is set up in full or bulk-logged recovery, but the log backups weren’t set up until the drive filled.  This is because the log needs to be backed up before it can be flushed, which means it will grow until it runs out of space.  It’s common to miss this when the backup plan needs you to specify which databases to back up, as opposed to doing all databases.  Once you take the first log backup it leaves you with a huge log file where you don’t expect to hit that size again, and it’s ok to shrink it.

2. Runaway transaction

The second large process is when someone runs something that didn’t go as planned.  A transaction, even an implied transaction for a single statement, stayed open forever, or close enough to forever to cause us a headache.  This caused a ton of excessive growth in the log files, and that space will only ever be used when someone says “oops” again.  In this case, evaluate if you’ll need that space for another process before you shrink because someone will say “oops” again someday.  If the drive is dedicated to log files and you only have one large database, consider shrinking it just enough to clear the warnings in your monitoring software.

3. VLF cleanup

The final reason is because you have too many VLFs.  When a log file is created or grows it splits up the new space into smaller chunks called Virtual Log Files that SQL Server will cycle through when writing logs.  If the database is growing in small increments it creates tons of these, slowing SQL Server down.  The point here is that you resolve the issue by shrinking the log (delete smaller VLFs), then grow the log to the desired size in larger chunks (create bigger VLFs).  The details on finding what size your VLFs are, what size new VLFs will be, and just about everything else VLF related is in my post Database Log VLFs.

What happens when log files grow?

If you’re going to shrink a log file then you’re increasing the chances that it will have to grow again, so we need to talk about growth for a second.

When a data file grows it can often take advantage of instant file initialization where it just reserves unallocated space on the disk, making it a quick process.  Without this being turned on or if you’re using Transparent Data Encryption, SQL Server would have to zero out the disks to get rid of data that used to be there.  Kimberly Tripp (b|t) goes into a lot more details than this in her post Instant Initializations – What, Why and How?.

Log files can’t use this feature.  They write those 0’s every time the log grows, and SQL Server is patient enough to wait for that to happen.  Here’s how that went over the last time I overheard one of my users “discussing” this with my biggest server.

SQL Server: My log’s full, hold on while I get some more space.

User: I don’t care, I just want my query to finish.

SQL Server: I’m told to grab 8,000 MB at a time, please be patient.

User: Not a problem, there’s 8,000 MB right there, please hurry.

SQL Server: I only use clean space, hold on while I ask the OS to write about 8,388,608,000 zeroes for me.  This isn’t a problem, I’m patient.

User: What?  NO!  Well, at least that gives me enough time to open a ticket for the DBA.

Ticketing System: Hello again, I haven’t seen you for several minutes.

The moral of this story is that you don’t want autogrowth to happen.  You want it turned on for emergencies and one-time growths, but it’s better to grow manually in a maintenance window.  Here’s how that maintenance window conversation goes:

DBA: I see you’ve hit 90% utilization on your log a couple times, how about we add 8,000 MB.

SQL Server: Oh, I didn’t notice, but I can add that space now.  I only use clean space, hold on while I ask the OS to write about 8,388,608,000 zeroes for me.  This isn’t a problem, I’m patient.

User: Zzzzzzzz….

DBA: There’s no rush, everyone can use the existing log space while you work on that.

Ticketing System: Zzzzzzz…..

The 8,000 MB I used here isn’t a made-up number, it’s a best practice due to VLF sizes on larger databases.  Most DBAs set autogrowth to less than 8,000 because they’d rather deal with smaller VLFs than irate users, and I’d recommend 1,000 MB for autogrowth on larger databases.  To be fair, that number drops to 500 MB starting in SQL 2012, but that’s still a lot of waiting during peak hours.

On smaller databases you aren’t going to want to grow log files by 8,000 MB off hours, or even 1,000 MB for autogrowth.  The growth should be something rather significant for the database yet size appropriate, and I typically like to do close to 50% of the current file size.  So, if I have a 500 MB log, I’ll set it to 250 MB autogrowth and do manual growths of the same size.  For smaller databases where 50% log growth would be less than 100 MB, I’m not always worried about manually growing the log files since there are diminishing returns on my time and effort.

My post Database Log VLFs gets into the details of why those are best practices and what it means to you.

Entry-Level Posts

Tim Ford (b|t) noticed how we moved to advanced topics and asked us to remember where we came from in his Entry-Level Content Challenge.  This is my second post to take on his challenge, and I hope others will take him up on this!

 

 

 

Tim Ford’s Entry-Level Content Challenge

All bloggers started as beginners.  We picked things up, learned them, and got to the point where we wanted to start sharing our knowledge.  The problem is that most of us moved on to talking about advanced topics and left people to fend for themselves on the difficult entry-level learning curve.  My blog, Simple SQL Server, was actually created with me saying I was going to fill that gap, right up until the time I figured out the best way to learn advanced topics was to write things down as I learned.

Entry-Level Content Challenge

Tim Ford (b|t) noticed how we moved to advanced topics and asked us to remember where we came from in his Entry-Level Content Challenge.

It’s very simple.  Create one blog post a month that someone just learning your job could understand and benefit from, use the #EntryLevel tag on your blog and notification on twitter, and link to his original post.

This is for everyone, I don’t care what job you have.  I’m a SQL Server DBA, Tim’s a SQL Server DBA, but every profession has beginners that could use some help.

We’re Outnumbered

One of the things, if not THE biggest thing, you hope to do with a blog is to make a difference.  Then you talk about advanced topics to help other people near your level keep moving forward.  You also realize that teaching the details on these topics accelerates your learning and confidence faster than you thought possible.  It’s great; I recommend everyone does it.

However, there are a couple problems with this logic.  By the time people feel confident enough to blog, they aren’t talking about the fundamentals anymore.  Also, for every advanced person in any field, there are many, many more entry-level people who could benefit from learning the fundamentals properly from the start.

Bloggers aren’t talking to the largest portion of their potential audience, who just happen to be going through the most difficult part of the learning curve.

It’s the Right Thing To Do

In no specific order, here’s why it’s the right thing to do.  This list contains selfish reasons, moral reasons, and everything in-between.

  • You want to reach a larger audience, and there’s more people who need this content.
  • You were a beginner once and learned off of others.
  • You wish you learned the fundamentals right from the start, but you didn’t always.
  • You learn everything you teach better, even if you consider it simple.
  • Tim challenged you, and it’s hard to say no to a challenge.
  • You always wanted to make a difference.

New Bloggers

This is also a great opportunity for new bloggers.  Talk about subjects you know well, even if you’re not at the level you feel comfortable teaching advanced topics.

The content is the only difficult part of blogging.  Setting up a blog and creating a post is practically identical to setting up a new email account and sending an email.  If you want to get into blogging and need help, let me know.

If Einstein Can Do It…

Einstein said “If you can’t explain it simply, you don’t understand it well enough.”  I took this quote to heart enough to name my blog after it.  Try explaining the fundamentals simply, you’ll see how much your understanding of the topic increases to accomplish that feat.

 

 

Shrinking Database Data Files

Normal DBAs will sum up shrinking a database data file in one word: NO!

I’ll talk to you about this subject because I’m not normal.  Still, my goal is that you’ll go from “why can’t I shrink my files” to “I don’t want to shrink my files.”

Truth be told, even when I HAVE to shrink files, I don’t want to.

Basically, you needed the space at one point in time, so you’ll probably need it again.  Unless you changed a process or something extremely rare happened, you’ll use the space again and there’s no reason to take on the issues involved with shrinking just to have a file grow again.  While this advice is mostly the same for log files, it’s different enough where I’m making a separate post for shrinking database log files

When should you shrink data files

First, a process changed where you’ll never need that amount of space again, such as the database used to keep 10 years of history but now an automated process keeps it pruned to 6 months.  This change also left you with a very significant amount of free space that you need for another purpose.

Really, that’s about it.

What happens when you shrink data files

The most common way to shrink a file is to have it reorganize pages before releasing free space, so I’ll cover that.  It basically has two steps:

  1. Fragment the file as much as possible (killing performance)
  2. Truncate the end of the file

By “reorganize” it means take the tail end of the file after the size you said you’d like the file to be shrunk to, find every populated page of data after that point, then move those pages to the first available spot you find in the file.  When the end of the file is empty, truncate it.

It’s an expensive process that leaves a big mess behind, but there are times you’ll need to go through with it (kicking and screaming).  If you do, schedule an index maintenance job immediately after this to undo most of the damage you just did and make sure the server still loves you.

Think about this, you’re considering kicking off a process where you have to tell the server you’re sorry afterwards.  You better have a good reason for this.  A 100 GB database with only 2 GB used may be a good reason to shrink.  A 100 GB database with 75 GB used is normal and healthy.

If you are going to shrink, make sure the database won’t be using that space again.  If you have a 100 GB database with only 2 GB used, does it populate, process, and purge 90 GB in an overnight process?  Find out before you shrink.

Manual Shrinking

If I haven’t scared you off yet (and I hope I did) then here’s how you shrink.

Here’s the link how to shrink a file using T-SQL: DBCC ShrinkFile

Here’s the link how to shrink a file using the GUI: SSMS Shrink File

There are Shrink Database options as well.  I don’t use it and don’t recommend it.  You know what files you want to shrink, don’t shrink them all.  When this is used it’s typically because people want to shrink their data files, but they mess up their log files while their at it.  Be nice to yourself, don’t use shrink database.

What about the Auto-Shrink option

No.

Just no.

It’s not a feature, it’s a threat.

If you have a certain amount of free space it will automatically shrink your database causing all the problems I just discussed, but it will probably kick in during your busiest time of day.  Then it won’t follow that up with index maintenance, so it just left you hopping all over your disk to find data.  The best part is that it didn’t check to see if you would need that space again, so it’ll probably grow tomorrow to get that space back.

In the example above of a 100 GB database that only uses 2 GB during the day but populates 90 GB for overnight processing, those 2 GB will be horribly fragmented, you’ll spend the resources shrinking, then you’ll take the performance hit growing the database every night as it puts the growth anywhere it can which typically means physical file fragmentation as well.  In the mean time your monitoring software will show that you have tons of free space, right up until you get the disk full error.  You can auto-shrink that database and get all those issues, or you can just let it do its job as you do yours without a maintenance nightmare.

The Connect Item on this is marked as fixed, stating that they’ll consider removing it in future versions.  This wasn’t a joke opened by some wannabe DBA.  This was serious and opened by someone who went on to become the president of the Professional Association for SQL Server.

To be fair, we all word our feelings on this differently.  I say no, Tom LaRock says never, Paul Randal says Turn It Off!, and Brent Ozar is colorful enough to come up with the term Hamster Wheel of Death.

As of the time of this writing, no one has used the term “Magical Unicorn” to describe this feature.

Sum it up

  1. Don’t shrink data files.
  2. If you didn’t listen to #1, why?
  3. Follow it up with index maintenance.
  4. Every time you try to automate this process a unicorn dies.

Entry-Level Content Challenge

I’m taking Tim Ford’s (b|tEntry-Level Content Challenge, and this is my first post with the #EntryLevel tag.  The challenge is to have one post that earns that tag each month, but I hope to exceed that.