Author Archives: Wes Brown

About Wes Brown

Wes Brown is Vice President at Cantata Health and maintains his blog at http://www.sqlserverio.com. Previous experiences include Product Manager for SQL Litespeed by Quest software and consultant to fortune 500 companies. He specializes in high availability, disaster recovery and very large database performance tuning. He is a frequent speaker at local user groups and SQLSaturdays.

Sometimes, you have to fix it yourself

The Problem

SQL Server is a huge product with lots of moving parts. Bugs happen. Microsoft has a place to voice your issues or problems. They allow you to vote on the issue and then decide when or if it will get fixed. I’ve used Connect when I hit a bug and I have voted on items that were important to me. Recently I hit a bug in sp_createstats. I use this system stored procedure generate statistics in an automated process I’ve got that manages statistics. I added a new vendor database to the system and on the first run hit “Column ‘DAYSOPEN’ in table ‘dbo.TBL_OPPORTUNITY’ cannot be used in an index or statistics or as a partition key because it is non-deterministic.”. Well, we all know you can’t create stats on a computed column! I quickly went to the connect site and someone else had already entered it. The down side was it had so few votes it was only slated to go into the next cumulative update/service pack. When I hit this issue they hadn’t yet announced service pack 4. I already had this procedure coded into my routines and really didn’t want to rewrite them to get past this one problem.

The Solution

!!WARNING!!

By doing what I am about to describe could break at a later date or randomly kill baby kittens.

Since it is a system stored procedure I am loathe to make any changes to it directly. There are ways to modify some system stored procedures but they involve the installation CD and creativity. With that door closed there was only one avenue open to me. Create my own system stored procedure with the fix in it. There is a problem with this solution as well, if it gets dropped due to a service pack or an upgrade anything calling it will break. The first thing I did was to see if the procedure text was available by executing sp_helptext sp_createstats. Luckily it was! Now all I had to do was figure out where it was broken. The procedure is pretty simple and uses some cursors to loop through all the objects and create column statistics where they don’t exist.

declare ms_crs_cnames cursor local for select c.name from sys.columns c  
     where c.object_id = @table_id  
     and (type_name(c.system_type_id) not in ('xml'))  
     and c.name not in (select col_name from #colpostab where col_pos = 1)  
     and ((c.name in (select col_name from #colpostab)) or (@indexonly <> 'INDEXONLY'))
    -- populate temporary table of all (column, index position) tuples for this table

It was pretty easy to spot. The weren’t checking to see if the column was computed so I added a line to the where clause.

and c.is_computed = 0

That’s it. One little check to see if it is a computed column. Now that I had fixed it I created a new procedure named sp_createstats_fixed in the master database. Just creating it in master doesn’t make it act like the original procedure or make it a system stored procedure. For that I had to execute EXECUTE sp_MS_marksystemobject ‘sp_createstats_fix’. This is an undocumented stored procedure and could change or go way any time. The only way to unmark it in SQL Server 2005 is to drop the procedure and recreate it. Now it acts just like the old procedure. Next I had to replace all references to the old proc with the new one. I made an entry into our bug tracking system about the change so we would have a record of what I did and why.

Conclusions

This wasn’t the most elegant solution. It could break later. The upside is it only took me about 30 minutes to fix and deploy versus the hours of re-coding and then testing that I would have had to do before. Do I think you should go around creating your own system stored procedures? Not at all. I don’t recommend you put anything in the master database period. If the problem had been more complex I would have redone the original routines to exclude the broken procedure. This time it just happened to be a very quick fix to a non-critical part of our system.

Quick Tip Of The Day

Over and over again we are told that the DMV’s only hold data since your last reboot. So, how do you know when your server was last rebooted? Well, every time your SQL Server service restarts tempdb is recreated every time. With a quick query to sys.databases we can get the creation date of tempdb! Armed with that little nugget you can then analyze what is in the DMV’s relevant to the last system restart.

SELECT

    create_date AS last_restart_time

   FROM

    sys.databases

   WHERE  name = 'tempdb'

Three Turns On The Road

Huh? My story? Okay. It was never easy for me. I was born a poor black child. I remember the days, sittin’ on the porch with my family, singin’ and dancin’ down in Mississippi. – The Jerk

This meme was started by Paul Randal (Blog | Twitter) who asks the question "What three things or events brought you to where you are today?" I was tagged by David Taylor (Blog | Twitter). I wasn’t born in Mississippi but we sure were poor. I grew up I a small West Texas town of farmers and ranchers. I grew up between two households. I am a single child but have a large extended family and was a middle child in that mix. I as never as tall or fast as my closest brothers but I have always had a very sharp wit and a quick thinker. With all of those factors competition for resources was pretty fierce.

Turn number one, being broke breeds invention!

My aptitude for mechanical things and electronics was my first love. Being handy with a socket set has a great value when you have to fix everything you own yourself. It also allowed me to scavenge parts where ever I could to build stuff I wanted. It wasn’t ever pretty but it was mine. The bike I rode during my middle years was made up of at least four others. My radio was also made up of three or four other radios. One of the great things was one of my best friends had a great hookup into technology, his dad owned the Radio Shack in town. I spent a lot of time there and got to hone my soldering skills there on a real Weller station with adjustable temp!. My mom saved and saved. She new computers were the future. Having a Radio Shack meant I got a TRS-80! I scraped up the money to buy a tape drive and eventually a modem. I’ll never forget them telling my mom that this thing would take me through high school and college! I spent the better part of my junior high and high school life on this and later a CoCo3. My High school had Apple II’s and eventually some very coveted Mac’s. My best friend through high school was one of two people that had those “IBM PC” things. I fell in love with those 8088’s and his dads “high end” 286 and got to spend a lot of time on those as well. Those few of us that had modems would dial into each others machines. Needless to say my folks weren’t real happy when the picked up the phone and a computer was screaming at them. I wasn’t to happy ether since they usually killed me right at the end of a two hour download 🙂

Turn number two, the college years and dial tones.

Alas, it wasn’t my goal to be a computer guy. I loved speech and theatre and did quite well in both. I dabbled in physics but quickly realized it was hard and I wasn’t very fond of the math. The other thing I majored in was partying. I slowly quit playing with computers all together and for about a year was just a college bum. My first real friend in college had a 286 and a modem as well. He showed me the local BBS’es and I was 100% hooked again. He gave me a old hard drive and I went down to a local computer store to buy a used computer and start learning all over again. The shop owner was a friend of my friend and knew who I was. I walked in with enough money to buy a 286 he had for sale. I told him I needed a computer and was planning on starting a real BBS. He smiled, and flatly refused to sell it to me. I argued with him for the better part of an hour. I left his store pretty mad but none of the other computer stores in town had a computer that met my limited budget. I went back the next day and started in on him again. He stuck to his guns telling me I wouldn’t be happy with that old clunker. My friend had told him I was handy with the soldering iron so he made me a deal. I put a down payment on a cutting edge 486 and worked the rest off fixing computers. He took me under his wing and helped fill in the gaps in my computer hardware education. He had a crazy huge stockpile of old stuff that I could tinker with. That is where I found out I truly loved computers and connecting with others via my BBS. Looking back on what Duane did for me is one of the biggest reasons I give back to the community today. Spending hours working on my board I learned to love DOS like no other. It very quickly grew into a rekindling of my passion of programming as well. I changed majors(again) and started the computer science thing. Having people come to the house to play their turns on BRE or Trade Wars, posting up on fidonet groups or sending mail to friends around the world was a huge kick for me. I met some of my closest friends through the board. The Internet will never replace the BBS in my heart. I had a loyal local group of BBS’ers with the ability to communicate globally. Eventually, I dropped out of school to work full time again. As the Director of Technology for a small school outside of San Angelo, Texas I had a huge impact on the students and helped bring computers and the Internet, into every classroom. I also participated in the speech and theater stuff with the high school students as a coach.

Turn number three, the big city.

360 Bridge, Austin, TX by MoralesDirect.

this image is property of Dan Morales
www.flickr.com/…/set-72157601003144486

After a shakeup at the school system I was working at occurred and the Superintendent of Schools left, I decided it was the right time to make a move to the big city. I had always loved Austin. I’d done a tour of service in the other major cities and found that Austin was the only one that felt like a real community. I did what lots of folks did in the late 90’s and interviewed at start ups. To be honest, they scared the crap out of me. Most of the time it was people younger than me, with more money than I’d ever seen, doing whatever the hell they wanted. It looked like a lot of fun, but not so much a solid way to build a business. So I did what everyone does, I went to work for Dell. Did it suck? Yeah, kind of. I met a great group of guys there though. It was also the first place I’d ever been that had what you might call a growth path. My boss and my team mates were great people to work with. I had been doing side work pretty much most of the 90’s small software projects or working in the new open source movement. I loved data, so I spend a lot of time working with databases or building custom data engines. When I had landed at Dell I’d already had several years of tinkering with SQL Server. We had a single server at the school and I worked with clients I met through the computer store as well. I also love a puzzle. I watched the calls that came in. Saw the patterns in them and started plugging them into a brand new SQL Server 7.0 database. Quickly I moved off the phones full time and started working on a special projects team analyzing call volumes, doing root cause analysis and coming up with solutions. Dell also opened my eyes to the possibilities and gave me the confidence boost to just strike out and blaze my own path. I did my time at Dell and moved on to another large company, Broadwing. Again, great group of guys, not so great working environment. But, for the first time I was a full on DBA. Not a DBA/developer/janitor/whateverthebossneeds. I LOVED IT. Lots of machines, lots of data. From there it has just been a natural progression always growing and learning but staying laser focused on SQL Server. Like many folks, the 90’s was an awesome decade for me.

Today

Well, I’m still with SQL Server. I love community work. I’m a family man. None of the things I take for granted today wouldn’t have happened if I’d never been exposed to computers at a young age, or had a stranger take a chance on me, or worked up the nerve to just move to another city when I had a stable job where I was at. Give back when you can. You never know how you will shape someone else’s road for the better.

Out Of My Comfort Zone: Building a Web App

If you read Fundamentals of Storage Systems – Stripe Size, Block Size, and IO Patterns you know I built a little web tool to help you with sizing and estimating your RAID array’s performance. This is way out of my area of expertise. Luckily for me I like a challenge and had a guiding hand from some friends. I haven’t done any web programming since I wrote a photo album mod for Snitz! forum package in 2002, using classic ASP. I still get thank you emails from folks that have been running it for years. Needless to say my skills are a little rusty. My first instinct was to fire up Visual Studio 2008 and build an ASP.net page tied to a back end SQL Server 2008 database. Well, ASP.net is just different enough that I was struggling to do the most basic things. So, I thought it would be good to “get back to basics”. A very good friend of mine has been a professional web developer for the same company since about the time my photo album came out. He lives and breathes web technology. The problem was he doesn’t do ASP.net at all. Everything he does is standards compliant HTML and JavaScript. I told him about my spread sheet of calculations and my desire to turn it into a web page. Joe quickly begged me to leave him alone. Once I calmed him down, he did a little sample page to get me on the right track. I took a look at it and thought it would be easy to goof around in this JavaScript stuff. I was wrong. What a convoluted world web developers live in.

I hacked on it the weekend before Christmas and was pretty happy with my handy work, right up until I showed it to some people. Comments like “Kill it with fire!” were common. I explained what it was and that took some of the hostility out of the feedback. The next Monday I asked the two lead web heads at work to look at it. Once Ben had washed his eyes out with bleach he told me under no certain terms could he fix it. My powers of persuasion, and a threat to never approve another schema change, helped bring him around. He explained several new technologies to me I wasn’t aware off, like CSS. Ben showed me quickly how to format the page so peoples heads wouldn’t burst into flames when they saw it. He also spoke of things like Ajax and JSON. Being the clueless data guy that I am I turned to my trusty friend yet again, Google.

Quick Definition List

JavaScript, the underpinning of the modern dynamic web 2.0 world.

Ajax (asynchronous JavaScript and XML, a group of interrelated web technologies that allow for dynamic web design.

JSON (JavaScript Notation), a lightweight data-interchange format, a way to structure data like XML implementing name/value pairs and ordered lists.

JQuery, a JavaScript library used to traverse HTML elements and build dynamic content easily.

JavaScript Implementation

What struck me was just how much had changed, yet stayed the same. One of the things I’ve always hated is no two browsers render the page the same way. If it worked in Firefox it didn’t work in IE. The other thing I didn’t realize for quite a while was any of the browsers would happily run the worlds worst code. I’m not talking poorly written code, I mean flat wrong. They will let any old JavaScript run. JavaScript is defined as a loosely typed language. The way it is implemented it goes from loosely to sloppy very quickly. Don’t get me wrong here. The language specification EMCAScript, is solid. The implementations are very poor in most browsers and that is where the problems developing for JavaScript really come in. I’m not saying JavaScript is slow, quite the opposite in fact. It is just difficult to make sure you are actually writing the correct code. Having worked with Visual Studio for quite some time, I’ve become spoiled with having my IDE tell me when the syntax is wrong before I hit the compile button. So, I was back to Google again looking for tools to help me out.

The Tools

I quickly settled on Aptana Studio 2.0 to do the actual code work. I also used Notepad ++, which is by far the best text editor in the world. with Aptana Studio allowing me to see errors in real time and use the preview function to look at page renders without having to have browser windows open. I did look at Microsoft’s new Expression Suite 3.0. It is nice. The designer is slick, but in typical Microsoft fashion it mangles some parts of the code that I then have to fix by hand. Aptana Studio also has a built in JavaScript minimizer that shrinks the size of your JavaScript files for quicker load times. Aptana Studio doesn’t enforce the use of line terminators and when I compacted my JavaScript it wouldn’t load. I did some additional digging and found an awesome Lint tool for validating JavaScript JSLint. My favorite thing about it is the quote and link on the front page “Warning: JSLint will hurt your feelings.” I knew I had found the right tool. After several passes I got my code to validate and minimize without any errors. I quickly looked for other Lint based tools and found one for JSON as well JSONLint.

The Work

Over the course of a few weeks I tinkered with the page, adding features, correcting mistakes and cleaning up the layout. The biggest thing that I learned is just how fast JavaScript can be. Even after I did the initial page I had no doubt a database would be involved at some point. I had compiled a small database of hard drives and their characteristics and wanted to add that to the page. The problem was I would basically have to redo the page again in asp.net or some other dynamic language to pull the data from the database. So, I cheated. As a proof of concept I built a JSON structure with a sample of the data to load the drop down list box and all the variables dynamically. I was stunned at just how fast the page was to respond. I hacked together some T-SQL that would generate the JSON object from my database, all 1200 entries. I cleaned it up a bit made sure that JSONLint validated it ran the minimizer on it and plugged it in. Holy cow, it worked! It was also still very vast. I just couldn’t believe it. The only drawback is having to update the JSON if I add new drives to the database.

What I’ve Learned

The main takeaways for me on this project are simple, web development stinks. Now I know why web folks have a hard time with T-SQL, they already have to be experts in five or six different technologies across two or more platforms. I use to introduce Joe as “My friend who does JavaScript”, now I know is is a hard core developer! What I build is simple and works. I think the layout is tolerable. If you have any suggestions or feed back just drop me a note.

Fundamentals of Storage Systems – Stripe Size, Block Size, and IO Patterns

If you have been following this series we have covered system buses, hard disks, host bus adapters and RAID. Along the way we also covered how to capture your IO patterns and the SQLIO tool. Now we will pull it all together.We move up the stack even further to the actual layout of the RAID stripe and the file system. How the stripe and file system are laid out on your disks has a huge impact on performance. One of the things that has really gotten some traction over the last few years is sector alignment. This one thing, if not done, could cost you 30% to 40% of your IO potential. Jimmy May has covered sector alignment in depth So I won’t hash it here again. Kendal Van Dyke also has a good series that covers offset, stripe size, and allocation units with different raid levels.

It Don’t Add Up…

Something I’ve seen, and been guilty of, is taking a drives base specifications and just multiplying out. Say the manufacturer says the drive will to 79MB/Sec minimum throughput, we have 10 drives so that is 790MB/Sec of throughput! We all know from experience that this isn’t so. What eats us up is how much slower it really can be. As we have seen throughout this series there is overhead associated to everything. Before we just throw a bunch of disks in an enclosure and press it into service it would be nice to have an idea of what the performance should be. It’s also recommended to do some of this work before you actually buy anything so you don’t have to go back to your boss and beg for more money and explain to him that your wild guess was wrong.

Always add a pinch of salt to whatever the disk manufacturer puts in the specifications. Most of the time they will be close enough. The problem lies in the fact they don’t always disclose the methods for archiving those numbers. For instance, when they report minimum and maximum throughput they are usually talking about a scan of the entire disk including all meta data stored between tracks, the best possible throughput possible. You won’t see those results in every day life. They also give you numbers that can be completely irrelevant like single sector read rates. very rarely do you read a single sector at a time. Personally, I would love if the drive makers gave the engineering specifications. I know that won’t happen, it would make my life easier though. The disk characteristics that are important are, sector size,spindle speed, seek times read and write, sequential times read and write. To a lesser extent sequential throughput in megabytes per second. With the single disk numbers we can move on to the RAID configuration.

Configuring your RAID Array

There are several factors that impact the RAID arrays ability to perform. The RAID level, size of the IO request, and stripe size. RAID level is the easy one, what kind of hits do you take on writes vs. capacity of the array. On the stripe size there is a direct corollary with the size of the IO request. If the IO request is bigger than the stripe size it will have to seek across another disk to satisfy the data request. If the IO request size is very small and random you may loose some IO performance if the requests pile up on one disk causing a hot spot. There are established calculations that you can perform to get an idea of how to configure you array. I’ve built a web page that you can use to do all the basic calculations, Disk Drive RAID Configuration Tool. These equations are base line estimates so you aren’t working completely in the dark. You can enter your own drive statistics or pick from one of 1100 hard drives in the database. This web calculator is based off of Peter Chen’s equations for estimating RAID performance and best stripe size. I’ll add more to it as I get time.

SQL Server IO Patterns and Array Performance

SQL Server works with two specific IO request size 8K and 64K in general. If you did your due diligence earlier you could also add any other request size that you saw come through. Focusing on the page size and extent size is a good place to start. Using the raid calculator tool I chose a Seagate Savvio 15K.2 drive as my base. One of the things my calculator can’t take into consideration is your system and RAID HBA. This is where testing is essential. You will find there are anomalies in every card, physical limits on throughput and IO’s. Since my RAID card won’t do a stripe bigger than 256k that is my cap for size. Reading through several IO white papers on SQL Server the general recommendation is for 2000/2005 a 64k or 128k stripe size and for SQL Server 2008 a 256k stripe size. I’ve found as general guidance, this is a good place to start as well. The calculator tells me for a RAID 10 array with 24 drives at a 256k stripe size and 8k IO request I should get 9825 IOs/Sec and 76.75 MB/Sec on average, across reads, writes, sequential and random IO requests. That’s right, 76 MB/Sec throughput for 24 drives rated at 122 MB/sec minimum. That is 2.5 MB/Sec per drive. The same array at a 64k IO request size yields 8102 IOs/Sec and 506 MB/Sec. A huge difference in throughput just based on the IO request size. Still, not anywhere near 122 MB/Sec. As an estimate, I find that these numbers are “good enough” to start sizing my arrays. If I needed to figure out how big the array needs to be to support say 150 MB/sec throughput or 10000 IOs/Sec you can do that with the calculator as well. Armed with our estimates it’s time to actually test our new RAID arrays. I use SQLIO to do synthetic benchmarking before running any actual data loads.

After doing a round of testing I found that in some cases the numbers were a little high or a little low. Other factors that are hard to calculate are cache hit ratios. Enterprise RAID HBA’s usually disable the write cache on the local disk controller and just use their own batter backed cache for all write operations. This is safer but with more and more disks on a single controller the amount of cache per disk can get pretty low. The HBA will also want you to split that between read and write operations. On my HP RAID HBA’s the default is 25% read and 75% write. In an older study I found on disk caches and cache size saw diminishing returns above 2 MB gaining between 1 and 2 percent additional cache hits per megabyte of cache. I expect that to flatten out even more as the caches get larger, you simply can’t get 100% cache ratios that would mean the whole drive fit in the ram cache or your IO request are the same over and over. Generally if that is the case you will find SQL Server won’t have to go to disk it will have what it needs in the buffer pool for reads. I find that if you have less than 20 percent write activity leaving the defaults is fine. If I do have a write heavy load I will set the cache to 100% writes.

The Results

Having completed my benchmarking I found that 128k or 256k stripe size was fine on average. Just realize that if you optimize for one IO pattern the others will suffer. Latency is also important and I have included it here as well. You find that the larger the IO request and the smaller the stripe size latency gets worse. Here are the results from my tests on a DL380 G5 with a P411 and 24 drives in a MSA 70 enclosure. I’ve included tests for an 8k to 256k stripe sizes.

As a footnote I’d like to thank Joe Handley, Ben Poliakoff, David Gosslin and Dale Davis for helping me get the Disk Drive RAID Configuration Tool together. I’m not a web guy!

WARNING! Lots of charts below!