SQL Server Database Consulting
I was asked to give a once over to a system not built by my team. SQL 2000 32-bit on Windows 2003 64 bit. 40GB RAM.
Min and Max set to 32gb, yet Perfmon is showing 40gb used by SQL Server. Hmm.
Freakshow noticed that the Min and Max Server memory settings seemed a little high at 33000000. 33000000/1024 = 31.4 Terabytes of RAM. Sweet.
Linchi Shea from the .server newsgroup noticed it as well.
We set it to a more conservative 36gb.
You want room for the O/S with that?
Kevin3NF
More translating of SQL stuff into less technical terms for the new folks…
SQL Server (and most other database systems) offer the option of using indexes on your data to help queries go faster. The purpose of this post is to give the new SQL dude a quick mental connector while learning the concept.
Clustered Index:
Think of the White pages phone book you have at the house. Now, find the entry that has your phone number in it. Mentally, you think of your last name (parameter 1 in a query), then your first name (parameter 2). So if your last name is Gates, you flip directly to the G section. If your first name is Bill, you go to the B section within the Gates area. At that point, there may be more than one Gates, Bill entry, so your eyes start scanning through them for some other piece of identifiable info, such as a street or city (parameter 3) until you find the correct entry. You then slide over to the right, look at the phone number and return that to your brain.
The White pages are a “Two-column” clustered index on lastname, firstname in alphabetical order (Ascending). The data itself (the names) IS the index. No extra pages at the back of the book. Speaking of extra pages….
Non-clustered index:
Think of every technical book or textbook you’ve ever read. There is almost always a collection of additional pages at the back of the book called the index. These pages do not contain any of the data about the topic at hand…just pointers to where in the book you can find what you are looking for.
Imagine you are holding a SQL Server 2005 administrators book, and you want to find every reference to “Replication.” Yes, you can look in the table of contents but that may leave out an entry found in the Backup/Restore chapter, or Performance troubleshooting. So, you go to the back of the book, look through the alphabetical list of Keywords for “Replication” and now you know that the word exists on pages 45, 255-298, and 453. You have a collection of pointers to the specific data pages in your book that may contain the information you need.
What if I don’t have any indexes?
No clustered index: Imagine finding your name in a white pages that was not sorted alphabetically. You would have to start at the first entry and read every single row until you hit it (a table scan in SQL speak). Ugh.
No non-clustered index: Imagine me telling you to find the phrase “SQL Profiler” in that SQL book you bought, after I rip the table of contents and index pages out. Sounds like a fraternity hazing ritual for IT geeks 😉
How many can I have?
Clustered: 1. How many ways can you sort and print the data in the SAME book? 1.
Non-clustered: More than one, depending on the database platform and version.
That’s all for today…what indexes to have is not a 101 level discussion, other than to say…whatever you join or search on is a good candidate.
Now you are ready for part of your interview 😉
Kevin3NF
SQL 101 – Replication vs. Log Shipping vs. Clustering
Continuing the “Englishification” of SQL Server for those new to the product…
These three terms are the most incorrectly used terms in all of SQL Server, not just by CEOs and pointy haired managers but by some very sharp developers and more than a few experienced DBAs. If you don’t know them, pull up a virtual chair for a 3 minute primer.
Clustering – its all about high availability and uptime
In its most basic form, a Windows/SQL “Cluster” is two or more servers (nodes) attached to a shared storage – a SAN. Only ONE of the nodes is running the SQL Server instance at any given time. So…you can have a 4 node cluster with one SQL Server instance, and it will NOT be running 4 times faster. If the node you are running on suffers a meltdown, the Cluster service moves it to another node. Key Point: This is NOT a fully redundant solution!!! If the SAN dies, your data is gone. Period. Go find your backups.
Log Shipping – move that data!
Log Shipping’s sole purpose is disaster recovery. There is a secondary benefit that you can use the destination server as a reporting server if you set it up in a specific configuration.
LS is nothing more than a glorified backup/copy/restore process with a GUI and jobs:
Backup the data on Server A.
Copy the backup files to Server B
Restore the files on Server B.
You cannot edit the data on Server B…just read it.
Replication (no, I won’t discuss the different types here):
Replication is all about distributed processing. This means having the data in two different places (Walla Walla, WA and Kissimmee, FL for example) so users don’t have to depend on the server and WAN in the remote city. Or for sales/field personnel entering data from their cars.
Replication can be a partial DR solution, but understand that not everything gets replicated (security changes), and only new data gets sent automatically. Schema changes, new tables, etc. do not.
Summary:
Yes, you can combine some of these. Set up two clusters in different cities and Log Ship between them. Now you have HA and DR. Expensive, but effective.
For all the SQL Experts that are chomping at the bit ready to scream that I left out what LUN is, or didn’t discuss Geoclustering, please see the post title…this is a 101 level post 🙂
Thanks,
Kevin3NF
A recovery model is simply a choice made on each database that determines how much data you can recover if your db goes “toes up”.
If you do not backup your database, you have chosen recovery model “terminated” or “update resume”
The 3 that are offered by Microsoft are:
Simple vs. Full is very simply a matter of how much data can you afford to lose if the database is lost and you are forced to restore from your backups.
Simple recovery model: does not allow backing up the transaction logs, therefore you cannot restore any of the data in them. You can only restore to the point of the Full backup and any one Differential you may have.
Full recovery model: You can restore from t-log backups (assuming you have them), right up to a specific point in time. Reduced data loss, assuming the backup files are valid.
When to use:
Simple: When you do not care about the data, such as in a Development or Test environment where you can regenerate from another source. Also useful in a static, read-only database (assuming you have a full backup).
Full: Pretty much any live production database that has Inserts, Updates and Deletes happening.
Switching from one to the other:
Simple to Full: Immediately take a Full or Differential backup
Full to Simple: No specific action required, other than verifying regular data backups are in place and working.
Maintenance plan considerations:
If you have both Simple and Full recovery model databases in your SQL instance, and you create a Maintenance Plan to back up data and logs, you may run into an issue (at least in SQL 2000) where the automated deletion of old t-log backups is failing. Make two plans: one for Full and one for Simple. I have no idea if this issue still presents in SQL 2005.
I hope this is clear…please feel free to comment.
Kevin3NF
SQL 101 – Understanding Transaction logs
I posted this in the microsoft.public.sqlserver.server newsgroup (response to questions about log file space):
If you build a bookshelf (physical .ldf file) and fill it up with books (transactions), its full.
If you loan 5 books to a friend (backup the t-log), there is space available on the shelf, but the shelf size didn’t change, correct?
If you buy another book (DML transaction), it goes where one of the others was.
If you fill up your bookshelf and then buy 3 more books, your only choices (besides stacking) are to expand the size of the shelf (grow the physical .ldf file) or add a 2nd one (MyDB_Log2.ldf) to the wall. Or return the books (failed transaction).
Chopping off the end of the bookshelf (Shrinking the .ldf file) makes no sense, nor does making a shelf that can hold 1000 books, when you’ll never have more than 100 there….wasted wall space (disk).
Hope that helps.
Kevin3NF
Dateline: Sunday morning, 2:30am (my week on call)…
Ring!!!!!
Kevin3NF: {snort} thisiskevin
Caller: Hey man…sorry to wake you but we have some t-log backups failing for Acme Widgets
Kevin3NF: whatstheerrormessagein the job (waking up a bit, not out of bed)
Caller: I can’t find the job in the SQL Agent job list
Kevin3NF: WhatIsTheExactAlertFromNetIQ? (slightly clearer now)
Caller: SQL Task failed: ‘Backup Job 17’ failed
Kevin3NF: The job list is alphabetical…did you sort by name?
Caller: Yeah…but theres like 2000 jobs…most have numbers and letters and stuff
Kevin3NF: Those are reporting services jobs…just ignore them and look for the Backup job
Caller: I’m looking for Job 17, but its not in the ‘J’s
Kevin3NF: Did you look under ‘B’, since the job’s name starts with ‘Backup…’ ?
Caller: silence
Caller: I’m an idiot
Kevin3NF: Goodnight.
Caller: Click
Kevin3NF: snore.
Still makes me laugh…I went back to sleep knowing I would post this little 2 minute interaction 🙂
Kevin3NF
