TempDB In Azure IaaS

Jim Donahoe shows how to use the temporary SSD on an Azure VM for SQL Server’s tempdb:

Remember, this disk is as the title of this section says…TEMPORARY! Do NOT put ANYTHING on this drive you cannot afford to lose. Don’t say nobody warned you either, because the drive itself contains a nice little txt file warning you, here is the EXACT text:

WARNING : THIS IS A TEMPORARY DISK.

Any data stored on this drive is SUBJECT TO LOSS and THERE IS NO WAY TO RECOVER IT.

Please do not use this disk for storing any personal or application data.

It’s good to see what you need to do to get this working.  I’ve found it just to be easier to set up a permanent SSD, but if you’re on a tight budget, this can save you some cash.

Docker Persistence: Multi-Level Mounting

Andrew Pruski gives us a third option for persisting SQL Server database files in Docker containers:

Last week in Part Two I went through how to create named volumes and map them to containers in order to persist data.

However, there is also another option available in the form of data volume containers.

The idea here is that create we a volume in a container and then mount that volume into another container. Confused? Yeah, me too but best to learn by doing so let’s run through how to use these things now.

Read through for the step-by-step description.

Separating Data And Log Files

Brent Ozar looks at an old chestnut:

So it’s time for a quiz:

  1. If you put all of a SQL Server’s data files & logs on a single volume, how many failures will that server experience per year?
    • Bonus question: what kinds of data loss and downtime will each of those failure(s) have?
  2. If you split a SQL Server’s data files onto one volume, and log files onto another volume, how many failures will that server experience per year?
    • Bonus question: what kinds of data loss and downtime will each of those failures have?

Think carefully about the answers – or read the comments to see someone else’s homework, hahaha – before you move on.

With SANs, this advice is not even that good on the performance side—especially with modern SANs which don’t let you dedicate spindles.  It definitely doesn’t fly on the reliability side.

S3 Versus HDFS For Spark Data Storage

Reynold Xin, Josh Rosen, and Kyle Pistor argue that you should use blob storage (S3, Azure Blob, etc.) instead of disk when building a cloud-based Spark cluster:

Based on our experience, S3’s availability has been fantastic. Only twice in the last six years have we experienced S3 downtime and we have never experienced data loss from S3.

Amazon claims 99.999999999% durability and 99.99% availability. Note that this is higher than the vast majority of organizations’ in-house services. The official SLA from Amazon can be found here: Service Level Agreement – Amazon Simple Storage Service (S3).

For HDFS, in contrast, it is difficult to estimate availability and durability. One could theoretically compute the two SLA attributes based on EC2’s mean time between failures (MTTF), plus upgrade and maintenance downtimes. In reality, those are difficult to quantify. Our understanding working with customers is that the majority of Hadoop clusters have availability lower than 99.9%, i.e. at least 9 hours of downtime per year.

It’s interesting how opinion has shifted; even a year ago, the recommendation would be different.

Selecting Into A Filegroup

Denis Gobo points out a new feature in SQL Server 2017:

With the CTP2 build of SQL Server 2017, you now have the ability of doing a SELECT INTO operation into a specific filegroup.

The syntax looks like this

SELECT * INTO TableName ON FileGroup FROM SomeQuery

What is the use of this you might ask? Well maybe you have some ad-hoc queries where you save some data but you don’t want it sitting on your expensive SAN. Or maybe you populate staging tables on the fly and you want it to end up on a specific SSD because you need the speed of the SSD disk for these operations.

This might also be helpful in migrating tables to different storage.

Re-Provisioning Volumes In Azure

Robert Bishop shows how to create data disks in Azure and attach them to VMs:

Recently a client found this article on “Best Practices for SQL Server in Azure Virtual Machines” and wanted to re-provision his volumes to adhere to them.

No my first thoughts was wait, I’m a DBA, not a System Admin that’s not my role! But thinking more about it I realized the client views this as a SQL Server issue and I am the SQL Server Consultant and that it is my job to remedy this problem.

Not being 100% confident in Azure, I spun up a VM SQL Server and attempted to add some volumes.  To my surprise, this was way too easy.

Click through for the steps.

Securing Azure Storage

Christos Matskas has an article on securing Azure blobs and containers:

All communication with the Azure Storage via connection strings and BLOB URLs enforce the use of HTTPS, which provides Encryption in Transit. You can enforce the use of “Always HTTPS” by setting the connection string like this: “DefaultEndpointsProtocol=https;AccountName=myblob1…” or in SAS signatures, as in the example below:

https://myblob1.blob.core.windows.net/?sv=2015-04-05&ss=bfqt&srt=sco&sp=rwdlacup&se=2016-09-22T02:21:41Z&st=2016-09-21T18:21:41Z&spr=https&sig=hxInpKBYAxvwdI9kbBglbrgcl1EJjHqDRTF2lVGeSUU%3D

To protect data at rest, the service provides an option to encrypt the data as they are stored in the account. There’s no additional cost associated with encrypting the data at rest and it’s a good idea to switch it on as soon as the account is created. There is a one-click setting at the Storage Account level to enable it, and the encryption is applied on both new and existing storage accounts. The data is encrypted with AES 256 cipher and it’s now generally available to all Azure regions and Azure clouds (public, government etc)

There’s some good information here, making it worth the read.

Removing Tempdb Files

Erin Stellato relates a painful experience of tempdb filling up a VM’s drive:

I made a mistake with a script today. I created three new tempdb files sized at 10GB each that filled up a hard drive.

Whoops.

Luckily it was in one of my own testing VMs, so it wasn’t awful. Fixing it, however, was a fun one.

**NOTE: All work was done in a test environment. Proceed with caution if you’re running these commands in Production and make sure you understand the ramifications.

It’s a good opportunity to learn from Erin’s experience.

PySpark Persistence

David Crook shows how to save data to disk from PySpark:

This is working on HDInsight v3.5 w/Spark 2.0 and Azure Data Lake Storage as the underlying storage system.  What is nice about this is that my cluster only has access to its cluster section of the folder structure.  I have the structure root/clusters/dasciencecluster.  This particular cluster starts at dasciencecluster, while other clusters may start somewhere else.  Therefor my data is saved to root/clusters/dasciencecluster/data/open_data/RF_Model.txt

It’s pretty easy to do, and the Scala code would look suspiciously similar.  The Java version of the code would be seven pages long.

Considerations For Reducing I/O Costs

Monica Rathbun gives a few methods for reducing how many I/O operations a query requires:

Implicit Conversions

Implicit conversions often happen when a query is comparing two or more columns with different data types. In the below example, the system is having to perform extra I/O in order to compare a varchar(max) column to an nvarchar(4000) column, which leads to an implicit conversion, and ultimately a scan instead of a seek. By fixing the tables to have matching data types, or simply converting this value before evaluation, you can greatly reduce I/O and improve cardinality (the estimated rows the optimizer should expect).

There’s some good advice here if your main hardware constraint is being I/O bound.

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