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      Check This Overlooked Setting to Troubleshoot ‘Strange’ Microsoft SQL Server Performance Issues


      As a SQL DBA or a system admin of highly transactional, performance demanding SQL databases, you may often find yourself perplexed by “strange” performance issues reported by your user base. By strange, I mean any issue where you are out of ideas, having exhausted standard troubleshooting tactics and when spending money on all-flash storage is just not in the budget.

      Working under pressure from customers or clients to resolve performance issues is not easy, especially when C-Level, sales and end users are breathing down your neck to solve the problem immediately. Contrary to popular belief from many end users, we all know that these types of issues are not resolved with a magic button or the flip of a switch.

      But what if there was a solution that came close?

      Let’s review the typical troubleshooting process, and an often-overlooked setting that may just be your new “magic button” for resolving unusual SQL server performance issues.

      Resolving SQL Server Performance Issues: The Typical Process

      Personally, I find troubleshooting SQL related performance issues very interesting. In my previous consulting gigs, I participated in many white boarding sessions and troubleshooting engagements as a highly paid last-resort option for many clients. When I dug into their troubleshooting process, I found a familiar set of events happening inside an IT department specific to SQL Server performance issues.

      Here are the typical steps:

      • Review monitoring tools for CPU, RAM, IO, Blocks and so on
      • Start a SQL Profiler to collect possible offending queries and get a live view of the slowness
      • Check underlying storage for latency per IO, and possible bottle necks
      • Check if anyone else is running any performance intensive processes during production hours
      • Find possible offending queries and stop them from executing
      • DBAs check their SQL indexes and other settings

      When nothing is found from the above process, the finger pointing starts. “It’s the query.” “No, it’s the index.” “It’s the storage.” “Nope. It’s the settings in your SQL server.” And so it goes.

      Sound familiar?

      An Often-Forgotten Setting to Improve SQL Server Performance

      Based on the typical troubleshooting process, IT either implements a solution to prevent identical issues from coming back or hope to fix the issue by adding all flash and other expensive resources. These solutions have their place and are all equally important to consider.

      There is, however, an often-forgotten setting that you should check first—the block allocation size of your NTFS partition in the Microsoft Windows Server.

      The block allocation setting of the NTFS partition is set at formatting time, which happens very early in the process and is often performed by a sysadmin building the VM or bare metal server well before Microsoft SQL is installed. In my experience, this setting is left as the default (4K) during the server build process and is never looked at again.

      Why is 4K a bad setting? A Microsoft SQL page is 8KB in size. With a 4K block, you are creating two IO operations for every page request. This is a big deal. The Microsoft recommended block size for SQL server is 64K. This way, the page is collected in one IO operation.

      In bench tests of highly transactional databases on 64K block allocation in the NTFS partition, I frequently observe improved database performance by as much as 50 percent or more. The more IO intensive your DB is, the more this setting helps. Assuming your SQL server’s drive layout is perfect, for many “strange performance” issues, this setting was the magic button. So, if you are experiencing unexplained performance issues, this simple formatting setting maybe just what you are looking for.

      A word of caution: We don’t want to confuse this NTFS block allocation with your underlying storage blocks. This storage should be set to the manufacturer’s recommended block size. For example, as of recently, Nimble storage bock allocation at 8k provided best results with medium and large database sizes. This could change depending on the storage vendor and other factors, so be sure to check this with your storage vendor prior to creating LUNs for SQL servers.

      How to Check the NTFS Block Allocation Setting

      Here is a simple way to check what block allocation is being used by your Window Server NTFS partition:

      Open the command prompt as administrator and run the following command replacing the C: drive with a drive letter of your database data files. Repeat this step for your drives containing the logs and TempDB files:

      • fsutil fsinfo ntfsinfo c:

      Look for the reading “Bytes Per Cluster.”  If it’s set to 4096, that is the undesirable 4K setting.

      The fix is easy but could be time consuming with large database sizes. If you have an AlwaysOn SQL cluster, this can be done with no downtime. If you don’t have an AlwaysOn MSSQL cluster, then a downtime window will be required. Or, perhaps it’s time to build an AlwaysOn SQL cluster and kill two birds with one stone.

      To address the issue, you will want to re-format the disks containing SQL data with 64K blocks.

      Concluding Thoughts

      If your NTFS block setting is at 4K right now, moving the DB files to 64K formatted disks will immediately improve performance. Don’t wait to check into this one.

      Explore INAP Cloud.

      LEARN MORE

      Rob Lerner


      READ MORE



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      How To Troubleshoot Issues in Redis


      Introduction

      Redis is an open-source, in-memory key-value data store. It comes with several commands that can help with troubleshooting and debugging issues. Because of Redis’s nature as an in-memory key-value store, many of these commands focus on memory management, but there are others that are valuable for providing an overview of the state of your Redis server. This tutorial will provide details on how to use some of these commands to help diagnose and resolve issues you may run into as you use Redis.

      How To Use This Guide
      This guide is written as a cheat sheet with self-contained examples. We encourage you to jump to any section that is relevant to the task you’re trying to complete.

      The commands and outputs shown in this guide were tested on an Ubuntu 18.04 server running Redis version 4.0.9. To obtain a similar setup, you can follow Step 1 of our guide on How To Install and Secure Redis on Ubuntu 18.04. We will demonstrate how these commands behave by running them with redis-cli, the Redis command line interface. Note that if you’re using a different Redis interface — Redli, for example — the exact outputs of certain commands may differ.

      Alternatively, you could provision a managed Redis database instance to test these commands, but note that depending on the level of control allowed by your database provider, some commands in this guide may not work as described. To provision a DigitalOcean Managed Database, follow our Managed Databases product documentation. Then, you must either install Redli or set up a TLS tunnel in order to connect to the Managed Database over TLS.

      memory usage tells you how much memory is currently being used by a single key. It takes the name of a key as an argument and outputs the number of bytes it uses:

      • memory usage key_meaningOfLife

      Output

      (integer) 42

      For a more general understanding of how your Redis server is using memory, you can run the memory stats command:

      This command outputs an array of memory-related metrics and their values. The following are the metrics reported by memory stats:

      • peak.allocated: The peak number of bytes consumed by Redis
      • total.allocated: The total number of bytes allocated by Redis
      • startup.allocated: The initial number of bytes consumed by Redis at startup
      • replication.backlog: The size of the replication backlog, in bytes
      • clients.slaves: The total size of all replica overheads (the output and query buffers and connection contexts)
      • clients.normal: The total size of all client overheads
      • aof.buffer: The total size of the current and rewrite append-only file buffers
      • db.0: The overheads of the main and expiry dictionaries for each database in use on the server, reported in bytes
      • overhead.total: The sum of all overheads used to manage Redis’s keyspace
      • keys.count: The total number of keys stored in all the databases on the server
      • keys.bytes-per-key: The ratio of the server’s net memory usage and keys.count
      • dataset.bytes: The size of the dataset, in bytes
      • dataset.percentage: The percentage of Redis’s net memory usage taken by dataset.bytes
      • peak.percentage: The percentage of peak.allocated taken out of total.allocated
      • fragmentation: The ratio of the amount of memory currently in use divided by the physical memory Redis is actually using

      memory malloc-stats provides an internal statistics report from jemalloc, the memory allocator used by Redis on Linux systems:

      If it seems like you’re running into memory-related issues, but parsing the output of the previous commands proves to be unhelpful, you can try running memory doctor:

      This feature will output any memory consumption issues that it can find and suggest potential solutions.

      Getting General Information about Your Redis Instance

      A debugging command that isn’t directly related to memory management is monitor. This command allows you to see a constant stream of every command processed by the Redis server:

      Output

      OK 1566157213.896437 [0 127.0.0.1:47740] "auth" "foobared" 1566157215.870306 [0 127.0.0.1:47740] "set" "key_1" "878"

      Another command useful for debugging is info, which returns several blocks of information and statistics about the server:

      Output

      # Server redis_version:4.0.9 redis_git_sha1:00000000 redis_git_dirty:0 redis_build_id:9435c3c2879311f3 redis_mode:standalone os:Linux 4.15.0-52-generic x86_64 . . .

      This command returns a lot of information. If you only want to see one info block, you can specify it as an argument to info:

      Output

      # CPU used_cpu_sys:173.16 used_cpu_user:70.89 used_cpu_sys_children:0.01 used_cpu_user_children:0.04

      Note that the information returned by the info command will depend on which version of Redis you’re using.

      Using the keys Command

      The keys command is helpful in cases where you’ve forgotten the name of a key, or perhaps you’ve created one but accidentally misspelled its name. keys looks for keys that match a pattern:

      The following glob-style variables are supported

      • ? is a wildcard standing for any single character, so s?mmy matches sammy, sommy, and sqmmy
      • * is a wildcard that stands for any number of characters, including no characters at all, so sa*y matches sammy, say, sammmmmmy, and salmony
      • You can specify two or more characters that the pattern can include by wrapping them in brackets, so s[ai]mmy will match sammy and simmy, but not summy
      • To set a wildcard that disregards one or more letters, wrap them in brackets and precede them with a carrot (^), so s[^oi]mmy will match sammy and sxmmy, but not sommy or simmy
      • To set a wildcard that includes a range of letters, separate the beginning and end of the range with a hyphen and wrap it in brackets, so s[a-o]mmy will match sammy, skmmy, and sommy, but not srmmy

      Warning: The Redis documentation warns that keys should almost never be used in a production environment, since it can have a major negative impact on performance.

      Conclusion

      This guide details a number of commands that are helpful for troubleshooting and resolving issues one might encounter as they work with Redis. If there are other related commands, arguments, or procedures you’d like to see outlined in this guide, please ask or make suggestions in the comments below.

      For more information on Redis commands, see our tutorial series on How to Manage a Redis Database.



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      How To Troubleshoot MySQL Queries



      Part of the Series:
      How To Troubleshoot Issues in MySQL

      This guide is intended to serve as a troubleshooting resource and starting point as you diagnose your MySQL setup. We’ll go over some of the issues that many MySQL users encounter and provide guidance for troubleshooting specific problems. We will also include links to DigitalOcean tutorials and the official MySQL documentation that may be useful in certain cases.

      Sometimes users run into problems once they begin issuing queries on their data. In some database systems, including MySQL, query statements in must end in a semicolon (;) for the query to complete, as in the following example:

      If you fail to include a semicolon at the end of your query, the prompt will continue on a new line until you complete the query by entering a semicolon and pressing ENTER.

      Some users may find that their queries are exceedingly slow. One way to find which query statement is the cause of a slowdown is to enable and view MySQL's slow query log. To do this, open your mysqld.cnf file, which is used to configure options for the MySQL server. This file is typically stored within the /etc/mysql/mysql.conf.d/ directory:

      • sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf

      Scroll through the file until you see the following lines:

      /etc/mysql/mysql.conf.d/mysqld.cnf

      . . .
      #slow_query_log         = 1
      #slow_query_log_file    = /var/log/mysql/mysql-slow.log
      #long_query_time = 2
      #log-queries-not-using-indexes
      . . .
      

      These commented-out directives provide MySQL's default configuration options for the slow query log. Specifically, here's what each of them do:

      • slow-query-log: Setting this to 1 enables the slow query log.
      • slow-query-log-file: This defines the file where MySQL will log any slow queries. In this case, it points to the /var/log/mysql-slow.log file.
      • long_query_time: By setting this directive to 2, it configures MySQL to log any queries that take longer than 2 seconds to complete.
      • log_queries_not_using_indexes: This tells MySQL to also log any queries that run without indexes to the /var/log/mysql-slow.log file. This setting isn't required for the slow query log to function, but it can be helpful for spotting inefficient queries.

      Uncomment each of these lines by removing the leading pound signs (#). The section will now look like this:

      /etc/mysql/mysql.conf.d/mysqld.cnf

      . . .
      slow_query_log = 1
      slow_query_log_file = /var/log/mysql-slow.log
      long_query_time = 2
      log_queries_not_using_indexes
      . . .
      

      Note: If you're running MySQL 8+, these commented lines will not be in the mysqld.cnf file by default. In this case, add the following lines to the bottom of the file:

      /etc/mysql/mysql.conf.d/mysqld.cnf

      . . .
      slow_query_log = 1
      slow_query_log_file = /var/log/mysql-slow.log
      long_query_time = 2
      log_queries_not_using_indexes
      

      After enabling the slow query log, save and close the file. Then restart the MySQL service:

      • sudo systemctl restart mysql

      With these settings in place, you can find problematic query statements by viewing the slow query log. You can do so with less, like this:

      • sudo less /var/log/mysql_slow.log

      Once you've singled out the queries causing the slowdown, you may find our guide on How To Optimize Queries and Tables in MySQL and MariaDB on a VPS to be helpful with optimizing them.

      Additionally, MySQL includes the EXPLAIN statement, which provides information about how MySQL executes queries. This page from the official MySQL documentation provides insight on how to use EXPLAIN to highlight inefficient queries.

      For help with understanding basic query structures, see our Introduction to MySQL Queries.



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