Planet MySQL

Planet MySQL - https://planet.mysql.com
  1. New small servers for performance testing

    My old NUC cluster found a new home and I downsized to 2 new NUC servers. The new server is NUC8i7beh with 16g RAM, 500g Samsung 860 EVO for the OS and 500g Samsung 970 EVO for performance. The Samsung 860 is SATA and the Samsung 970 is an m.2 device. I expect to wear out the performance devices as I have done that in the past. With the OS on a separate device I avoid the need to reinstall the OS when that happens.The new NUC has a post-Skylake CPU (i7-8559u), provides 4 cores (8 HW threads) compared to 2 cores (4 HW threads) in the old NUCs. I disabled turbo boost again to avoid performance variance as mentioned in the old post. I am not sure these have sufficient cooling for sustained boost and when boost isn't sustained there are frequent changed in CPU performance. I also disabled hyperthreads out of concern for both the impact from Spectre fixes and to avoid a different syscall overhead each time I update the kernel.I might use these servers to examine the impact of the ~10x increase in PAUSE times on InnoDB with and without HT enabled. I might also use them for another round of MySQL performance testing when 8.0.14 is release.I am a big fan of Intel NUC servers. But maybe I am not a fan of the SATA cables they use. I already had one of my old NUCs replaced under warranty after one of the SATA wires was bare. In the new NUCs I just setup a few of the SATA cables appear to be cut and I wonder if that eventually becomes bare.
  2. Hangs when using trylock reader writer lock functions

    The pthreads reader writer locks have a couple of problems that can lead to application hangs. The trylock functions can cause other threads to hang indefinitely in the rdlock or wrlock functions even after the lock is no longer held.  If these hanging threads hold other locks, the application can deadlock and grind to a halt.  This bug was reported in glibc bug 23844 and has existed since glibc 2.25 was released.  The 2.25 version includes a new reader writer lock implementation which replaces large critical sections with compare and exchange atomic instructions.  The code paths for an uncontended lock should be very fast.  Unfortunately, a couple of bugs that can hang the application need to be fixed.The trywrlock function is a non-blocking function that tries to grab a write lock on a reader writer lock.  Unfortunately, this function is missing logic that the wrlock function has when the phase of the reader writer lock transitions from read phase to write phase.  This missing logic is intended to synchronize the state of the lock with other reader threads so that when the write lock is released, blocked reader threads are awakened.  Since the logic is missing, the reader threads are not awakened.The tryrdlock function is a non-blocking function that tries to grab a read lock on a reader writer lock.  Unfortunately, this function is missing logic that the rdlock function has when the phase of the reader writer lock transitions from write phase to read phase.  This missing logic awakens reader threads that may be waiting for the read phase.  It just so happened that the thread executing tryrdlock got there first.  Since the logic is missing, the reader threads are not awakened.There are patches to the trywrlock and tryrdlock functions in the glibc bug report that fix these bugs.  Hopefully, these patches will be included in a new glibc release soon.The MySQL server uses reader writer locks for various purposes.  I wonder if it is affected by these reader writer lock bugs.
  3. Amazon RDS Aurora Serverless – The Basics

    When I attended AWS Re:Invent 2018, I saw there was a lot of attention from both customers and the AWS team on Amazon RDS Aurora Serverless. So I decided to take a deeper look at this technology, and write a series of blog posts on this topic. In this first post of the series, you will learn about Amazon Aurora Serverless basics and use cases. In later posts, I will share benchmark results and in depth realization results. What Amazon Aurora Serverless Is A great source of information on this topic is How Amazon Aurora Serverless Works from the official AWS  documentation. In this article, you learn what Serverless deployment rather than provisional deployment means. Instead of specifying an instance size you specify the minimum and maximum number of “Aurora Capacity Units” you would like to have: Once you set up such an instance it will automatically scale between its minimum and maximum capacity points. You also will be able to scale it manually if you like. One of the most interesting Aurora Serverless properties in my opinion is its ability to go into pause if it stays idle for specified period of time. This feature can save a lot of money for test/dev environment where load can be intermittent.  Be careful, though, using this for production size databases as waking up is far from instant. I’ve seen cases of it taking over 30 seconds in my experiments. Another thing which may surprise you about Amazon Aurora Serverless, at the time of this writing, is that it is not very well coordinated with other Amazon RDS Aurora products –  it is only available as a MySQL 5.6 based edition and is not compatible with recent parallel query innovations either as it comes with list of other significant limitations. I’m sure Amazon will resolve these in due course, but for now you need to be aware of them. A simple way to think about it is as follows: Amazon Aurora Serverless is a way to deploy Amazon Aurora so it scales automatically with load; can automatically pause when there is no load; and resume automatically when requests come in. What Amazon Aurora Serverless is not When I think about Serverless Computing I think about about elastic scalability across multiple servers and resource usage based pricing.   DynamoDB, another Database which is advertised as Serverless by Amazon, fits those criteria while Amazon Aurora Serverless does not. With Amazon Aurora Serverless, for better or for worse, you’re still living in the “classical” instance word.  Aurora Capacity Units (ACUs) are pretty much CPU and Memory Capacity. You still need to understand how many database connections you are allowed to have. You still need to monitor your CPU usage on the instance to understand when auto scaling will happen. Amazon Aurora Serverless also does not have any magic to scale you beyond single instance performance, which you can get with provisioned Amazon Aurora Summary I’m excited about the new possibilities Amazon Aurora Serveless offers.  As long as you do not expect magic and understand this is one of the newest products in the Amazon Aurora family, you surely should give it a try for applications which fit. If you’re hungry for more information about Amazon Aurora Serverless and can’t wait for the next articles in this series, this article by Jeremy Daly contains a lot of great information. —Photo by Emily Hon on Unsplash
  4. MySQL Cluster Manager 1.4.7 released!

    MySQL Cluster Manager 1.4.7 is now available for download from My Oracle Support. Overview MCM 1.4.7 continues to improve MySQL Cluster Manager by adding some sought after features – and some important bugfixes. The 1.4.7 release bundles MySQL Cluster 7.6.8.…
  5. Fun with Bugs #75 - On MySQL Bug Reports I am Subscribed to, Part XII

    From the lack of comments to my previous post it seems everything is clear with ERROR 1213 in different kinds and forks of MySQL. I may still write a post of two about MyRocks or TokuDB deadlocks one day, but let's get back to my main topic of MySQL bugs. Today I continue my series of posts about community bug reports I am subscribed to with a review of bugs reported in November, 2018, starting from the oldest and skipping those MySQL 8 regression ones I've already commented on. I also skip documentation bugs that should be a topic for a separate post one day (to give more illustration to these my statements).These are the most interesting bug reports from Community members in November 2018: Bug #93139 - "mysqldump temporary views missing definer". This bug reported by Nikolai Ikhalainen from Percona looks like a regression (that can appear in a bit unusual case of missing root user) in all versions starting from 5.6. There is no regression tag, surely. Also for some reason I do not see 8.0.x as affected version, while from the text it seems MySQL 8 is also affected. Bug #93165 - "Memory leak in sync_latch_meta_init() after mysqld shutdown detected by ASan". This bug was reported by Yura Sorokin from Percona, who also made important statement in his last comment (that I totally agree with):"In commit https://github.com/mysql/mysql-server/commit/e93e8db42d89154b37f63772ce68c1efda637609 you literally made 14 MTR test cases ignore ALL memory problems detected by ASan, not only those which you consider 'OK' when you terminate the process with the call to 'exit()'. In other words, new memory leaks introduced in FUTURE commits may not be detected because of those changes. Address Sanitizer is a very powerful tool and its coverage should be constantly extending rather than shrinking." Bug #93196 - "DD crashes on assert if ha_commit_trans() returns error". It seems Vlad Lesin from Percona spent notable time testing everything related to new MySQL 8 data dictionary (maybe while Percona worked on their Percona Server for MySQL 8.0 that should have MyRocks also supported, should be able to provide native partitioning and proper integration with data dictionary). See also his Bug #93250 - "the result of tc_log->commit() is ignored in trans_commit_stmt()". Bug #93241 - "Query against full text index with ORDER BY silently fails". Nice finding by Jonathan Balinski, with detailed test cases and comments added by Shane Bester. One more confirmation that FULLTEXT indexes in InnoDB are still problematic. Bug #93276 - "Crash when calling mysql_real_connect() in loop". Nice regression in C API (since 8.0.4!) noted by Reggie Burnett and still not fixed. Bug #93321 - "Assertion `rc == TYPE_OK' failed". The last but not the least, yet another debug assertion (and error in non-debug build) found in MySQL 8.0.13 by Roel Van de Paar from Percona. You already know where QA for MySQL happens to large extent, don't you? Bug #93361 - "memory/performance_schema/table_handles have memory leak!". It's currently in "Need Feedback" status and may end up as not a bug, but I've never seen 9G of memory used for just one Performance Schema table so far. It's impressive. Bug #93365 - "Query on performance_schema.data_locks causes replication issues". Probably the first case when it was proved that query to some Performance Schema table may block some important server activity. Nice finding by Daniël van Eeden. Bug #93395 - "ALTER USER succeeds on master but fails on slave." Yet another way to break replication was found by Jean-François Gagné. See also his Bug #93397 - "Replication does not start if restart MySQL after init without start slave." Bug #93423 - "binlog_row_image=full not always honored for binlog_format=MIXED". For some reason this bug (with a clear test case) reported by James Lawrie is still "Open". Bug #93430 - "Inconsistent output of SHOW STATUS LIKE 'Handler_read_key';". This weird inconsistency was found by Przemysław Skibiński from Percona. Thinking about the future of MySQL 8 somewhere in Greenwich... To summarize this review: I obviously pay a lot of attention to bug reports from Percona engineers. It seems memory problems detected by ASan in some MTR test cases are deliberately ignored instead of being properly fixed. There are still many surprises waiting for early adopters of MySQL 8.0 GA :)  That's all I have to say about specific MySQL bugs in 2018. Next "Fun with Bugs" post, if any, will appear only next year. I am already subscribed to 11 bugs reported in December 2018. Stay tuned!