A conversation with 5 Facebook MySQL gurus

A conversation with 6 Facebook MySQL gurusFacebook, the undisputed king of online social networks, has 1.23 billion monthly active users collectively contributing to an ocean of data-intensive tasks – making the company one of the world’s top MySQL users.

A small army of Facebook MySQL experts will be converging on Santa Clara, Calif. next week where several of them are leading sessions at the Percona Live MySQL Conference and Expo. I had the chance to chat virtually with four of them about their sessions: Steaphan Greene, Evan Elias, Shlomo Priymak and Yoshinori MatsunobuMark Callaghan, who spoke at Percona Live last year, also joined our conversation which included a discussion of Facebook’s use of MySQL and other open source technologies.


Tom: What’s Facebook’s view of Open Source?

Steaphan: Facebook was built on open source software, and we still invest heavily in open source today. We understand the power these communities have to drive innovation – they allow us to focus on new challenges, as opposed to reinventing the wheel over and over again. And contributing as much as possible back to open projects is in everyone’s best interest.


Tom: Why MySQL? Wouldn’t NoSQL databases, for example, be better suited for the massive workloads seen at Facebook?

Mark: MySQL is great for many of our important workloads. We make it even better with our expertise in MySQL operations and engineering, and by working with the community and learning from their experience.

Yoshinori: I have not been able to find a transactional NoSQL database better than InnoDB. And it’s easy to understand how MySQL Replication works, which makes much easier to fix problems in production.


 Tom: How does Facebook make MySQL scale?

Steaphan: Sharding, automation, monitoring, and heavy investment in operations and performance engineering.


 Tom: What other things help Facebook run smoothly?

Steaphan: Our completely open culture, and the freedom all engineers here have to try any idea they have.


Tom: What is the top scaling challenge(s) Facebook faces in 2014 – and beyond?

Mark: Our biggest challenge is to make things better (performance, efficiency, availability) in the future at the rate we made things better in the past.

Yoshinori: Availability has improved a lot so far for us. Come to my session at Percona Live to hear about that. For me personally, efficiency is the biggest challenge for 2014 and 2015. This includes reducing space and optimizing for newer-generation hardware.


Tom: Facebook deployed MySQL 5.6 last year – including on critical environments – long before many other large organizations. What prompted such a move so soon? And where there any major concerns?

Steaphan: The same thing that prompts most efforts on the Facebook infra team: We will consider any technology that will help us improve performance, efficiency, or reliability, and we’re willing to accept the risk that sometimes comes along with adopting things like 5.6 very early on. But that’s only half the story here. The other half is that Facebook encourages its engineers to go after big bets like these — in this case it was just one engineer who made this happen. And we had the MySQL engineering talent we needed to work with the Oracle team to get 5.6 ready for production at our scale.

Yoshinori: At Facebook, we have three MySQL teams — Operations, Performance and Engineering. Facebook is one of the very few MySQL users that has internal MySQL developers. We all worked hard to adapt 5.6 to our scale and ensure that it would be production-ready. We found some issues after production deployment, but in many cases we could fix the problem and deployed new MySQL binary within one or two days. When deploying in production, we expected that we encountered MySQL 5.6 specific issues, which was typical when releasing new software. We were just confident that we could fix issues immediately.

Our 5.6 deployment step was not all at once. At first rollout, we disabled most major 5.6 features, such as GTID and binlog checksum. We gradually enabled such features in production.


Tom: Where there any significant issues in that move to MySQL 5.6? Any lessons learned you like to share – along with best practices you’d like to share?

Yoshinori: Performance regression of the CPU intensive replication was a main blocker for some of our applications. I wrote a blog post about this last year. We have several design plans to fix the problem on MySQL side. One of the most effective plans is grouping multiple transactions into one, since the most expensive part is writing to InnoDB system table at transaction commit. This optimization would be done when writing binary log, or by SQL thread. It may take longer time to test and deploy in production. For existing applications, we optimized to group multiple transactions from application side to mitigate the problem.


Tom: Performance monitoring is usually challenging at any organization. How do you do that at Facebook, which has tens of thousands of MySQL instances?

Yoshinori: Top-N monitoring is very important for managing a huge number of instances. Average statistics (for example: average innodb_rows_read across all instances) is not always useful since ~1% of problematic instances won’t noticeably change average numbers. p99 gives better indicators, but in our environment we typically have fewer than 0.1% instances causing problems, in which case p99 is not helpful either. We have several graphical and command-line tools to efficiently list up top-N bad behaving hosts. After listing up bad instances, the way to investigate root cause is pretty straightforward, like what MySQL consultants usually do. Server failure is something we expect and plan for at Facebook. For example, typical MySQL DBA at small companies may not encounter master instance failure during employment, because recent mysqld and H/W are stable enough. At Facebook, master failure is a norm and something the system can accommodate.


Tom: Evan, you and Yoshinori will present on Global Transaction ID (GTID) at Facebook. GTID is very tricky to deploy to an existing large-scale environment – how, and why, did you decide on adopting it?

Evan: Our primary motivations for adopting GTID all relate to either failover or binlog backups. When a master fails, getting replicas in sync with GTID is substantially simpler, faster, and less error-prone than previous methods of diffing binlogs. For backups, GTID is a cornerstone in building cross-datacenter point-in-time recovery, without needing redundant binlog streams from every region.

The “how” question is a bit more involved, and we’ll be covering this in detail during the session. The GTID project was a joint effort between three of Facebook’s MySQL teams. Santosh added new functionality to the MySQL server to make online rollout possible, and Yoshinori improved MHA to seamlessly support GTID-based failover. I added GTID support to all of our other in-house automation, and also scripted the rollout procedure across our many thousands of replica sets. A lot of validation logic and monitoring functionality was involved to ensure the safety of the rollout.


Tom: Shlomo, your session is titled “Under the Hood – MySQL Pool Scanner (MPS).” As you point out in your talk, Facebook has one of the largest MySQL database clusters in the world, comprising thousands of servers across multiple data centers. You must have an army of DBAs – or is there some secret you’d like to share? 

Shlomo: We do have an army, yes — it’s an “army of one.” We have one person on call on the MySQL Operations team at a given time, and they don’t even need to do all that much most days. We built “robots” to do our day to day jobs. The largest and most complex robot we have is MPS, an automated system to do most of the work a DBA might otherwise spend time on, such as replacement of broken or overflowing servers. Among other things, MPS also allows a human to initiate complex bulk operations with a few keystrokes, and it will follow up and complete the operations over the course of days or weeks.

I’ll be describing some of the complex MySQL automation systems we have at Facebook, and how they fit together during my talk.


Tom: Shlomo, what does a typical day look like for you there at Facebook?

Shlomo: The team’s work mostly focuses on maintaining those robots I’ve mentioned, as well as developing new ways to improve the reliability of our databases for Facebook’s users. This year the team also spent a lot of time making sure the new MySQL features such as GTIDs and semi-sync are deeply integrated in our automation. Every day, we work hard to to make ourselves obsolete, but we haven’t gotten there just yet!

On a typical day, I probably spend much of the time coding, mainly in Python. I also spend a significant amount of time working on capacity-related projects, such as thinking of ways to optimize the way we distribute the data across our fleet of servers.
Even after 2.5 years at Facebook, I am still in awe of the number of servers we manage. The typical small-scale maintenance operation at Facebook probably involves more servers than all the companies I’ve previously worked for had, combined. It really is pretty amazing!


Tom:  What are you looking forward to the most at this year’s conference?

Evan: There are plenty of fascinating sessions this year. Just to mention a handful: Jeremy Cole and Davi Arnaut’s session on innodb_ruby, since it’s a very unique way to interactively learn about InnoDB’s internals. Baron Schwartz’s session on using Go with MySQL, as VividCortex is blazing the trail here. Peter Boros and Kenny Gryp’s talk on scalability and benchmarking, which I’m hoping will include recent developments of Percona Playback. Tom Christ’s session on my former project Jetpants, to see how it has evolved over the past year at Tumblr. And several talks by Oracle engineers about upcoming functionality in MySQL 5.7.

Steaphan: In addition to the conference sessions, I look forward to the birds of a feather session with the MySQL team.  Last year, it proved to be a valuable opportunity to engage with those upstream developers who make the changes we care about, and I expect the same this year.


Tom: If you could talk to a DBA or developer on the fence about attending this year’s conference, what would be your top 3-5 reasons for making it over to Santa Clara for this event?

Evan: I’m based in NYC, so I’m traveling a bit further than many of my colleagues, but I can still confidently say that Percona Live is well worth the trip. The MySQL ecosystem is very healthy and constantly evolving, and the conference is the best place to learn about ongoing developments across a wide spectrum of companies and contributors. It’s also a perfect opportunity to personally connect with all of the amazing engineers, DBAs, users, and vendors that make MySQL so unique and compelling.


 The Percona Live MySQL Conference and Expo 2014 runs April 1-4 in Santa Clara, Calif. Use the code “SeeMeSpeak” when registering and save 10 percent. The inaugural Open Source Appreciation Day is on March 31 – this full-day event is free but because space is limited I suggest registering now to reserve your spot.

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ScaleArc: Benchmarking with sysbench

ScaleArc recently hired Percona to perform various tests on its database traffic management product. This post is the outcome of the benchmarks carried out by Uday Sawant (ScaleArc) and myself. You can also download the report directly as a PDF here.

The goal of these benchmarks is to identify the potential overhead of the ScaleArc software itself and the potential benefits of caching. The benchmarks were carried out with the trunk version of sysbench. For this reason, we used a very small set of data, so the measurements will be fast, and it’s known that caching has huge benefits when the queries themselves are rather expensive. We decided that we would rather show that benefit with a real-world application, which is coming later is this series. And if you’re in the Silicon Valley area, be sure to join us this evening at the first-ever Open Source Appreciation Day – I’d be happy to discuss the findings presented here in this post. Admission is free but due to limited space you should register now. I’ll also be available throughout the Percona Live MySQL Conference and Expo all this week.

sysbench_image1.2

In this summary graph it’s visible that in terms of throughput (read-only benchmark, which is relevant for read mostly applications), ScaleArc doesn’t have any significant overhead, while caching can have potentially huge benefits.

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The situation is pretty similar with response times. ScaleArc doesn’t add any significant overhead, and caching can mean huge benefit in terms of response time as well.

In case of this particular workload (which is read only sysbench), using caching means a roughly 3x increase in throughput and a roughly 80% drop in response time.

Overall, ScaleArc is a good product in terms of performance and features as well. I would definitely recommend it.

About ScaleArc for MySQL
ScaleArc for MySQL is a software appliance that drops in transparently between applications and databases to improve application availability and performance. It requires no changes to applications or databases and delivers:

  • Instant scale up – transparent connection pooling and multiplexing, TTL-based transparent caching, surge protection
  • Transparent scale out – read/write split, load balancing, query routing, sharding
  • Automatic high availability – automatic failover
  • Real-time actionable analytics

Benchmarking setup
The client machines are running the benchmarking software like sysbench in case of these benchmarks.

CPU: 2 x Intel(R) Xeon(R) CPU E5-2620 v2 @ 2.10GHz (6 cores, chip multithreading off)
Memory: 64G

We used 2 clients. The results of the 2 clients are graphed separately, so it’s visible that they put the same amount of workload on the database or ScaleArc software.

Database machines
CPU: 2 x Intel(R) Xeon(R) CPU E5-2620 v2 @ 2.10GHz (6 cores, chip multithreading off)
Memory: 64G

Running MySQL Community Edition 5.6.15

MySQL configuration

[mysqld]
   max_allowed_packet = 64M
   thread_cache = 256
   query_cache_size = 0
   query_cache_type = 0
   max_connections = 20020
   max_user_connections = 20000
   max_connect_errors = 99999999
   wait_timeout = 28800
   interactive_timeout = 28800
   log-error=/var/lib/mysql/mysql.err
   back_log=60000
   innodb_buffer_pool_size = 3G
   innodb_additional_mem_pool_size = 16M
   innodb_log_buffer_size = 8M
   innodb_flush_log_at_trx_commit = 0
   innodb_flush_method = O_DIRECT
   innodb_open_files = 2000
   innodb_file_per_table
   innodb_log_file_size=2G
   innodb_log_files_in_group=2
   innodb_purge_threads=1
   innodb_max_purge_lag=0
   innodb_support_xa=0
   innodb_locks_unsafe_for_binlog = 1
   innodb_buffer_pool_instances=8
   sql_mode=NO_ENGINE_SUBSTITUTION,STRICT_TRANS_TABLES

The buffer pool of the database is intentionally small, so it’s easy to generate a disk-bound workload.

Please note that the following settings are not recommended in production.

innodb_support_xa=0
   innodb_locks_unsafe_for_binlog = 1

 

We used these settings to drive the node to its peak performance, avoiding any possible overhead which might be required on a production system. In typical production settings, these are not set, and binary logging is enabled, which potentially reduces ScaleArc’s overhead further.

ScaleArc software appliances
CPU: 1 x Intel(R) Xeon(R) CPU E5-2620 v2 @ 2.10GHz (6 cores, chip multithreading off)
Memory: 64G

The machines were running ScaleArc for MySQL 3.0.

Network
The machines were connected using 10G connections.

Measurements
All of the measurements were done with a very small database that completely fits in memory.

--oltp-table-size=10000
  --oltp-tables-count=64

 

In these benchmarks, we expected both the database and ScaleArc to be CPU bound. In case of a disk-bound workload, ScaleArc would shine even more than in this benchmark. If the queries are more expensive (they have to hit storage), the overhead in % is smaller, and in case of caching the query-by-query benefit is bigger.

We measured 3 different setups, both on read-only and read-write cases. These are the following.

  • Direct connection to the database.
  • Connection to the database through ScaleArc, where ScaleArc only acts as a pass-through filter (since it’s a load balancer that speaks the MySQL wire protocol, all the mechanics for that are still in place). Please note that this setup doesn’t make sense in real life. The purpose of this setup is to show the potential overhead of using ScaleArc and uncovering potential limitations of the ScaleArc software itself.
  • Connection to database through ScaleArc, where ScaleArc is allowed to cache. Caching in ScaleArc is TTL (Time To Live)-based caching, meaning that a read query’s results are cached in ScaleArc. If that read query is seen again before expiring, the query is not run again on the database server but rather served from the cache. Once the timer for the cached query expires, the query will be issued on the database again. Caching of course only works for reads, which are not in an explicit transaction (autocommit is on and no START TRANSACTION is issued). Because of that, we used –oltp-skip-trx during cached benchmarks (read-only case). In case of these benchmarks, the TTL was 1 hour, because we wanted to saturate the ScaleArc software while serving cached queries. An 1 hour TTL might be unrealistic for some applications, while for other applications even an 1 day TTL is something they can live with for some queries. In this case, we wanted to measure the cache’s performance, so we wanted the queries to be cached during the entire benchmark run to show the potential gain even in case of very small queries.

TTL-based caching
It’s important to note that the cache’s expiration is controlled by a TTL value – there is no other invalidation, so it’s possible to read stale data when the query results is changed, but the cache is not expired. Reading stale data alone is ok for most applications, it can happen with a regular, asynchronous slave if it’s lagging behind the master (and it always lagging behind somewhat). Otherwise, the cache is pretty similar to MySQL’s query cache, which doesn’t suffer from the stale read problem, but it has a coarse invalidation (if a table is written, the cache entries belonging to the given table are flushed). While the cache is flushed, the query cache mutex is held, which blocks reads even. Because of the mutex, the built-in query cache is a very usual performance bottleneck. ScaleArc’s cache doesn’t suffer from this.

It’s important to note that ScaleArc caches nothing by default. Also, there are other ways to invlidate cache entries apart from waiting for the TTL to expire.

  • API Call based invalidation (you can clear the cache for an entire query pattern rule with one API call)
  • Query comment based invalidation (you can put a comment /*wipe*/ before a query and wipe and refresh the cache)
  • Cache Bypass (you can send a comment /*nocache*/ and bypass the cache for that specific query)

Read-only
Sysbench throughput

sysbench_image6

In the lower region of threads (up to 32), we see that the TPS value significantly drops in case of going through ScaleArc. That’s nothing to be surprised about, the reason for that is network roundtrips. Because ScaleArc is a software appliance, it adds a hop between the database and the application, which introduces latency. If the number of threads is higher (32 and up), this starts to matter less and less, and performance is almost identical which is very impressive. It means that around the optimal degree of parallelism for these machines, ScaleArc introduces very little (barely measurable) overhead.

Sysbench response time
sysbench_image7
This graph contains the response times belonging to the previous benchmarks. This is really hard to read because at 4096 threads, the system is overloaded, and the response time is much more than in the maximum throughput region. Because it’s multiple orders of magnitude higher, the interesting response times are not readable from this graph.

sysbench_image8

The following graph is the same as above, except that the y axis is limited to 250 ms, so the region which is not visible on the graph above is visible here. What we see there regarding the overhead is pretty much the same as we saw in case of the throughput graph, which means that ScaleArc by itself introduces immeasurably low latency (which explains the difference in cases when parallelism is low). Usually applications which are utilizing the database server are using significantly more than one thread (in MySQL a single query always uses a single thread, in other words there is no intra-query parallelism). The latency from 32 threads above is actually somewhat lower when going through ScaleArc (the exact tipping point can be different here based on the number of CPUs). The reason for that is ScaleArc itself uses an event loop to connect to MySQL, so at a high concurrency, and can schedule sending the traffic to MySQL differently. This only matters when otherwise the MySQL server is saturated CPU-wise.

CPU utilization

sysbench_image9

Last but not least, this graph contains the CPU utilization of the different setups. The left-hand side shows the CPU utilization when connecting directly to the database, and the right-hand side shows connecting through ScaleArc. In both cases, the database server’s CPU is the bottleneck. It’s visible that the client node’s CPU is more than 75% idle (only client1 is graphed to improve readability, client2 is practically the same). From 32 threads and up, the blue bar (CPU user%) is relatively high on the database servers, as is the green (CPU sys%). From 64 threads, the idle time is practically 0, until the systems are overloaded. On the right hand side, we can see that ScaleArc at this load still had 50% idle CPU, which means that we could practically do the same benchmark on another set of boxes through the very same ScaleArc, and only then it will be fully utilized. We are talking about 3000 sysbench tps here. One more interesting thing to note is the relatively high system time of ibd. This is also because of the way ScaleArc connects to the database (see the previous paragraph).

[  17s] threads: 64, tps: 3001.98, reads/s: 41962.70, writes/s: 0.00, response time: 35.22ms (95%)

 

These threads are from a single client, which means that ScaleArc could keep up with parsing roughly 84000 statements / second with utilizing half of its CPU, which is impressive. Please note that the ScaleArc software in this case was tuned towards this type of workload, which means we had more query processing threads. In case of caching, we will have more cache handler threads.

Effects of caching on read-only workload
Sysbench throughput

The next set of graphs will compare the cases when cache is used and not used.

sysbench_image11

The preceding TPS graph contains reads / second (because we measured with –oltp-skip-trx), so roughly 42000 reads corresponds to roughly 3000 transactions in the earlier setup (14 reads in a transaction). On the left-hand side of the graph, the cached throughput is visible with green – on the right-hand side, the non-cached throughput is visible with red (direct access) and blue (access through ScaleArc as a pass-through filter). It’s visible that caching improves the speed drastically, but when ScaleArc becomes overloaded (8192 client threads, 4096 from each client), the performance becomes somewhat inconsistent, which is understandable considering how few cores ScaleArc was running on. On the graph, the dots are translucent, which means the colors are brighter in the areas that have more samples. Even in the overloaded case, the majority of the samples are in the region of 100k+ reads / second across two clients, which means that the performance degrades very gracefully even under heavy load.

Sysbench response time

sysbench_image12

Like in the case of a non-cached workload, the response times are not too readable because of the very high response times when the systems are overloaded. But from the overloaded response times visible, it seems like using caching doesn’t make response times worse.

sysbench_Image13

Like in the case of non-cached workload, this graph is the zoomed version of the previous one. Here the maximum of the y axis is 100 ms. From this graph, it’s visible that at lower concurrency and at the optimal throughput, caching actually helps response time. This is understandable, since in case of a cache hit, ScaleArc can serve the results, and the client (in our case here sysbench) doesn’t have to go to the database, so a roundtrip and database processing time is spared. It’s also worth mentioning that the data “comes from memory,” it doesn’t matter if we hit the ScaleArc cache of the database. When the ScaleArc cache is used, the response time is lower because the additional roundtrip to the database and potential database work (like parsing SQL) is avoided. This means that caching can have benefits even if the database fits in the buffer pool. The improvement is always subject to the workload – caching helps the most when it can cache relatively expensive queries like aggregations and queries hitting the storage.

CPU utilization

sysbench_image14

Similarly to the previous case, the preceding graph shows CPU utilization of the various components. In case of the cached workload, the client itself is much more utilized (since it gets responses sooner, it has to generate the traffic faster). With this kind of workload, when using only one client, we would hit the client’s CPU as the performance bottleneck. The database is interesting too. With caching, its CPU is barely used. This is because if a query is served from the cache, it never gets to the database, so the database’s CPU utilization will be lower. In other words, using the cache helps to offload the database. If offloading is visible on ScaleArc’s graphs, when caching is used, the CPU on the server hosting ScaleArc is much more utilized. For this benchmark, the ScaleArc software was tuned to handle a cached workload, which means more cache handler threads.

Read-write
For read-write benchmarks, we had to create oltp_nontran.lua, which is the same sysbench benchmark as oltp.lua, except that it does the reads outside of the transaction and does only the writes in transaction, so caching can have an effect on read. The rest of the benchmarking setup is the same as the read-only case.

Sysbench throughput

sysbench_image15

Similarly to the read-only case, at a low concurrency, the overhead of ScaleArc is coming from the additional network roundtrip. At the optimal concurrency, the overhead is barely measurable (the dots are plotted practically on top of each other).

Sysbench response time

sysbench_image16

sysbench_image17

The case is pretty similar with the response times as in the read-only case. Similarly, the second graph is a zoomed version of the first one, which a 250 ms maximum.

CPU utilization

sysbench_image18

The CPU utilization graph shows that in this case, the database server’s CPU is the bottleneck. What is interesting is that ScaleArc is using less CPU than in the read-only case. This is understandable, since a transaction now contains writes as well, which are expensive on the database side, but they are still just statements to route on the ScaleArc side.

Effects of caching on read-write workload
Measuring caching here is interesting because the workload is no longer read-only of mostly reads. We have a very significant amount of writes.

sysbench_image19

For 30k reads, we get 8,5k writes. It’s expected that caching won’t help as much as in the previous case, because writes can’t be cached and while they are in process, the benchmarking threads can’t proceed with reads. Please note that this means that roughly 25% of the traffic is write, a typical application scaling out with additional slaves for reads doesn’t have this kind of read-to-write ratio.

Sysbench throughput

sysbench_image20

The first graph shows that in terms of total throughput, caching still helps.

Sysbench response time

sysbench_image21

sysbench_image22

Similarly to the read-only case, caching also helps response time, because it reduces the time needed for the read part of the workload.

CPU utilization

sysbench_image24

This test really stresses the database server’s CPU when not caching. With caching on, similarly to the read-only case, the client’s workload increases somewhat (but not as much), and the database server’s CPU usage decreases significantly. In the last row, the CPU utilization of ScaleArc shows that although it’s somewhat higher with caching, it’s still not that much higher.

From these tests it’s visible that caching can still be beneficial even if the write ratio is as high as in this test.

Conclusion
Engineering is always about making the right tradeoffs. If one wants features that needs a protocol-level load balancer like ScaleArc, the price should be paid in the overhead of Layer 7 parsing and decision making. ScaleArc’s engineering team did a great job minimizing this overhead. ScaleArc itself is very well tunable for different workload types (if caching is important, ScaleArc can be tuned for caching – if query rewriting, ScaleArc can be tuned for that).

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