Set infinite or very long timeout, and have at least two semisync readers. Facebook enhancements to semi-synchronous Replication we spent a lot of time for testing Semi-synchronous replication in 2013. We found some S1 bugs, serious performance problems, and some administration issues. Our mysql engineering team and Performance team worked for fixing issues and finally our Operations team deployed Semisync in production. Here are our major enhancements. Backporting Loss-Less Semisync from.7 As described above, loss-Less Semisync is needed to prevent data loss and phantom reads, so we backported Loss-Less Semisync patch from official mysql.7 to our Facebook mysql.6 branch. It will be merged to webScalesql branch soon.
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reducing write amplification. Write volume can be reduced a lot, even less than half in some cases. This is important especially when using flash devices, because less write volume increases flash life expectancy. Requirements for Semisync Deployment to make semisync work, you need kindle at least one semisync reader (slave with semisync enabled) within the same (or very close) datacenter as the master. This is for latency. When semisync is enabled, round-trip time(RTT) between master and one of the semisync slaves is added to transaction commit latency. If none of the semisync slave is located within close datacenter, rtt many take tens or hundreds of milliseconds, which means you can commit only 10100 times from single client. For most environments, this will not work. You need a slave within close datacenter. To make fast failover work without data loss, you need to make sure semi-synchronous Replication is always enabled. Mysql semisync has a couple of points where optionally semisync is disabled: - exceeding timeout (exceeding rpl_semi_sync_master_timeout milliseconds to get ack from all of the semisync slaves) - no semisync slave (can be controlled via - executing set global rpl_semi_sync_master_enabled0 If you want to enable.
With Semi-synchronous replication, you can immediately start failover when master is down. When promoting a degenerative slave to the new master, identify the latest slave (highly likely one of the semi-synchronous slaves but not guaranteed) and apply differential logs to the new master. Master's durability does not matter here, because there is no way to access master's data during failover. So you can safely reduce durability. Reducing durability has a lot of benefits. reducing latency on (group) commit because it doesn't wait for fsync. reducing iops because the number of fsync calls is significantly reduced: from every commit to every second. Overall disk workloads can be reduced. This is especially helpful if you can't rely on battery/flash backed write cache.
But it's much less likely to happen compared to normal semisync. To avoid data loss and phantom reads, normal Semisync can't meet your expectations. Using Loss-Less Semisync is needed. With Loss-Less Semi-synchronous replication, committed data should be on one of the slaves, so you can recover from the latest slave. You can always do fast failover here. Reduced Durability When you do fast failover, you can set reduced durable settings about on master as well as slaves. Reduced durability means innodb_flush_log_at_trx_commit!
Normal semisync wait (after_commit on normal semisync(after_commit committing to Innodb is done before waiting for ack from semisync slave, so the committed rows are visible from applications, even though semisync slaves may not have received the data. If master is crashed and none of the slaves received the data, the data is lost but applications may have seen them. This is called phantom reads, and in many cases it's problematic. Loss-less semisync (after_sync) avoids the problem. Loss-less semisync commits Innodb after getting ack from one of semisync slaves. So when committed data is visible from applications, one of the semisync slaves have received that. Phantom read risk is much smaller: if both master and the latest semisync slave are down at the same time, data is lost.
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Loss-Less Semisync is a new Semisync feature supported in official mysql.7. Original implementation was done by Zhou zhenxing as marathi ". Enhanced Semisync " project, and also filed as a bug report. Oracle implemented based on his idea, and named Loss-Less semisync for. So enhanced Semisync and Loss-Less Semisync have same meanings. I say loss-Less semisync in this post.
Normal semisync and loss-less semisync work as below. Binlog prepare (doing nothing). Innodb prepare (fsync). Binlog commit (writing to fscache). Binlog commit (fsync). Loss-less semisync wait (after_sync). Innodb commit (releasing serving row locks, changes are visible to other users).
If you do not care about data loss risk, there is no reason to use semi-synchronous replication. You can use normal mysql replication and do fast failover with mysqlfailover or mha. Facebook is one of the companies to care about data loss risk with. Mysql, so that's why we were interested in Semi-synchronous replication a lot. Semisync replication was originated from, google in 2007.
Official mysql supported from.5. Actual implementation algorithm was substantially different from google's. Mysql cluster and Galera offers synchronous replication protocol in different ways. I do not cover them in this blog post. Semi-synchronous Replication currently has two types of different algorithms - normal Semisync and Loss-Less Semisync. Let me explain the differences. Differences between Normal Semisync and Loss-Less Semisync.
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There is a risk of losing some data but you don't shmoop have any other choice. This "safer" approach has two issues. This is because you have to wait for master's recovery. you can't eliminate risks of losing data. If master is dead and never recovers, your risk of losing data is the same as doing fast failover. So, in bad cases, you have to suffer from both longer down time and losing data. Semi-synchronous Replication is helpful to prevent from losing data.
always set fully durable settings on master. By fully durable i mean setting and sync_binlog1. on master crash, wait for a while (1030 minutes) until the crashed master recovers. Recovery takes long time because it involves os reboot, storage resume and filesystem recovery, and Innodb crash recovery. if the crashed master recovers, you can continue services without losing any data. Since all data exist on the master, slaves can continue replication. Btw official.6 had a bug causing all slaves broken in this scenario, but this bug was fixed.6.17. if the crashed master doesn't recover (H/W failure etc you need to promote one of slaves to a new master.
supports gtid. Both mysqlfailover and mha rely on mysql replication. Mysql replication is asynchronous. So there is a very serious disadvantage - potential data loss risk on master failover. If you use normal mysql replication and do automated master failover with mha/mysqlfailover, you can do failover quickly (a few seconds with mha but you always have risks of losing recently committed data. If you don't want to take any risk of losing data, you can't do fast master failover with normal mysql replication. You have to do the following steps in case of master failure.
First, let me describe why the objective is difficult without semisync. There are a couple of fast slave promotion (master failover) solutions. Mha covers both fully automated and semi-automated mysql failover solution. Fully automated means both failure good detection and slave promotion are done automatically. Semi automated means failure detection is not done but slave promotion is done by one command. Time to detect failure is approximately 10 seconds, and actual failover is taking around 5 to 20 seconds, depending on what you are doing during failover (i.e. Forcing power off of the crashed master will take at least a few seconds). Total downtime can be less than 30 seconds, if failover works correctly.
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After intensive testing and hack, we started using Semi-synchronous mysql replication at analysis Facebook production environments. Semi-synchronous Replication itself was ready since mysql.5 (GA was released.5 years ago! but I'm pretty sure not many people have used in production so far. Here are summary of our objective, enhancements and usage patterns. If you want to hear more in depth, please feel free to ask me at Percona live this week. Objective / Why semisync? The objective of the semi-synchronous Replication is simple - master failover without data loss, without full durability.