I have been working on the server migration of a legacy ecommerce application using PHP 5.6.
The switch involved two Dedicated 32 servers from Linode.
One server is for NginX + PHP and the other is for MySQL only.
The legacy application leverages memcached.
After the switch, I can see a heavy internal traffic caused due to private inbound and outbound connections.
So far this element didn't cause any problem on performance.
However, I was under the impression that the queries would be cached on the local machine, and not on the remote.
Because if the query is cached on the remote host, it sill has to transmit the result set over the private network, instead of retrieving from RAM or the local SSD.
Am I assuming this wrong?
It may be that I am missing the point where the private inbound traffic is more beneficial for overall performance when compared to a local cache.
MySQL has a feature called the Query Cache, but this caches query result sets in the mysqld server process, not on the client. If you run the exact same query again after the result has been cached in the Query Cache, it will copy the result from the Query Cache and avoid the cost of running the query again. But this will not avoid the time to transfer the result across the network from mysqld to your PHP application.
Also keep in mind that the MySQL Query Cache is being deprecated and retired.
Alternatively, your application may store data from query results in memcached, but typically this would be done by the application code (I know there are UDF's to read and write memcached from MySQL triggers, but this is a bad idea).
If your memcached service is not on the same host as your PHP code, it would result in network transfer twice: Once when querying the data from MySQL the first time, then again transferring the data into memcached, then later every time you fetch the cached data out of memcached.
PHP also has some features to do in-memory caching, such as APCu. I don't have any experience with this, and it's not clear from a brief scan of the documentation where it stores cached data.
PHP is designed to be a "shared nothing" language. Every PHP request has its own data, and data doesn't normally last until the next request. This is why a cache is typically not kept in PHP memory. Applications rely on either memcached or the database itself, because those will hold data longer than a single PHP request.
If you have a fast enough network, it shouldn't be a high cost to fetch items out of a cache over a network. The performance architects at a past job of mine developed this wisdom:
"Remote memory is faster than local storage."
They meant that if the data is in RAM on a server, then reading it from RAM even with the additional overhead of transferring it across a network is usually better than reading the data from persistent (disk) storage on the local host.
Related
We have a 2GB Digital Ocean server, and it is dedicated for a MySQL server of other two PHP servers. we are using Percona MySQL Server 5.6 on this server. We configured MySQL replication and these configuration is working fine
Our issue is sometime our site monitoring tools reporting that some of the URL hosted with this server is down (May be this is happening once in a week or two). When I am checking, I could see that Mysql Master server load is too much high (May be 35 - 40), so the MySQL server was not responded. # that I usually do a MySQl service restart, this restart cause to server load become normal and the sites started working after service restart.
This is a back-end MySQL database server of 20-25 PHP applications (WordPress, Drupal and some custom applications server).
Here are my questions,
Why this server load automatically goes down, after a spikes happens?
Is there any way in which database is causing issues? So that I can identify the application too.
How can I identify the root cause of this issues
Depending upon your working dataset, a 2GB server providing access for 20-25 PHP applications (WordPress, Drupal and some custom applications server) could be the issue.
For example, if you have a 1.4GB buffer pool (assuming all tables are InnnoDB) and 10GB of data, then your various applications could end up competing for resources, such as I/O, buffer pool pages, Adaptive Hash Index, query cache. They could also, assuming caching is used, be invalidating theit caches within a similar timeframe, thus sending expensive queries to the database.
Whilst a load of 50 is something that you would normally want to avoid, the load average is not something that you should concern yourself with if showing in isolation.
The use of the uninterruptible state has since grown in the Linux
kernel, and nowadays includes uninterruptible lock primitives. If the
load average is a measure of demand in terms of running and waiting
threads (and not strictly threads wanting hardware resources), then
they are still working the way we want them to.
http://www.brendangregg.com/blog/2017-08-08/linux-load-averages.html
If the issue is happening once per week then it is starting to sound like a batch process, or cache expiration issue - too much happening at once for the resources available.
The best thing to do is to monitor and look for the cause. Since you are already using Percona Server, using PMM should give you the perfect insight to find the cause, although it works with Oracle MySQL, MariaDB, Aurora, etc. You can try a demo to see the insights that you can gain:
https://pmmdemo.percona.com. The software is Open Source and free to use.
You can look in QAN to find the most expensive queries, whilst looking at Prometheus data to give an insight into the host itself. There are some recommendations to get the most from PMM, depending upon your flavour of MySQL.
I have wordpress website running in a VPS server and is handling about 150 mysql queries / second.
Ocassionaly when we notice a spike in traffic to about 200 mysql queries a second the https requests to the site is extremely slow.
The site loads decently with http but with https it takes 20+ seconds.
Gradually over a period of an hour after the spike, the load times get better and then it gets back to normal again.
There server load and memory looks fine. There is only a spike in mysql queries, firewall traffic and eth0 requests. There are no mysql slow queries
Any help would be appreciated.
Thank You
I think your answer is in "disk latency" and "disk utilization" charts.
MySQL works well under small loads, when it can cache all data it needs. But when your results or queries get too big, or you request too many of them, it will start doing many disk I/O operations. This enables you to handle huge loads and very big data, but in the moment you exceed your MySQL allocated memory, you will need to read and write everything to disk.
This would not be so bad if you ran on a local SSD drive. But from the device name I see that you are running on an EBS volume, which is not a real hard drive. It is networked drive, so all the traffic overloads your network connection.
You have several options:
1.) install mysqltuner, let the server operate for some time and then run it and see what it suggest. My guess is, that it will suggest you to increase your MySQL memory pool, decrease number of parallel connections or restructure your queries.
2.) use EC2 instance type with actual local storage (e.g. m3 or r3) and write to local SSD. You can do RAID on several SSD drives to make it even faster.
3.) use EBS optimized instance (dedicated EBS network bandwidth) and correct volume type (some EBS volume types have I/O credits, similar to CPU credits for t-type instances, and when you run of those, your operations will slow down to crawl).
I have a Rails 4.x application running on server A and MySQL on server B.
Using ab to do a load test of my API calls I notice that the MySQL server is showing CPU activity. So I go back to the code and check, but no SQL statements are triggered, to be sure I also deactivate all before filters, but still the MySQL server shows CPU load.
I went to MySQL and run
show processlist;
but that also shows no active SQL statements
Why would there be load on the DB server?
A Rails application initializes connection pools to the configured database on app load and also loads basic schema data for each ActiveModel defined to populate runtime mappings from the DB to instances of that model.
These connections/queries will happen as soon as you have loaded the application and running traffic.
If this is not what is responsible for the activity on your database server, you will need to use other tools to see what is responsible. For example, NewRelic's system monitoring tools are great for snapshotting CPU/memory usage over time correlated to what processes were running. This will help you rule out MySQL itself using resources vs. other things running on the DB server.
According to this article, storage engines like Innodb may have their own per thread and/or global memory allocations which is probably accounting for the CPU overhead. If this is a stock (non-tuned) MySql install, you're probably just seeing baseline CPU activity. The article mentions a number of places to look that might indicate areas that can be tuned to reduce this footprint.
I have 2 servers for my website
Web Server
For sending dynamic content, mostly created with PHP
A lot of RAM and a fast processor, only a few GBs of hard drive space.
and a
File Server
For sending static content, images, videos etc..
A few TBs of hard drive space, not as much RAM and a slower processor.
I want to Use the speed of the Web Server, but the space of the File Server. But I heard the overhead of NFS will make it so slow it will not matter...
I will be using MySQL and I want to know how I should optimize the database so I can keep the data on the File Server, but have the queries preformed, and processed by the Web Server.
The advice you received is correct in my experience... running mysqld on one box and using a remote server via NFS for file storage is not very fast (if the remote storage were a SAN that would be a different matter).
You can reduce the number of times your database is hit and leverage the RAM on the web server by caching on that tier. Look into introducing something like memcached to help with the most expensive MySQL operations.
If some of your tables are small but used frequently, you could consider running a second instance of MySQL on your web server just for those tables. Keep in mind, though, that you will have two separate points of database failure that need to be managed (appropriate backups, security updates, etc.).
I am writing a db logging ruby gem which will simply take out a job from a Beanstalk queue and write it in the DB.
That is one process on Server A puts a job (that it wants to log) in the Beanstalk queue on Server B, and my logging process on Server B takes it out and writes it to the mysql DB on Server B.
I want to know if this is worth it?
Is putting a job in the Beanstalk queue faster than writing to the DB. Or can my process that wants to log to DB directly write it to DB instead of using the logging process.
Note that both the beanstalk server and DB are on another server.
Beanstalk internally makes a socket call from Server A to Server B.
I believe mysql would need to do the same as well?
So therefore is mysql to another server going to be slower than putting in the beanstalk queue.
It'll be much faster, primarily because Beanstalkd jobs, by default, are stored in-memory and are lost if, for example, you lose power on your server, whereas MySQL is a strongly ACID-compliant relational database, and hence will go to a lot of effort and flush each of your logs to disk.
I think you'll find that, after your do some benchmarking with a lot of logs being made by your system, that disk I/O will be your limiting factor, rather than the speed of TCP/IP sockets. Your current system's advantage is that when server A files a log on Server B's beanstalkd instance it takes up very little of Server A's time, and Server B can periodically flush our many logs at once from beanstalkd to MySQL, making the process more efficient. The disadvantage is that, the more you batch up the logs, the more logs you will lose in the event of a software / power failure, unless you use beanstalkd's "-b" parameter which makes jobs durable by writing them to disk (and hence making the process slower).
Of course, the only way to truly settle this question is to benchmark!