Data access is expensive for your application. It often requires CPU, network and disk access, all of which can take a lot of time. Using less computing resources, particularly in the cloud, results in decreased overall operational costs, so caches provide real value by avoiding using those resources. You need an efficient and reliable cache in order to achieve the desired result. Your end users also care about response times because this affects their work productivity or their enjoyment of your service. This post describes some of the most common cache methods for MySQL.

Popular cache methods

The MySQL query cache

When the query cache is enabled, MySQL examines each query to see if the contents have been stored in the query cache. If the results have been cached they are used instead of actually running the query.. This improves the response time by avoiding the work of the query. If you are getting the impression that I’ve just introduced you to the query performance magic bullet, unfortunately, I haven’t.

The problem with the query cache is “coarse invalidation”. That is, as soon as you change a single row in any table, the query cache entries for every query which accessed that table must be invalidated. If a frequently executed or expensive query is invalidated, response time will be significantly impacted.

The invalidation frequency is controlled by the rate of change in the database tables. This results in unpredictable and therefore, undesirable, performance. The other big problem with the query cache is that it is protected by a single mutex. On servers with many cores, a high volume of queries can cause extensive mutex contention. Percona Server even has a state in the processlist ‘waiting on query cache mutex’ so that this is easier to spot.

External cache (Memcached)

In order to eliminate the unpredictable nature of the the query cache, external caches like Memcached are usually employed. When rows in the database change, the old query results remain available in the cache, but they are now “stale”. Until the cache key expires, the contents are available immediately and you avoid performing work in the database.

Eventually the cache contents expire. When this happens, the application attempts to get the cached value and fails. At this time, the application must compute the results, then place them into the cache. Additionally, memory pressure may cause unexpected invalidation of items, once again resulting in unpredictable, and therefore undesirable performance.

If the cache is emptied (perhaps due to a restart, crash, upgrade or power loss) then all of the results are invalidated, which can cause very poor performance.

Cache invalidation is a problem

With both of these cache methods, once the cache is invalidated or expires the entire result must be recalculated. This recalculation may be very expensive. For example, suppose we need to calculate the total count of items sold. A query may have to access many millions of rows of data to compute that result and this takes time.

Another problem which can occur is the “cache stampede” aka a “miss storm”. This happens when multiple requests need data for the same key, but the key recently expired. The stampede impacts performance because multiple requests try to recompute the contents at the same time. These cache stampedes are essentially the cause of the unpredictable performance of the MySQL query cache, since the rate of invalidation can not be controlled, and multiple cache entries may be invalidated by a single table change.

Peter’s advice in the miss storm blog post suggests that for best performance one should pre-compute the data for expensive keys. That is, the keys which are frequently accessed, or those that are expensive to recompute. For the queries that are expensive to recompute, the pre-computation normally takes the form of summary tables, which are discussed next.

If request frequency is the problem (mostly likely because the response time goes up due to increased concurrency) but the time to compute the contents is low, then using a mutex around the content generation is a possible solution to the problem. This forces one query to do the computation while others wait for the result. There are also probabilistic methods to enqueue items to be rebuilt with some increasing probability as the request time approaches the expiration time. This does not, however, offer very much improvement for keys which are not accessed very frequently.

Use what you need

Both Memcached and the MySQL query cache are limited in size. If you try to cache more information than you can store, space will need to be freed in order to store the new information in the cache. In order to ensure cache efficiency, you must only place information in the cache that you intend to retrieve again. Storing data in the cache that you won’t read again also increases response time in your application because it wastes a round trip to the cache server. It wastes CPU and other resources too.

Pick an efficient cache representation

If you can cache an entire HTML block instead of the rows used to create it, then do so. This avoids the CPU usage on your web server to create the block from the rows again and again. If you are paying for compute cycles in the cloud, this can be very beneficial as may reduce the number of instances you need.

Don’t make too many round trips

Asking the cache for many different pieces of data to satisfy a single request is not very efficient. Use multi_get when possible. Once again, caching entire portions of pages is a good way to reduce the number of round trips to the cache.

Summary tables

Queries that access a lot of data usually face two bottlenecks: disk IO and CPU usage. Disk IO is expensive, and even if the disk bottleneck is eliminated, the sorting, aggregation and join operations are still CPU intensive and single threaded. In order to avoid these operations, the results can be pre-aggregated into a summary table.

Ideally, the summary tables can be updated with only the information that changed since they were last populated. MySQL includes two statements that make this easier: CREATE TABLE .. SELECT and INSERT .. SELECT. These SQL commands can be used to either replace the contents of, or insert new data into the summary table.

One advantage of summary tables is that they are persistent unlike the query cache or Memcached. There is no risk of unexpected invalidation, either. The summary table always exists, and should be fast to access, since it can be indexed appropriately for your queries.

Using INSERT .. SELECT for summary tables

The INSERT .. SELECT approach works best when there is some sort of log table, such as a log of clicks, a web access log, metric data from a monitoring system, or the like, which is to be aggregated over time. With this type of source data, one does not expect to see very many (if any) updates to the data once it has been collected.

This method does not usually work well when database tables may be updated or when rows may be deleted. Such changes may happen if a log is accidentally imported twice, for example and then the duplicate items are deleted. When this happens, decreasing the counts in summary tables may not be possible and thus they may be out of sync with the underlying data.

If there are frequent changes to the data then other options for maintaining summary tables must be used. Either they must be rebuilt from scratch each time (like a memcache miss) or they must be updated. Updating the summary tables efficiently is a hard problem to solve. My latest post on using Flexviews addresses this problem in a dedicated post.

Conclusion

If the least expensive queries are the ones you never run, then the most expensive queries very well may be the ones you have to run when the cache is empty. When talking about cache, the miss path is at least as important as the hit one. In order to make the miss path less expensive, use a layered approach to your caching. Cron jobs and summary tables can be used to make the miss path much less expensive. If you don’t pre-compute, and your website performance is dependent on your cache, then you could have a serious performance problem if your cache is unexpectedly emptied.

Effective caching is important. It is important to only cache things you know you will need again. Cache data in the form that it makes most sense to your application. Don’t cache the results of every query, simply because it is easy to do so.

View full post on MySQL Performance Blog

Before installing Linux onto your PS3, you need to first decide which version of Linux you want to use. There’s Yellow Dog Linux, Ubuntu, Fedora, Xubuntu, Edubuntu, and a few others. With so many choices, it’s easy to see why a lot of people feel confused and lost and get frustrated with the process.

The best version of Linux for PS3 is probably Yellow Dog because it was specifically desgined for use on the PS3. Yellow Dog Linux has the most features and is also the easiest to install. The other version that is also pretty good is Ubuntu Linux for PS3. It’s possible to install the others that I mentioned above, but I recommend you stick with Yellow Dog or Ubuntu.

Once you’ve decided on the version of Linux you wish to use, you need to decide if you want to go with a hardware modification, or a soft-mod. I personally recommend staying away from a hard-mod because it’s very risky and you could ruin your PS3 for good if you do something wrong. A mod chip will also void your warranty which is never good. Soft-mods are much easier and safer to perform when it comes to figuring out how to install Linux on PS3.

So now that you know which version of Linux you’re going to install, and you’ve hopefully decided on going with a soft-mod, all that’s left to do before you’ve successfully figured out how to install Linux on PS3 is download the software and files and a good instruction manual to guide you through the process.

This part is very important because if you download the wrong files or follow a bad guide, you will be left with more headaches than it’s worth. I know this from experience after trying to follow some of the terrible guides written on different forums. There are even files out there that are corrupted and come with viruses attached to them. This is an easy way to really screw up your PC or PS3.

After a lot of testing and running into my fair share of problems trying to learn how to install Linux on PS3 myself, I finally found a simple solution that I’m going to share with you. It’s very safe, quick, and easy to perform the Linux installation using this method.

I wrote more in-depth about it on my site, so if you want to learn more about how to install Linux on PS3, head on over to my site here: How to Install Linux on PS3

Even though web development services have varied dimensions, the tips and tricks of the trade have always been a unique set, set aside by web development experts down the years. Whether you hire on-site employees or hire offshore staff, the set of tips for custom web development remains the same. Hereon, we discuss some such points that will help you in constructing fast yet efficient and productive web development solutions.

Know HTML better – You may know HTML better than several other web development experts, but strive to know better. Learning and understanding the code will be of great help in troubleshooting as well as appending your code or design in the near future.

Do Regular Updates – Regularly updating your web site, no matter how small or insignificant that change may be, is a great feature implemented by web development experts. This does not only make your site popular among the visitors but also among the search engine crawlers, an advantage most web development services target to achieve!

Take Your Time, Yet Be Efficient Enough – Don’t sprint your web site into the launch. Meticulous checking and testing is necessary for a site code to work, whether it is a MySQL development project or PHP development project. Always remember that an ill functioning site will always damage reputation, both of the site owner as well as of the web development experts who had worked on it. Clawing your way through the site code’s validation processes, update methods, etc. yields much better results than just hurrying and putting your site over the net with innumerable existent errors.

Make a Lasting Impression – With the huge array of sites of the similar category available to the user in a jiffy via the search engine result pages (SERPs), none of the visitors will go through your whole web site at the first go. If at the first glance (usually lasting about 10 seconds) a visitor does not like your site, he/ she will leave for better options. Using enhanced code, graphics and prominent calls to action, you can retain the visitor and thus automatically increase your conversion rate as well.

Add ‘Links’ Page – Adding the links page on your web site allows you to enhance your Search Engine Optimization tasks, as for link exchange services and making your web site more user-friendly.

Custom web development experts and other web service providers try and follow these tips to enhance their search engine favourability and optimization status to score over their rivals. It might not be a complete assurance for better productivity and return (as web site optimization depends on several other factors as well), but it will surely help your site to achieve better results than the existing optimization and revenue scenario it is currently experiencing.

Configuring MySQL cache sizes properly is one the common ways to expand the available RAM. If you noticed that your MySQL server instance is using too much memory, you can decrease the MYSQLcache sizes. And if its getting slower due to larger requests you can you can increase the chache size as per your needs.

One more way to increase the performance of the Linux VPS is to disable the control panels. Everyone likes to use the most popular control panels such as Cpanel & Plesk. But if you want to free your resources you should only use the control panels when necessary. You can install them again by running a small PHP script or using shell prompt. This will free up about 120MB of RAM.

Disable the unwanted features, modules and plug-ins such as Apache that are enabled in software packages. By disabling unnecessary modules or plugins will decrease the system memory that server softwares such as Apache requires, which will provide you more resources for the software that are more in need.

One of the top way to make your Linux VPS responsive is to disable the system services that are unnecessarly in use. The services which are not used not only consumes RAM and CPU space but they also make your server unsecured.

Apache server is a well-known for its role in the development of the World Wide Web and never been confusing to the clients. To free up the memory as per the needs check the memory apache is using and adjust the Startservers.

PHP development along with MySQL development has become one of the most popular web development services over the Internet today. With rapid updates and secure changes being made in the PHP development scene, it is bound to become one of the commanding scripting languages in no time. In custom web development services time is more important, with web development experts being constantly on the lookout for better tools to enhance speed in PHP development.

Here we shall discuss the five most effective tools to speed up PHP development for better results.

A superior editor or an IDE – Are you still using editing tools like Notepad or WordPad? Then its time to change to better alternatives, such as EditPlus2 or PHPEdit, which are absolutely free. A better editor gives you time-saving advantages such as -

Color highlighting, hence easier spotting and correction operations.

Search, locate and replace operations.

Another way is to use an extensive IDE (Integrated Development Environment), though these come with a high stake of license fees.

Use a basic relevant framework – Most PHP development projects are built on a similar kind of framework. Therefore, instead of starting off from square one on every project, use the skeletal framework and build on it, thus saving oodles of time on your project, which you can actually use to troubleshoot or customize your project better!

Recycle and Re-use – Ready solutions and answers to questions or issues that have risen and solved in previous projects should be put to use. Most of the existing scripts not only provide great ideas to imitate, they are free too! Extending an existing PHP script for custom web development services or modifying it to suit your needs will save you a lot of time and money.

Simplicity is the Best Policy – Keep your code simple and objective. The more complicated the codes in PHP MySQL development projects or custom web development projects are, the harder it is for the programmer to find the glitches and remove them. Re-do the code if necessary in order to keep it simple. Time is a luxury you cannot afford to waste otherwise your whole effort as well as the expensive work of your hired offshore staff will go down the drain.

Proper Documentation – The proper documenting of your code, including comprehensible comments, hints and other such tools is mandatory to save time. Troubleshooting is made much easier this way, especially while pondering over the complicated parts of the code.

PHP development or PHP MySQL Web Development projects are a rage today in the online services arena, and saving precious time while the development process is underway is a great way to cut down on your custom web development budget. Using at least two to three of these tips is bound to make your PHP development work pace speed up.

Furthermore, it might also result in inaccessibility of complete database components records. In such situations, if you have a backup of the InnoDB database, then you can overcome the problem very easily. But if in case, you have not maintained any backup or the backup file is inaccessible, then the only way to isolate the problem is by repairing the corrupt InnoDB table by using an efficient third-party mysql database repair software.

As a practical instance, you receive the below error message, while attempting to access your InnoDB table:

“InnoDB: Database page corruption on disk or a failed
InnoDB: file read of page 97234.
InnoDB: You may have to recover from a backup.
090106 0:04:13 InnoDB: Page dump in ascii and hex (16384 bytes):
len 16384; hex 5f5bbb4802017bee01017bda00017c0b0000000768da392745bf00000000000000000000014100001ea9ffffffff”
The same error message pops up when you try to access your InnoDB table. Furthermore, the database table records become inaccessible after the above error message appears.

Cause

The main cause for the occurrence of the above error message is corruption of metadata structure of the InnoDB table. Few main reasons for the corruption can be improper server shutdown, virus attack, and application malfunction.

Resolution

For systematic isolation of the above error message and accessing of table records, you will need to opt for a powerful MySQL database repair software. A MySQL Repair utility repairs almost all MySQL database components, without making any changes in the original data. The powerful repairing algorithms ensure maximum repair of MySQL database after any logical corruption situation. With to the point and detailed user-documentation, the software is easy to use without any prior technical knowledge.

Database Recovery For MySQL supports repair of MySQL database components created in InnoDB and MyISAM database engines. Designed for almost all Windows operating systems (7, Vista, XP, 2003 and 2000), the read only software supports  MySQL 5.x and 4.x. The demo version of the MySQL Repair software also provides a preview of all repairable database components.