blocks|key|3036988|text|Velocity等非常棒，特别是当您的SQL服务器独立运行时。我们一直在使用ASP.NET的内置缓存，但希望迁移到Velocity。多个web服务器与SQL集群通信，缓存确实有助于提高可伸缩性并减少SQL负载。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3036989|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Velocity, et al, are GREAT especially when your SQL server lives on its own box. We've been using ASP.NET's built-in caching but hope to move to Velocity. Several web servers talk to a SQL cluster and the caching really helps with scalability and reducing the SQL load.

blocks|key|3037017|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3037018|因此，如果SQL+server有自己的缓存，那么外部Memcached+(或类似的)服务器有什么好处？|blockquote|3037019|3037020|3037021|是的，SQL+Server有自己的缓存，但他只缓存：|3037022|3037023|Query+plans|unordered-list-item|3037024|pages+from+the+database+files+|3037025|3037026|但他不缓存：|3037027|3037028|results+from+a+query|3037029|3037030|例如，你有一个复杂的查询，它使用了对大量数据的聚合(想想:我们的客户数据库中有多少不同的国家:按国家从客户组中选择不同的国家)|3037031|SQL+Server将扫描整个customer表，但您的结果集只有几个条目。当您重新发出查询时，SQL+Server将重用查询计划并重新扫描customer表(如果幸运的话，页面仍在内存中)|3037032|当您使用memcached时，您可以存储结果集的几行，并在不连接到数据库服务器的情况下反复重用它们。因此，它会从数据库服务器中承担一些负载。|3037033|注意:如果您的数据在SQL服务器上发生更改，请注意一些过时的数据!！|3037034|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|-4|4|5|6|14|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|15|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|16|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|17|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|18|7|@]|8|@]|9|$]]|$1|H|3|-4|4|5|6|19|7|@]|8|@]|9|$]]|$1|I|3|J|4|K|6|1A|7|@]|8|@]|9|$]]|$1|L|3|M|4|K|6|1B|7|@]|8|@]|9|$]]|$1|N|3|-4|4|5|6|1C|7|@]|8|@]|9|$]]|$1|O|3|P|4|5|6|1D|7|@]|8|@]|9|$]]|$1|Q|3|-4|4|5|6|1E|7|@]|8|@]|9|$]]|$1|R|3|S|4|K|6|1F|7|@]|8|@]|9|$]]|$1|T|3|-4|4|5|6|1G|7|@]|8|@]|9|$]]|$1|U|3|V|4|5|6|1H|7|@]|8|@]|9|$]]|$1|W|3|X|4|5|6|1I|7|@]|8|@]|9|$]]|$1|Y|3|Z|4|5|6|1J|7|@]|8|@]|9|$]]|$1|10|3|11|4|5|6|1K|7|@]|8|@]|9|$]]|$1|12|3|-4|4|5|6|1L|7|@]|8|@]|9|$]]]|13|$]]

<blockquote>
 So if SQL Server has it's own cache, what is the benefit of an external Memcached (or similar) server?
</blockquote>

Yes SQL Server has its own cache but he caches only: 
- Query plans 
- pages from the database files 

but he does NOT cache: 
- results from a query

e.g. you have a complex query which uses some aggregation on a lot of data
( think of: how many different countries we have in our customer database :
 SELECT DISTINCT Country from Customers GROUP BY country )

SQL Server will scan th WHOLE customer table, but your resultset will only a few entries long.
When you reissue your query, SQL Server will reuse the query plan and will rescan the customer table,
( and if you are lucky the pages are still in memory )

When you use memcached you may store the few rows of your resultset and reuse them over and over again without connecting to the database server. So it takes some load from your database server. 
NOTE: Beware of some stale data, if your data changes on the SQL server !!

blocks|key|3037053|text|另一个好处是，SQL+Server的扩展成本较高，而添加新的web/缓存服务器的成本较低。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3037054|我们在应用程序级别使用缓存来存储所有类型的数据，也不是所有的数据都来自数据库。例如，您可以在代码中操作数据对象，然后添加到缓存中。|3037055|如果需要，您甚至可以存储标记(输出缓存)。|3037056|在一天内，通过使用缓存，我们将我们的网站从能够处理150个并发会话，而压力测试提高到800多个。我强烈推荐使用它！|3037057|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

Another benefit can also be that SQL Server is expensive to scale whereas adding a new web/caching server can be cheaper to achieve.

We use caching at an application level to store all sorts of things, not all of them from a database either. You may manipulate data objects in your code then add to cache for instance.

You can even store markup if necessary (output caching).

In one day by using caching we moved our site from being able to handle 150 concurrent session while stress testing to well over 800. I strongly recommend using it!!

I've been reading a lot of articles that suggest putting a Memcached (or Velocity, etc) in front of a database is more efficient than hitting the database directly. It will reduce the number of hits on the database by looking up the data in a memory cache, which is faster than hitting the database.

However, SQL Server has it's own memory cache for objects in the database. When data is retrieved, SQL Server maintains its cache and will (if necessary) pull the row from it's memory and not hit the disk.

So if SQL Server has it's own cache, what is the benefit of an external Memcached (or similar) server?

Most of the articles I have been reading are around social networking sites, that mostly use MySql. However, an <a href="http://www.baselinemag.com/c/a/Projects-Networks-and-Storage/Inside-MySpacecom/" rel="noreferrer">article</a> about MySpace, that uses SQL Server, suggests caching is used on that system as well.

This <a href="http://weblogs.asp.net/fbouma/archive/2006/08/31/Why-a-cache-in-an-O_2F00_R-mapper-doesn_2700_t-make-it-fetch-data-faster_2E00_.aspx" rel="noreferrer">article</a> explains when caching should be used and this <a href="http://aspadvice.com/blogs/ssmith/archive/2006/09/01/Caching-in-O_2F00_R-Mappers-and-Data-Layers.aspx#22311" rel="noreferrer">article</a> is a counterpoint.

Memcached vs SQL Server cache

压力测试

SQL Server

数据库

服务器

我已经阅读了很多文章，这些文章建议将Memcached (或Velocity等)放在数据库前面比直接访问数据库更有效。它将通过在内存缓存中查找数据来减少对数据库的命中次数，这比命中数据库的速度更快。但是，SQL Server为数据库中的对象提供了自己的内存缓存。检索数据时，SQL Server将保留其缓存，并将(如有必...

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