What is Blob Cache? How to configure it ?

SharePoint Server 2010 provides three types of caches that help improve the speed at which Web pages load in the browser: the BLOB cache, the page output cache, and the object cache.

SharePoint Server 2010 provides a disk-based cache that stores files that are used by Web pages to help them load quickly in the browser, and reduces the load on the database server when it uses those files. These files are known as binary large objects (BLOBs), and the cache is known as the BLOB cache.

Configuring cache settings for a Web application.

You enable and configure the BLOB cache, and make configuration changes to the page output cache profiles and the object cache in the Web.config file in the Web application to which you want to apply those changes. The changes you make to the Web.config file will be applied to all site collections within the Web application

Note : Please take a backup of the web.config before you make any changes to the file

To configure BLOB cache settings

1.      Verify that you have the following administrative credentials: You must be a member of the Administrators group on the local computer to configure the BLOB cache settings.

2.      Click Start, point to Administrative Tools, and then click Internet Information Services (IIS) Manager.

3.      In Internet Information Services (IIS) Manager, in the Connections pane, click the plus sign (+) next to the server name that contains the Web application, and then click the plus sign next to Sites to view the Web application or applications that have been created.

4.      Right-click the name of the Web application for which you want to configure the disk-based cache, and then click Explore. Windows Explorer opens, with the directories for the selected Web application listed.

5.      Right-click web.config, and then click Open.

6.      If the Windows dialog box appears, select Select a program from a list of installed programs, and then click OK.

7.      In the Open With dialog box, click Notepad, and then click OK.

8.      In the web.config Notepad file, find the following line: <BlobCache location="" path="\.(gif|jpg|jpeg|jpe|jfif|bmp|dib|tif|tiff|ico|png|wdp|hdp|css|js|asf|avi|flv|m4v|mov|mp3|mp4|mpeg|mpg|rm|rmvb|wma|wmv)$" maxSize="10" enabled="false" />

9.      In this line, change the location attribute to specify a directory that has enough space to accommodate the cache size.

Microsoft strongly recommends that you specify a directory that is not on the same drive as where either the server operating system swap files or server log files are stored.

10.  To add or remove file types from the list of file types to be cached, for the path attribute, modify the regular expression to include or remove the appropriate file extension. If you add file extensions, make sure to separate each file type with a pipe (|)

11.    To change the size of the cache, type a new number for maxSize. The size is expressed in gigabytes (GB), and 10 GB is the default. 

It is recommended that you not set the cache size smaller than 10 GB. When you set the cache size, make sure to specify a number large enough to provide a buffer at least 20 percent bigger than the estimated size of the content that will be stored in the cache.

12.  To enable the BLOB cache, change the enabled attribute, from "false" to "true".

13.  Save the Notepad file, and then close it.

When you save a change to the web.config file, the Web application in Internet Information Services (IIS) 7.0 automatically recycles. This recycling can cause a brief interruption in service to sites contained in that Web application, and users can lose session state

When you enable the BLOB cache on your front-end Web server, you reduce the load on the SharePoint Server 2010 database server created by read requests from Web browsers.

http://technet.microsoft.com/en-in/library/cc261797%28v=office.14%29.aspx#section3