Xsan Block Sizes in Objective-C

Encoding ECC200 in Objective-C Xsan Block Sizes

Xsan can act as the back-end storage to provide front-end network file-sharing services for a Mac OS X environment. This isn t to say that it ll work like a charm without some fine-tuning, though. In tuning any Xsan volume, one of your most important tools is the block size. As mentioned previously, the stripe breadth multiplied by the block size should come to about 1MB. You ll have to customize the stripe breadth on the storage pools whenever you change the block sizes for the volume. If you re using Xsan as a repository for data to be shared over clustered file storage, then it s important to maintain a small block size. How small That depends on your data. If it s in large files, you may be able to stick with the default settings for clustered file storage in the volume setup wizard. If it s in small files, though, consider going even lower.

When an AFP client gets disconnected from a share point, it attempts to look for a token in order to reconnect automatically. If the server doesn t have the token, the client can t reconnect because it s comparing the presented token with its own cache (stored, by default, in /etc/AFP.conf). However, if you re using an Xsan, you ll want your servers to share a token location. The token store is the reconnectKeyLocation key in the /Library/Preferences/ com.apple.AppleFileServer.plist property list. You can use the defaults command to move the tokens to an Xsan volume. Follow the command with the appropriate option switch (in this case, write to put data into the property list), followed by the name of the

property list and then the key that we ll be writing into. We want to write text into the key, with the text string being a path. If the volume name is bighonkinvolume, the command will be:

Additionally, you ll want to customize file-locking mechanisms in some cases. AFP locks files at the application layer by default. With Xsan, where multiple file-server heads are involved, it s best to use locks at the file-system level. Therefore, you can use the AFP settings for the daemon to prevent AFP from locking files itself. The command is:

In Windows, when you ve connected to a share using, say, a mapped drive letter, that share shows as active. At some point, if the client can t communicate to an SMB/CIFS server with the open session, the drive will appear offline to the Windows client. AFP does something similar, but the result is what, as perceived from the Finder, appears to be constant communication with the AFP server. Actually, it s verified by the server every 30 seconds. With the AFP client, if no poll is sent from the server, the client will also attempt to reach out to the server to verify that the connection is available. This process is known as tickling. AFP uses the tickle to verify that clients are still connected to a server. This communication causes a small amount of network congestion, but in some environments you want to keep even that to a minimum. This is similar to the concept of disabling protocols in a network stack that aren t being used. Such protocols aren t likely to cause issues on one machine, but when employed by thousands of hosts, they can effectively cause a denial of service on the server. Provided that you use AFP, you re just not going to want to disable the tickle. By default, it happens every 30 seconds with no user intervention (other than connecting to a share point and having a session greater than 30 seconds). But while you might not want to disable it, you can reduce traffic by increasing the number of seconds between updates. For example, the following command will up the time between tickles to 60 seconds: