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Virtual FileSystems


Disk filesystems
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Flash filesystems

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NFS was invented by Sun Microsystems. It became widespread largely because it's quite easy to implement. In return for its simplicity, it tends to give relatively poor performance and a nearly complete lack of safety. These are both largely due to its connectionless nature. When you request data from a file, the server sends you the requested information, but does NOT keep track of which clients have which files open. To keep you from seeing (terribly) out-of-date information from a file, the data you read has an "expiration date". If you refer to the data from more than, say, a minute, it will expire and your client will request the data from the server again, whether it's changed or not. If you write data to the file, you have no way of knowing whether somebody else has updated the information between your reading and writing your data, so you may overwrite things they've written with older data. To ensure at least a little bit of safety, the server is supposed to actually commit data you write to disk before it returns to you.

In other words, NFS works pretty well for read-only access to things like executables on a server. For things like on-line databases, it's essentially a disaster waiting to happen (and it usually doesn't wait very long).

More recent versions of the NFS spec have cured most of these problems, but support for these updates is still (years later) somewhat uneven.


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