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File Fragmentation, SANs, NAS and RAID RRS feed

  • Question

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    Stripe sets work well in the following environments:

    1. When users need rapid access to large databases or other data structures.

    2. Storing program images, DLLs or run-time libraries for rapid loading.

    3. Applications using asynchronous multi-threaded I/O's.

    Stripe sets are not well suited in the following situations:

    1. When programs make requests for small amounts of sequentially located data. For example, if a program requests 8K at a time, it might take eight separate I/O requests to read or write all the data in a 64K strip, which is not a very good use of this storage mechanism.

    2. When programs make synchronous random requests for small amounts of

    data. This causes I/O bottlenecks because each request requires a separate

    seek operation. 16-bit single-threaded programs are very prone to this problem.

    It is quite obvious that RAID can exploit a well written application that can take advantage of asynchronous multi-threaded I/O techniques. Physical members in the RAID environment are not read or written to directly by an application. Even the Windows file system sees it as one single "logical" drive. This logical drive has (LCN) logical cluster numbering just like any other volume supported under Windows. As an application reads and writes to this virtual environment (creating new files, extending existing ones, as well as deleting others) the files become fragmented. Because of this fact, fragmentation on this logical drive will have a substantial negative performance effect. When an I/O request is processed by the file system, there are a number of attributes that must be checked which cost valuable system time. If an application has to issue multiple "unnecessary" I/O requests, as in the case of fragmentation, not only is the processor kept busier than needed, but once the I/O request has been issued, the RAID hardware/software must process it and determine which physical member to

    direct the I/O request. Intelligent RAID caching at this layer can mitigate the negative impact of physical fragmentation to varying degrees but will not solve the overhead caused to the operating system with the logical fragmentation. So the question now becomes how does a defragmenter affect this? The defragmenter sees the RAID environment just as the file system does. That is, it defragments the logical drive, improving the speed and performance of a RAID environment by eliminating wasteful and unnecessary I/Os from being issued by the file system. This occurs because the file system sees the files and free space as being more contiguous. The file system will spend less time checking file attributes which means more

    processor time can be dedicated to doing real useful work for the user/application. In addition, these I/O requests are now more likely, depending on the RAID, to fill the entire 64K chunk (RAID stripe) size with the I/O now taking full advantage of the RAID. To gauge the impact of fragmentation on a RAID system employ performance monitoring technologies and examine Average Disk Queue Length, Split I/O’s, and %

    Disk Time. Additional disk performance tuning information can be found in Microsoft’s online resources.

    Tuesday, March 20, 2007 8:50 AM