Compaq 108164-003 - ProLiant - 800 White Paper page 15

Note: Compaq uses coalescing
algorithms to optimize disk
performance.
ECG025.0997
W P
HITE APER
(cont.)
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Table 7 lists the Host bus transfer rates for the following Compaq servers:
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Server Name
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. . ProLiant 5000, 6000, 6500 and 7000
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ProLiant 1500, 2500 and 4500
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The last example discussed how READ performance is increased. Now let's discuss how
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. . WRITE performance is enhanced on a system by taking advantage of posted writes.
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Posted writes take place when file system or disk controller caching temporarily holds one
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. . or more blocks of data in memory until the hard disk is not busy. The system then
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combines or "coalesces" the blocks of data into larger blocks and writes them to the hard
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disk. This results in fewer and larger sequential I/Os. For example, a network server is
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used to store data. This server is responsible for completing hundreds of client requests.
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. . If the server happened to be busy when data was being saved, the server's file system
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cache tells the application that the data has been saved so that the application can
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. . continue immediately without having to wait for the disk I/O to complete.
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Coalescing is also commonly referred to as "Elevator seeking." This coined phrase
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became popular because it provides the perfect analogy for describing coalescing. For
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instance, an elevator picks up and drops off passengers at their requested stop in the
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most efficient manner possible. If you were on level 6, the elevator on level 2, and other
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passengers on levels 1 and 7, the elevator would first stop on level 1 to pick up the
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passenger going up. Next, the elevator would stop on level 6, then 7 and then take
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everyone to level 9, their destination. The elevator would not perform all of the requests
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individually, instead it reorders then completes those requests in a more efficient manner.
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This same analogy applies to coalescing when writing data to different sectors on a disk.
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As an example, Joe saves or "writes" data B to the hard disk, then he saves data A to the
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same disk. And finally, he saves data C as well. Instead of completing 3 separate I/Os
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for B, A, then C, the system reorders the write requests to reflect data ABC then performs
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a single sequential I/O to the hard disk, thus improving disk performance.
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15
Table 7:
Host Bus (Memory) Transfer Rates
Transfer Rate
540 MB/s
267 MB/s