Chart 5 - Striped Dumps To Disk Arrays - Compaq 219700-001 - ProLiant - 1500 White Paper

Compaq Backup and Recovery for Microsoft SQL Server 6.x
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As expected, performance increases as the number of storage devices (controllers, SCSIports, spindles)
used is increased. The performance trend when dumping to RAID-0 arrays however, differs from that
seen with RAID-5 arrays. For the RAID-0 arrays, when the number of storage devices is doubled from
the first test to the second - throughput jumps about 50% (from about 32 to 48 GB/hr). After that only
a marginal increase is seen, because at this point the limit of the source (read) controller to deliver
data is already reached. With a throughput of nearly 50 GB/hr on the third test however, we have
reached the maximum throughput predicted in the previous section (allowing for a small additional
overhead for write processing)
Performance for the RAID-5 arrays at first lags significantly behind that of the RAID-0, but scales
quickly as storage devices are added. The performance doubles as the number of storage devices are
doubled from the first test to the second (from 16 to 32 GB/hr). This is because the throughput
limitation of the source controller has not yet been reached - at this point our bottleneck is at the Smart-
2 controllers used to write the data to; RAID-5 is very write-intensive and tends to saturate the data
channels on the controller when large (64KB) writes requests are rapidly being issued
second to the third tests, the number of controllers has been doubled again, so that the performance
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The throughput for the 3rd RAID-0 test may have yielded a higher number if SQL Server had been reading
the database from two Smart-2 controllers combined into a Windows NT stripe set, as with the 3rd null device
test in the previous section.
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Writes to a RAID level 5 disk array may involve up to 4x additional I/O. When transferring data to a RAID-5
volume, 4 physical I/O's (2 reads and 2 writes) must normally occur for every logical write request so that the
parity information may be re-created. If purely sequential writes occur to the array however, the Smart-2
controller can sometimes recognize the contiguous data pattern and thus perform fewer additional I/O's for the
parity (i.e: parity can be generated for each "stripe" across all drives instead of for each write to any one drive).
For a discussion on the performance implications of RAID technology, see the Configuring Compaq RAID
Technology for Database Servers white paper.

Chart 5 - Striped Dumps to Disk Arrays

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