HP AutoRAID 12H automating the cache, parameters, time to implementation:, formatting the array, performance

section 1	hp storage white paper
The HP Virtual Array accepts new disks while	automating the cache	1
the array is up and running and accepting I/Os	parameters
as with some higher-end traditional arrays.
However, the HP Virtual Array takes it one step
further. Once the disk is inserted, the array	Configuring a traditional array typically requires
	setting the cache parameters such as the percentage
automatically includes that disk into the existing	of read and write cache, the size of the cache
disk space and stripes all LUNs across that disk.
	pages, and, in some cases, the allocation of cache
This means that even without the creation of any
	to specific LUNs. In making these determinations,
additional LUNs, the array performance will
	there is ample opportunity for error.
improve because of the additional available
spindle. Only the HP Virtual Array automatically	With HP’s Virtual Arrays, all of this is preset and
adds the new disks to existing LUNs. Further,
	automatic. And this means that all the parameters
any newly created LUNs are also automatically
	within the array are tuned to work in unison with
spread across all the disks in the array, including
	the stripe size and the array hardware. First, the
the additional disk.
	cache is set at 80% read and 20% write, is
time to implementation:	shared between controllers, and is treated as a
	“pool.” Second, the cache page size is set at
formatting the array
	64K and is set to automatically destage to disk
As mentioned earlier, after new disks are added	every 4 seconds whether the page is full or not.
	The 64K size minimizes the number of I/Os to
to a traditional array, it then takes several hours	the back-end in sequential environments and
to complete the formatting of the RAID group.	provides a carefully calculated balance within the
During this format phase, no data can be written	array between the number of cache pages and the
to the new LUNs. With some implementations,	speed of the back-end in random environments.
the array is offline until all the LUNs have been
formatted. In other implementations, I/Os can	performance
be written to already formatted LUNs even while
other LUNs are going through the format	Traditional arrays are susceptible to “hot spots” and
process, although performance is very slow.	to changes in the environment that make the initial
Because executing the disk format command	configuration obsolete. The HP Virtual Array virtually
	eliminates these critical performance issues.
uses up so much of the array’s internal bandwidth,
array performance is greatly reduced until all	First, the HP Virtual Array is far less likely to
of the disk formatting has been completed.	experience a hot spot—in other words, it will
With HP’s Virtual Array Technology, the array is	almost never experience a condition where a
immediately available as soon as the LUNs have	few disk drives become a performance bottleneck
been configured. The disk formatting is done as	in the array. Here’s why: the virtual array
the writes are done. In other words, as writes	always (and automatically) stripes all of the
are sent to disk, the formatting is accomplished	LUNs across all of the disks in the RAID group.
for only those blocks being written to. This means	For example: assume a virtual array loaded with
that while there is a small hit to performance for	a total of 60 disks had 30 disks in each of its
that individual write, there is very, very little	two RAID/redundancy groups. Every LUN in
impact on overall array performance.	that group would be spread across all 30 disks.

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