04-26-2011
Well, it is a complex world, with security and speed in opposition. One oddity of expanding sidk sizes is that one new big disk may be overwhelmed with the level of I/O that used to be handled by 8 disks, so size attracting query and churn is a negative! Striping allows the bandwidth of many drives to be applied to the combined storage, with supports more buffering with faster buffer fills, if things are sequential often enough. If everything was sequential and failure was no worry, you could stipe all together for max bandwidth, but you might do better with 2 or more virtual volumes so copying, database joining and such can be sequential on each virtual device. So, there are sometimes ways to force smart parallelism, the ability to join huge sets without seeks. However, RAM and 64 bit VM have made buffering so ample that it may dilute that sort of approach. RAID has not entirely freed us from failure worry, since with all the layers of software and hardware and vendors, it seems RAID errors often never get heard until they are 2 devices down. Rebuild time is not inconsequential, either. So, your approach should go beyond hot spots to maximizing the bandwidth of a managable number of virtual volumes. Along the way, look at the pathways and how they figure into the redundancy and striping. If a controller handles both sides of a mirror, and goes wonky . . . . If striping runs across all controllers, scsi cables, then any controller or cable bottleneck is diluted. Intellegent use of simple mirror for high churn and RAID-N for low churn is nice, too! Sometimes, this discussion can be extended down into the app, as DB2 append tables with insert never update or delete are churn free except at the end. Disk is cheap and 100% history is wise. Churn-free data might even migrate to some hierarchical read-only store like DVD arrays. Assuming control of chaos is someone else's job can be a luxury.
But, yeah, it seems like it is still good, but might not be sufficient, and an approach with sufficiency might not make it necessary.
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re(7) Miscellaneous Information Manual re(7)
NAME
re - SWXCR RAID interface
SYNOPSIS
2100 Server Model A500MP DEC SWXCR
controller xcrn at * vector xcintr
device disk renn at xcrn drive nn
DESCRIPTION
The re driver is for the SWXCR RAID Array controller.
The following rules are used to determine the major and minor numbers that are associated with an re type disk. There are two major num-
bers used to represent re disks. The major numbers are 11 for block devices and 44 for character (raw) devices.
The minor number is used to represent both the unit number and partition. A disk partition refers to a designated portion of the physical
disk. To accomplish this reference, the 20-bit minor number is divided into three parts. The lowest six bits of the minor number specify
a disk partition. The partitions use a letter, a through h, for their name.
The next three bits of the minor number specify the RE unit number for a unit attached to an SWXCR controller. The final 11 bits specify
the controller number.
The device special file names associated with re disks are based on conventions that are closely associated with the minor number assigned
to the disk. The standard device names begin with re for block special files and rre for character (raw) special files. Following the re
is the unit number and then a letter, a through h, that represents the partition. Throughout this reference page, the question mark (?)
character represents the unit number in the name of the device special file. For example, re?b could represent re0b, re1b, and so on.
The unit number can be calculated if the major and minor numbers of an re disk are provided. For example, suppose you have a device spe-
cial file rre6a, with a major number of 44 and a minor number of 384. The partition is represented by the lower six bits of the number
384. These lower six bits of the number 384 are 0, which specifies the a partition. The next three bits of the minor number 384 specify
the unit number, which is 6. The next eleven bits specify the controller number, which is zero. Putting these three pieces together
reveals that the major/minor number pair 44/384 refers to the a partition of unit 6 attached to controller number 0.
A disk can be accessed through either the block special file or the character special file. The block special file accesses the disk using
the file system's normal buffering mechanism. Reads and writes to the block special file can specify any size. This capability avoids the
need to limit data transfers to the size of physical disk records and to calculate offsets within disk records. The file system can break
up large read and write requests into smaller fixed size transfers to the disk.
The character special file provides a raw interface that allows for direct transmission between the disk and the user's read or write buf-
fer. A single read or write to the raw interface results in exactly one I/O operation. Consequently, raw I/O may be considerably more
efficient for large transfers.
For systems with RE disks, the first software boot after the system is powered on may take longer than expected. This delay is normal and
is caused by the software spinning up the RE disks.
Disk Support
The RE driver handles all disk drives that can be connected to the SWXCR controller. To determine which drives are supported for specific
CPU types and hardware configurations, see the Installation and Configuration Guide for the StorageWorks RAID Array 200 Subsystem Family.
SWXCR RAID Controllers are viewed in all cases as RE type disks. There are some notable differences that should be taken into considera-
tion when configuring a RAID device: Currently only sector sizes of 512 bytes are supported. Logical Volume sizes are not fixed sizes as
compared to other disk devices. The size of the Logical Volume is configurable based on needs. The dynamic nature of Logical Volume sizes
is dealt with by defining RAID devices as DYNAMIC. Only partitions a, b, c, and g are defined. If necessary, the disklabel(8) command can
be run to change and define partitions for RAID devices.
Usually, the re?a partition is used for the root file system and the re?b partition as a paging area. The re?c partition can be used for
disk-to-disk copying because it maps the entire disk.
The starting location and length (in 512 byte sectors) of the disk partitions of each drive are shown in the following table. Partition
sizes can be changed by using the disklabel(8) command.
SWXCR (RAID) partitions for systems
based on the Alpha AXP architecture
disk start length
re?a 0 131072 re?b 131072 262144 re?c 0 end of media re?d 0 0 re?e 0 0 re?f 0 0
re?g 393216 end of media re?h 0 0
FILES
/dev/re??? /dev/rre??? /etc/disktab
RELATED INFORMATION
disklabel(8), MAKEDEV(8), uerf(8) delim off
re(7)