🚀 go-pugleaf

In article <31938d64.16703900@news.execpc.com> emjay@execpc.com (Tom Kustner) writes:
>Questions:

>Are RAID 5 controllers slower, the same, or faster than, say, a normal
>Fast Wide SCSI-2 controller?
>Which one does writes faster, RAID 5 or duplexing?

RAID 5 controllers have beaten duplexing/spanning for some time.
Reads have almost always been faster and with caching controllers
the RAID 5 write penalty is removed.

>What is preferrable, the "software" protection of duplexing as done by
>NetWare, or the "hardware" protection as done by RAID 5, or are they
>the same?
>Is RAID 5 harder to manage than duplexing?
>Are there any disadvantages to RAID 5 I haven't listed?

These questions indicate a broader question of data availability.
Can you justify hot-swap (under load) components? Do you need
hot spares. Study the following from NetWire:



RAID Summary

Disk arrays and RAID (Redundant Array of Independent Disks) and associated
technology may be confusing. This document will attempt to summarize. This
is not a tutorial - see reference documents for more detail. SCSI is also 
used as a common example. Implementations may differ.

While it is true that any collection of disk drives may be called a disk array,
it is more common to associate the redundant technology of RAID with a disk array
in order to provide data protection in case of drive failure. This can also
be called disk fault tolerance.

RAID Level Summary (all protect against drive failure except RAID 0)
---------------------------------------------------------------------------
RAID 0: Data Striping - data spread across disks (great performance)
RAID 1: Mirroring - data mirrored between drive pairs.
RAID 3: Dedicated Parity Drive - best for large (>64K), sequential I/O
RAID 4: Dedicated Parity Drive - parity drive is write bottleneck
RAID 5: Parity Spread - best for concurrent, smaller requests
RAID 10 - striping across (on top of) mirrored pairs (1 + 0)
RAID 50 - striping across (on top of) RAID 5 units (5 + 0)
Parity is used to determine missing data during drive failure & drive rebuild
RAID 3&5 is cost effective over RAID 1 above approx. 20GB
(other nomenclature and features are vendor specific)

Additional Technology (often associated with RAID)
--------------------------------------------------------------------------
Hot Plug - Bus quiet but power still on while drive replaced (aka Warm Swap)
Hot Swap - Active bus while drive replaced
Hot Spare - Standby automatically used as replacement drive
Hot Expansion - Ability to add & format drives while system under load
Redundant Fans
Redundant Power Supplies
Redundant Controllers
Redundant Power Cords
Caching Controllers - Write-back or write-through (opt. battery b/u)
SCSI - Fast, Wide, Tagged Command Queuing
Fault Notification - Alerts via SNMP, audible alarms, beepers, etc

Three Levels of RAID Implementation
--------------------------------------------
I. Software
Advantages:    Low cost (may be included in OS)
       High performance, especially when combined w/striping (spanning)
       Highest performance when mirroring across adapters (duplexing)
Disadvantages: Usually no "hot" technology
       Usually no other redundant components
       CPU intensive, especially when computing parity
       May not protect all storage (such as DOS boot)
       Mirroring may use many controllers (slots) when using 4 drives@ max.

II. Host Bus Controller (single or dual channel)
Advantages: Integrated in cabinet
       Computing parity does not impact host CPU
       Rebuild rate may be user definable
       RAID 5 performance is superior when R-W ratio >70%
       Write performance may be enhanced by caching (esp. sequential)
       Different drive groups may be different RAID levels
Disadvantages: Usually no other redundant components
       Unable to add drives (expansion)
       Each board uses a slot & can only have disk drives
       Unique drivers for each board on each OS
       Host bus dependent (EISA, MCA, PCI, etc)

III. Subsystem Controller
Advantages: Computing parity does not impact host CPU
       Rebuild rate may be user definable
       RAID 5 performance exceeds software when R-W ratio >70%
       Write performance may be enhanced by caching (esp. sequential)
       Different drive groups may be different RAID levels
       Highest performance when using RAID 5 + 0
       Multiple subsystems can use one host adapter (1 slot)
       Closest to meeting needs for continuous data availability
       Can easily be connected to standby server
       Standard host adapters may be used
       Other devices (tape, cd-rom) can share bus
       Independent of host bus (EISA, MCA, PCI, etc)
       Maximum subsystem capacity of (5 SCSI chan.@ 7x4GB) 130GB
       Maximum host adapter capacity of (7 SCSI ID@130GB) 910GB
       Maximum host capacity of (4 HA @910GB) 3640GB
Disadvantages: Higher cost can be justified
       Distance may require differential SCSI (popular in computer rooms)

Cost Justification
-------------------------------
Network downtime costs avg. $78,000/hr. in medium-large networks (Strategic Research)
Computer downtime costs avg. $78,000/hr. and avg. 4 hours (COMPUTERWORLD 8/10/92)
Productivity losses due to storage related failures: $420,000 (St. Res.)
Storage accounts for 55% of server failures (St. Res.)
Power accounts for 28% of server failures (St. Res.)
Fans account for 8% of server failues (St. Res.)
Server consolidation can reduce costs by 44% over 3 years (Gartner Group)
Fully featured RAID can raise productivity 1.25% (Interpose)
70% of IS executives say client/server storage should be centralized (EMC)
72% of client/server adopters say their storage is growing rapidly (EMC)

More Info
-------------
The RAID Advisory Board, 13 Marie Lane, St. Peter, MN 56082
The RAIDbook                                     $29.95
The RAIDprimer                                    $4.95
RAID Enclosure Primer                             $9.95
RAIDtutorial                                    $250.00
RAID: Basics and Futures                         $95.00
Disk Array Functional Test Specification        $490.00
Disk Array Performance Test Specification       $290.00
Host Interface Tutorial                         $350.00
RAID Ready Drive Profile                         $75.00
Storage Analysis: Optical Rewritable            $495.00
Storage Analysis: Disk Storage/2000             $395.00
Storage Analysis: Removable Disk Storage        $995.00
Storage Analysis: Form Factor Evolution	        $295.00
Phone: (507) 931-0967
Fax: (507) 931-0976
E-mail: MCI MAIL ID 470-6032


Courtesy Symbios Logic, 3718 N. Rock Road, Wichita, KS 67220 800-440-5606 
(06/21/95)