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Power Tools 1993 November - Disc 2
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Power Tools Plus (Disc 2 of 2)(November 1993)(HP).iso
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wsdamtbf.txt
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1993-07-23
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Analysis of Workstation Disk Array High Availability Modes
Steve Watt / Network Server Division
In order to increase capacity and enhance performance in
today's workstation environment, a disk storage system
requires multiple disk mechanisms. However, when
configuring numerous disk mechanisms into a storage system,
the combined Mean Time Between Failure (MTBF) decreases
making users more vulnerable to downtime and data loss due
to a disk mechanism failure.
To address this concern in critical environments, disk
arrays offer protection from disk mechanism failures with
several high availability modes. With each of the high
availability modes, if one of the disk mechanisms fail, user
data remains accessible and the array can reconstruct
missing information upon replacement of the failed
mechanism.
HP WORKSTATION DISK ARRAY HIGH AVAILABILITY MODES:
RAID 0/1, Mirroring
-------------------
RAID 0/1 offers data protection through disk mirroring in
which a disk array with six mechanisms uses three mechanisms
for data storage and three for mirrored data. This
operating mode provides the highest level of data protection
because each disk has a mirrored copy.
RAID 3, Byte Striping with Parity
---------------------------------
RAID 3 configurations utilize five disk mechanisms. User
data is striped on a byte basis across four of the disks and
the fifth disk is used to store parity information. The
disk array creates the parity on the fifth disk by using an
"exclusive-or" encoding scheme.
RAID 5, Block Striping with Parity
----------------------------------
RAID 5 also utilizes five disk mechanisms and the
"exclusive-or" parity scheme. However, with RAID 5 parity
information is spread across all five disks. As with
RAID 3, RAID 5 makes 80% of the total storage capacity
available for user data.
High Availability modes enable customers to connect large
capacitites (up to 228 Gbytes) to their S/700 workstation or
server while reducing the risk of downtime and data loss due
to a disk mechanism failure. In fact, when configured in a
high availability mode, HP workstation disk arrays provide
FOUR to FIVE TIMES the protection from downtime, and even
greater protection from data loss, when compared to an equal
number of independent HP disk drives.
MTBF / MTBDNA CALCULATIONS:
Component MTBF (hrs)
--------- ----------
5.25" Mechanisms 150,000 per mechanism
3.5" Mechanisms 150,000 per mechanism
Power Supply 1,000,000
Fan 300,000
Controller 300,000
Misc. (Cable, chasis, etc.) 5,000,000
The following calculations were used to determine the
magnitude of protection from downtime. Since Disk Arrays in
a high availability mode provide access to data despite a
disk mechansism failure, the measure of Mean Time Between
Data Not Available (MTBDNA) is more relevant to users than
Mean Time Between Failures (MTBF). When a disk array is not
in a high availability mode, MTBDNA for the array equals
it's MTBF.
The MTBDNA calculations assume a 24 hour restoration time
(replacement and rebuild of data) for a failed disk
mechanism.
Model 420SA (3.5" disk mechanisms, Qty 5; 1 cabinet)
-----------------------------------------------------
MTBF = 1 / [5/150,000 + 1/1,000,000 + 1/300,000 +
1/300,000 + 1/5,000,000]
MTBF = 24,271 hrs
------
RAID 3 or 5:
MTBDNA = 1 / [(4*5*24/150,000*150,000) + 1/1,000,000 +
1/300,000 + 1/300,000 + 1/5,000,000]
MTBDNA = 126,775 hrs
-----------
Model 420SA (3.5" disk mechanisms, Qty 6; 1 cabinet)
-----------------------------------------------------
MTBF = 1 / [6/150,000 + 1/1,000,000 + 1/300,000 +
1/300,000 + 1/5,000,000]
MTBF = 20,891 hrs
----------
RAID 0/1:
MTBDNA = 1 / [(3*2*24/150,000*150,000) + 1/1,000,000 +
1/300,000 + 1/300,000 + 1/5,000,000]
MTBDNA = 127,015 hrs
-----------
Model 1350SA (5.25" disk mechanisms, Qty 5; 2 cabinets)
--------------------------------------------------------
MTBF = 1 / [5/150,000 + 2/1,000,000 + 2/300,000 +
1/300,000 + 1/5,000,000]
MTBF = 21,962 hrs
----------
RAID 3 or 5:
MTBDNA = 1 / [(4*5*24/150,000*150,000) + 2/1,000,000 +
2/300,000 + 1/300,000 + 1/5,000,000]
MTBDNA = 81,824 hrs
----------
Model 1350SA (5.25" disk mechanisms, Qty 6; 2 cabinets)
-------------------------------------------------------
MTBF = 1 / [6/150,000 + 2/1,000,000 + 2/300,000 +
1/300,000 + 1/5,000,000]
MTBF = 19,157 hrs
----------
RAID 0/1:
MTBDNA = 1 / [(3*2*24/150,000*150,000) + 2/1,000,000 +
2/300,000 + 1/300,000 + 1/5,000,000]
MTBDNA = 81,924 hrs
----------
The calculations show that when in a high availability mode,
chances of downtime due to a disk mechanisms failure are
nearly eliminated. Downtime becomes a function of the
failure rates of the power supply, fan, controller and
miscellaneous components. Since disk mechanisms failures
typically account for 75-80% of all disk storage system
failures for configurations this size, the high availability
modes increase protection from downtime due to disk failure
by FOUR to FIVE times!!
PROTECTION FROM DATA LOSS:
In addition to increasing the availability of data, disk
arrays also provide protection from data loss caused by a
failure. It is dificult to estimate the magnitude of the
protection from data loss provided by an array, however,
since the most probable type of failure to cause data loss
is a disk mechanism failure, it follows that a high
availability mode will offer at least FOUR to FIVE times the
protection from data loss. If a system has the ability to
identify and avoid data loss in the event of a power supply,
fan or controller failure, the increased protection from
data loss with a disk array in a high availability mode will
be even greater.
SUMMARY OF WORKSTATION DISK ARRAY HIGH AVAILABILITY MODES
RAID DATA STORAGE TOTAL STORAGE MTBF MTBDNA
LEVEL MODEL (GB) (GB) (hrs) (hrs)
------ ----- ------- ------- ------- ------
0/1 1350SA* 4.05 8.10 19,157 81,924
0/1 420SA* 1.27 2.53 20,891 127,015
3 or 5 1350SA 5.40 6.75 21,962 81,824
3 or 5 420SA 1.69 2.11 24,271 126,775
* - customer must purchase a 6th disk mechanism for RAID 0/1