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| PC Server 500 - Using the advanced
functions |
| Service Hints &
Tips |
Using the
Advanced Functions
This section gives the procedures for
using the advanced functions, such as changing the write
policy, changing the RAID parameters, and formatting a
drive.
Warnings appear throughout this section to alert
you to potential loss of data and should be heeded before
answering yes to the confirmations requested by the RAID
configuration program.
Changing the Write
Policy
When you
configure a logical drive, the RAID adapter automatically sets
the write policy to write-through (WT) mode, where the
completion status is sent after the data is written to the
hard disk drive. To improve performance, you can change this
write policy to write-back (WB) mode, where the completion
status is sent after the data is copied to cache memory, but
before the data is actually written to the storage
device.
Although you gain performance with write-back
mode, it creates a greater risk of losing data due to a power
failure. This is because the system gets a completion status
message when the data reaches cache memory, but before data is
actually written to the storage device.
To change the write
policy:
1. Start
the RAID configuration program. See 'Starting the RAID
Configuration Program´ on page 17.
2. Select Advanced
functions from the Main Menu.
3. Select Change write policy
from the Advanced Functions menu. The cursor will be active in
the Logical Drive list.
4. Select the logical drive whose
write policy you want to change. The Advanced Functions screen
appears.
Note
Your information might be
different from that shown in this screen.
The Logical
Drive list shows you the logical drive ID, the size in
megabytes of each logical drive, the RAID level you assigned
to that logical drive, and the date you created it. The status
of the logical drive is also shown. Good means that all is
well with the drive; Critical means that you must replace the
hard disk drive and rebuild the logical drive. (You will have
received a message telling you what has happened to the
drive.) Offline means that the logical drive is unrecoverable;
the data in that drive is lost.
5. Locate the Wrt pol
(Write Policy) field in the Logical Drive list. The write
policy is shown as either WT (write-through, which is the
default setting) or WB (write-back).
6. Use the Up Arrow
(« ) key or the Down Arrow (» ) key to select the logical
drive whose write policy you want to
change.
Warning
If you change the write
policy to write-back, wait at least 10 seconds after your last
operation before you power-off the server. It takes that long
for the system to move the data from the cache memory to the
storage device. Failure to follow this practice can result in
lost data.
7. Press Enter to change the write policy.
Notice that WT changes to WB. You can press Enter to alternate
between WT and WB.
8. When you have made your choice,
press Esc to return to the Advanced Functions menu.
9.
Select Exit. The Confirm pop-up window appears asking you to
confirm your action.
10. To return the setting to its
original state, select No. To save your changes, select
Yes.
11. Back up the disk-array configuration information
to diskette. Refer to 'Backing Up Your Disk-Array
Configuration´ for more information.
Formatting
Drives
You can
perform a low-level format on drives with RDY (ready), OFL
(offline), or UNF (unformatted)
status.
Note
The Format drive choice on the
Advanced Functions menu provides a low-level format. If you
install a new hard disk drive that requires a standard format,
use the Format command provided by your operating
system.
The Format program works like the low-level
format program in the advanced diagnostics portion of the
system programs. It is provided in the IBM RAID configuration
program so that you can perform a low-level format on a drive
controlled by the RAID adapter.
To perform a low-level
format:
1. Start
the RAID configuration program. See 'Starting the RAID
Configuration Program´.
2. Select Advanced functions from
the Main Menu.
Warning
A low-level format
erases all data and programs from the drive.
3. Select
Format drive. The low-level format program starts.
4.
Follow the instructions on the screen.
You can perform
a low-level format on more than one drive at a
time.
Changing the RAID Parameters
You can change the RAID parameters
using the advanced functions by selecting Change RAID
parameters.
The default settings are: Stripe unit size
-- 8K
The stripe unit size is the amount of data
written on a given disk before writing on the next disk. To
maximize the overall performance, this stripe unit should be
chosen such that the stripe-unit size i close to the size of
the system I/O request. The default is set to 8K data
bytes.
Warning
Once the stripe unit is chosen
and data is stored in the logical drives, the stripe unit
cannot be changed without destroying data in the logical
drives.
Rebuild priority -- Equal.
Rebuild
priority can be set to equal, high, or low. When set to equal,
the rebuild I/O request and system I/O request get equal
priority in the execution order.
When set to high, the
rebuild I/O request will get a higher priority than a system
I/O request. In a heavily loaded system (with a high rate of
system I/O requests), the high-priority rebuild can
significantly reduce the disk rebuild time at the expense of
degraded handling of I/O requests.
When the rebuild
priority is set to low, the rebuild I/O requests can execute
only if there is no pending system I/O requests. In a moderate
to heavily loaded system, low rebuild priority will increase
the disk rebuild time significantly and provide better system
performance.
Note
Rebuild priority can be
changed without affecting data in the logical
drives.
Parity placement --
RA.
Warning
Once a parity placement scheme is
chosen and data stored, it cannot be changed without
destroying data.
Parity placement defines how parity is
placed in the disk array with respect to data. The following
illustration shows both the Left Symmetric (LS) and Right
Asymmetric (RA) parity placement in a four-drive disk array.
Here AAA, BBB, and CCC are the data stripe units, and PP0 is
the corresponding parity. Similarly DDD, EEE, and FFF are the
data stripe units, and PP1 is the corresponding
parity.
Right Asymmetric (RA) Left Symmetric (LS)
Disk Disk Disk Disk Disk Disk Disk Disk
1 2 3 4 1 2 3 4
PP0 AAA BBB CCC AAA BBB CCC PP0
DDD PP1 EEE FFF EEE FFF PP1 DDD
GGG HHH PP2 III III PP2 GGG HHH
JJJ KKK LLL PP3 PP3 JJJ KKK LLL
In some situations you may
want to try LS parity placement to improve performance. The
default parity placement is RA. Read ahead -- On. Normally the
IBM SCSI-2 Fast/Wide PCI-Bus RAID Adapter transfers data from
disk to its local cache in steps of stripe-unit size. This
provides excellent overall performance when workloads tend to
be sequential. However, if the workload is random and system
I/O requests are smaller than stripe-unit size, reading ahead
to the end of the stripe unit will result in a wasted SCSI bus
bandwidth and wasted disk utilization. When read-ahead is set
to Off, the size of data transfer from the disk to local cache
is equal to the system I/O request size, and no read-ahead to
the end of the stripe unit is
performed.
Notes
1. The Read-ahead setting
can be changed without destroying data in a logical
drive.
2. When the configuration is saved on a diskette,
the RAID parameters are saved also. |
| Document
ID: DDSE-3UXNC8 |
| Last
Modified: 1999-06-09 |
| Copyright © 2000
IBM Corporation | | |
