Thanks so much to
alan_at_nabeth.cxo.dec.com,
golden_at_falcon.invincible.com Dave,
rockwell_at_rch.dec.com Paul E. Rockwell,
aidan_at_cse.unsw.edu.au Aidan Williams, and
j_dubois_at_mtl.digital.ca jean Dubois
for their kind, fast and helpful responses. I do appreciate it.
What I've done is that I did the installation without enabling the write
back cache especially because I have the KZESC controller, i.e. the one
without cache battary backup, and I don't have yet a reliable UPS connected
to the system.
Do you think it's safe to enable it when I have the UPS running with the
right software to shutdown the system when power fails ? I appreciate your
comments.
My original message was :
>I have a problem after setting up the RAID array on 2 3-port KZESC
>with RAID 5 on 2100 4/275 machine, attempt to boot the factory installed O/S
>to further sertup the system caused an error message ( WRITE BACK CACHE NOT
>SUPPORTED ) to appear at the time of the initialization of the two RAID
>controllers. I did'n proceed with the rest of the installation yet.
>
>I put back the RAID setup to WRITE THRU policy. Is there a way to enable the
>WRITE BACK CACHE with DU 3.2C.
>
>NOTE :-
> the FIS was on a disk located in the RAID array, but I have moved it
>to an external BA350, as so it's needed & it's not gonna be part of the
>RAID. So I'm reinstalling DU 3.2C from the CD anyway.
>
>Can you help ! I have to make this installation tomorrow !
================================================================================
The answer I got from alan_at_nabeth.cxo.dec.com :
> Some of versions of the RAID Array 200 family support
> a battery backed up write-back cache. Recent versions
> of Digital UNIX should be willing to trust the cache
> if they can determine that it does have battery backup.
> (I don't know that they are willing though).
>
> If you don't have the battery backup, then it wouldn't
> be wise to use the cache as write-back. A power failure
> will lose data and probably corrupt the parity. That's
> why Digital UNIX doesn't trust the cache and disables it.
================================================================================
The answer I got from Dave :
>The write back cache message is kind of a "left over". When
>the KZESC and KZPSC (PCI) controllers first came out, there
>were no battery backup units available. Running writeback
>cache with no battery opens the door to losing one "cache"
>worth of data in the event of a power failure. That's why the
>message gets printed out if you enable writeback. Please note
>that writeback STILL works, you just have to ignore the message.
>
>The good news is that the batteries are now available so you
>can safely run writeback. I'm using a battery on my PCI card
>now and the message still prints out every time I boot.
>
>Dave
================================================================================
The answer I got from Paul E. Rockwell :
>Write back cache is supported on the KZPSC (the PCI version of the
>KZESC) *only* if it is equipped with the cache battery backup option.
>
>Write back cache is not supported on the KZESC at all.
>
>On both the KZPSC and KZESC, you can enable it without the battery backup,
>but if you do, and you lose power to your system, your RAID-5 sets
>are turned into mush. The contents of the cache are lost, and as the contents
>have not been written back to the disk, you will have to restore the disks
>from your backup.
>
>
>--------
>+---------------------------+tm Paul E. Rockwell
>| | | | | | | | Northeast Region SBU Technical Support
>| d | i | g | i | t | a | l | Digital Equipment Corporation
>| | | | | | | | 500 Enterprise Drive
>+---------------------------+ Rocky Hill, CT 06067
>
>Internet: rockwell_at_rch.dec.com Phone: (860)258-5022
============================================================================
==========
The answer I got from Aidan Williams :
>Not that I know of.
>It isn't supported by Digital -- primarily because the battery that turns
>the RAM on the RAID controller into NVRAM is not there. Consequently,
>a power failure is almost certain to result in a corrupt filesystem.
>Since metadata is the disk data most likely to be in the cache (because
>it is accessed most frequently) you are likely to lose a fair amount.
>
>regards
> aidan
============================================================================
==========
The answer I got from jean Dubois :
>Hi,
>
>If the RAID controler is answering that it cannot support Write Back, it
>probably cannot set the write back enable on the disk. You might try to
>set these manually:
>1) connect your drive to an OSF box that boots
>2) use the following commands for each drive to enable WB:
>scu
>scu> switch /dev/rrz?c
>scu> change page error-recovery
> (skip till you get AWRE and set it to 1)
>scu> set page error-recovery
>scu> change page cache-control
> (skip till you find "write cache enable" and set to 1)
>scu> set page cache-control
>
>
>Be aware that this can cause some data loss and corruption of your
>meta-data on the file system that uses these disks. This is wy DEC ships
>it's disks with this OFF by default (while HP, and others put it on).
>
>Hope this helps,
>/jean
Thanks again for everyone who is willing to help ... Is good to live in such
a world.
Don't you agree with me ..........
*****************************************************************
* Amr G.F. Ahmed ... From Sunny Dubai *
*****************************************************************
Received on Thu Nov 02 1995 - 01:46:36 NZDT