Re: What is the Difference between Shadow and Mirrored disk?
From: Hans Vlems (hvlems.dotweg_at_zonnet.nl)
Date: 02/28/05
- Next message: kenneth.randell_at_verizon.net: "Re: Point Secure"
- Previous message: john_20_28_2000_at_yahoo.com: "What is the Difference between Shadow and Mirrored disk?"
- In reply to: john_20_28_2000_at_yahoo.com: "What is the Difference between Shadow and Mirrored disk?"
- Next in thread: JF Mezei: "Re: What is the Difference between Shadow and Mirrored disk?"
- Reply: JF Mezei: "Re: What is the Difference between Shadow and Mirrored disk?"
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Date: Mon, 28 Feb 2005 17:46:01 +0100
<john_20_28_2000@yahoo.com> schreef in bericht
news:1109608065.016739.142670@g14g2000cwa.googlegroups.com...
> Is a Shadow disk in OpenVMS the same as a Mirrored disk in RAID for
> Windows or other OS? And does anyone know why a Quorum disk cannot be
> shadowed. Thanks.
>
Err, yes and no. I can almost see you looking at the ceiling thinking "wise
ass":-) but there's a reason for at least a little confusion.
A VMS shadow set is a software based solution. The OS recognizes the disk
devices and bundles them into a shadow set. There is a decicated device
driver that does all the work. Once the shadow set device is created, the
users and system administrators access the shadow set through that driver.
Example: under VMS a SCSI disk uses the DKdriver and the shadow set is
implemented by the DSdriver. Consequently a shadow set called, say, DSA3:
may consist of one, two or three harddisks, say, $2$DKA0:, $2$DKB100: and
$2$DKB300:
This technique is called host based shadowing.
Now there's also a thing called controller based shadowing. A RAID
controller drives the harddisks and the RAID sets are defined thru firmware
tools built into that controller. The main difference here is that the OS no
longer controls the actual disk hardware, no longer knows whether a RAID
volume is RAID-1 or RAID-5, let alone how many disks the RAID volume
contains. VMS tends to know that controller based RAID is used, these disks
are under control by DRdriver (and are called DRAn: accordingly). Other OSes
might not even know. Windows 2000 Server just recognizes a RAID-1 set as C:
and has no clue about the mirroring.
In terms of recovery and speed there's not much difference between the two,
other than that host base shadowing uses main processor cycles and RAID
controllers use there own processor abilities. But the difference lies in
system management opportunities. With VMS you are able to dismount one
member from a shadow set, and back it up or mount it on an entirely
different system. Each member of the set is identical. That is not
necessarily the case with RAID controllers, nor can you be sure that a
RAID-1 member can be used on a normal standalone system with a regular SCSI
controller.
Complicated? Not at all, just wait until we start discussing HSC50's and HSC
based shadowing....
Under VMS a shadow set (DSAn:) device cannot be used as a quorum disk. IIRC
a RAID set member (DRAn:) can be used.
Hans
- Next message: kenneth.randell_at_verizon.net: "Re: Point Secure"
- Previous message: john_20_28_2000_at_yahoo.com: "What is the Difference between Shadow and Mirrored disk?"
- In reply to: john_20_28_2000_at_yahoo.com: "What is the Difference between Shadow and Mirrored disk?"
- Next in thread: JF Mezei: "Re: What is the Difference between Shadow and Mirrored disk?"
- Reply: JF Mezei: "Re: What is the Difference between Shadow and Mirrored disk?"
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Relevant Pages
|
