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Conference ssag::ask_ssag

Title:	Ask the Storage Architecture Group
Notice:	Check out our web page at http://www-starch.shr.dec.com
Moderator:	SSAG::TERZAN

Created:	Wed Oct 15 1986
Last Modified:	Fri Jun 06 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	6756
Total number of notes:	25276

6660.0. "Shadowing unlike drives, YES or NO?" by OTOOA::LAVIGNE () Tue May 06 1997 13:38

    Can competely different drives be mirrored or shadowed either via
    Software or hardware based  Raid 1 on an Open VMS VAX or Alpha system.      
    The idea is to offer a mirror/shadow set of a Solid state disk to a
    regular spinning disk.  Leaving out the why's and who would want to, I
    need to know if it's possible and supported.  It appears Volume
    shadowing does not support it but I can't find out if controller based
    shadowing supports drives of different geometries.
    
    Regards,
    
    JP Lavigne

T.R	Title	User	Personal Name	Date	Lines
6660.1		COOKIE::FROEHLIN	VMS...riding into the setting sun!	`Tue May 06 1997 13:48`	3
	OpenVMS host based volume shadowing requires identical disks (by geometry). Guenther
6660.2	Yup	SSDEVO::RMCLEAN		`Tue May 06 1997 14:09`	2
	Yes HSx40/50's will do that. The result will be the size of the smallest disk.
6660.3	Maybe I missed something...	XDELTA::HOFFMAN	Steve, OpenVMS Engineering	`Tue May 06 1997 17:00`	2
	DIGITAL solid state disks already have in-built mirroring...
6660.4	I do?	SUBSYS::VIDIOT::PATENAUDE	Ask your boss for ARRAY's...	`Tue May 06 1997 19:08`	14
	Built in "retention" yes, built in mirroring is a stretch ;^) To answer the base question. 1) VMS Shadowing requires EXACT geometery. This also means the controllers involved also have to present the drive the same to VMS. You cannot shadow an RZ28M on a KZPSC to an RZ28M on a KZPAA or HSZ40. But you can shadow a RZ28M on a KZPAA to an RZ28M on a KSPSA or KZPDA. (BTW: RZ28,RZ28B and RZ28M all have the exact same geo) 2) Controller Shadowing does not "always" require the same geometry, they will take the lowest capacity member and apply to rest of members. IE: 2GB+4GB=4GB being used, or 2GB data space available (mirrored) + 2GB wasted.
6660.5	Give your system VD:	EVMS::EVERHART		`Tue May 13 1997 10:51`	10
	One of the vddriver variants on the freeware CD will work with volume shadowing and will invent geometries or let you do so if you want. I believe LDdriver will also do this, in either case permitting parts of drives to be shadowed with host based shadowing. (Some of my other virtual disks also do their own 2 volume shadowing and one at least implements "shadowing" to a global section so reads come from there also.)
6660.6	customer is doing it!!	DV780::THURY	Making it so...	`Tue May 27 1997 10:02`	10
	A customer of mine has successfully done exactly what the base-noter asked. He has mirrored an SSD (EZ56?) with an RZ26 (within an HSJ40). They have since experienced a SSD failure, and the RZ26 alone was able to service the IO requests (although not as fast as as the SSD). Upon repairing/replacing the SSD, everything returned to normal! Bottom line is that controller-based mirroring of mixed disk drives has been demonstrated to work! Denny