From: weikl@ee5sy1.ilt.fhg.de (Dr.B.Weikl)
Newsgroups: comp.os.vms
Subject: Re: Striping and Shadowing
Date: 23 Jan 1993 11:37:35 GMT
Organization: ilt-fhg
Distribution: world
Reply-To: weikl@ilt.fhg.de
NNTP-Posting-Host: 192.44.11.11


>	We are considering buying DEC's disk striping and/or shadowing
>	software and would be interested in other people's experiences	
>	with these products.   Our VMS environment will probably be
>	a VAX 4000/100 with 4 (or possibly 5) SCSI disks.

>	1) Disk Striping:
>		- how effective in improving disk I/O ?
>		- how reliable is it ?
>		- any disadvantages ?

Sorry, I have no experience with striping.

>	2) Volume Shadowing:
>		- how effective in improving disk I/O ?

You'll find and improvment of disk I/O only if you have mainly reads. If you have 1/3 write and 2/3 read operations (common mix of a userdisk) you'll see the same I/O performance as with an unshadowed disk.

The main advantage of shadowing is increasing the reliability.

>		- how reliable is it ?

No failures of shadowing in 1 year with 4 shadow pairs of 1.3 GB drives.

>		- How easy is it to install and use ?

Fairly simple with normal disks a bit tricky with system disks.

>		- can you have more than one shadow volume ?

I do not understand what you mean with this question. You can build volume sets which are shadowed (no problem, we have them). A shadow set can contain one to three members (one is not very useful though).




>	3) Mixture of 1 & 2:
>	  
>	   If both Disk Striping and Volume Shadowing are installed,
>	   e.g. a shadow set of 2 disks is striped across 2 other disks,
>		- how easy is this to set up ?
>		- would we get improved I/O from shadowed reads *plus*
>		  improved I/O from striped reads ? 
	
Sorry, can't comment on that.

>	Will either of these products work with non-DEC drives ?

We've done it with Emulex MD4160 drives (1.3 GB, RF71 compatible). Over all they perform quite good. We had some trouble to use them as system disk because of a wrong EPROM (not related to shadowing - solved now). Twice we had bad errors: the disks went off line while running which caused the shadow sets to hang (no I/O possible). Switching power off/on on the BA400 storage expansion resolved this. The cause was an interface error on the MD4160s. 

One word according to merging: with VMS 5.5-2 came a new version of disk shadowing. Some HSC and RF73 disks now support mini merge. With mini merge
only those blocks of a shadow set are merged which were accessed recently (doesn't work for system disks). 
DEC says a mini merge is finished within one minute, which is a big improvement compared to standard merge. This costs hours. And contrary to what DEC says you see a performance degradation with merging. We do not have RF73 compatile drives, so I could not test mini merge. If you buy new disks you'd be probably better off with RF73 compatible drives.

>	Thanks.

You're welcome.

>						== Geoff Hill
>		- How easy is it to install and use ?

What is easy in life?

Bruno

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