> 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.
