[Info-vax] what is a good value for /CLUSTER_SIZE? (INITIALIZE and qualifiers)

[Phillip Helbig---undress to reply]

In article
<3afe2964-6917-446a-a33c-4bc1571f58a0 at k7g2000yqj.googlegroups.com>, Hein
RMS van den Heuvel <heinvandenheuvel at gmail.com> writes: 

> On Feb 5, 1:02 pm, hel... at astro.multiCLOTHESvax.de (Phillip Helbig---
> undress to reply) wrote:
> 
> > Up until now, I have usually taken the defaults of INITIALIZE, except
> > where something else (e.g. /SYSTEM) was absolutely necessary (and, of
> > course
> 
> The defaults used to be 'odd'. Literally.
> They've become better: Even, multiples of 16.

When was the change?  My disks still have the old value, but they were 
probably originally initialised under 7.1 or something.

With DDS, presumably if I initialise a new, larger disk to add to the 
shadow set, its values of /HEADERS and /CLUSTER_SIZE won't be 
overwritten by the shadow copy.  Or will they?  If they are, then it 
becomes difficult to use DDS and DVE to grow a shadow set to a larger 
size with sensible values for the cluster size.

> All of that playing together you stack the deck just a little to
> improve the odds that things, like fragments, line up nicely.
> No more, no less.
> 
> I specify /HEADERS as much larger than the default; I use 50,000
> > per GB as a rule of thumb).
> 
> So you expect an average file size of about 40 blocks,

Right.

> and your are fine pre-allocating 450,000 headers (1/4 GB) on a 9GB
> drive. Fine by me.

Right; that's not too much, especially when the consequences of having 
it too low are bad.

> >  Since I plan to do some stuff with DDS and DVE soon,
> 
> DVE = Dynamice Volume Expansion. DDS = ?

Dissimilar-device shadowing.

> > I need  to specify /LIMIT.  This will give me a cluster size of 8 unless
> I
> > specify something else.
> 
> So? 8 is fine, but if you don't like it specify your own.
> Personally I like larger and powers of 2 : 32? 128?
> 
> The price of large clusters is wasted space in the last cluster of a
> file.

> The price for small clusters is potentially extra (free space)
> fragmentation and just more 'work' to manage everywhere.
> 
> Pick your poison. 

Aye, there's the rub.

> Don't pick 42 not 17. Pick 32 or 16.

I think I'll go with 16 rather than 8.  That's close to what I have now
(17) and I think wasting a bit (well, more than a bit, actually) more
space is less of a problem than additional overhead, fragmentation etc. 

That's for a 9-GB disk.  For smaller disks which might one day grow to 9 
GB or more, this would also make sense.  (In my case, small disks which 
I expect to grow to 9 GB or larger tend to have fewer, larger files, so 
the waste is less.)  For disks which I expect will grow to 4 GB at most, 
I think I should go with 16 as well.  Ideal would be cluster size the 
same as the file size, and the average file size is not smaller on my 
smaller disks.  Such a big (compared to now) cluster size will mean more 
wastage, but my disks smaller than 9 GB have more free space, so waste 
is not a problem there.  (My 9-GB disks don't have much free space.  I 
hope to get some 17- or 36-GB SBB disks soon so I can use DVE to 
increase the size of these shadow sets.)

Probably only for disks where I expect the average file size to be much 
larger should I choose a cluster size larger than 16, probably 128.

> > Should I stick with the default implied by /LIMIT, i.e. a cluster size
> 
> NEVER accept the default if you worry about it or try to understand
> it.
> Always specify, if only for documentation

Right.

> The real answer? Oracle!  To support their build environment.

Presumably Oracle Classic and not Oracle Rdb?!