Contiguous Disk Space

[Sk Noorul Hassan]

To check Contiguous disk Space, I used the following command,

MCR SYSGEN CREATE DRV$DAT:[0,0]TEST.LIS/SIZE=50000/CONTIGUOUS and I deleted the file after it is success.

My Question is : After I delete the file, Is the size of contiguous disk blocks will remain same or will it be fragmented.

[Karl Rohwedder]

Just after deletion, the space will be contigous, but if the disk is in use, it will get fragmented.

To check for the fragmentation of a disk, you can use the free DFU tool (from the freeware CD or the OpenVMS freeware pages). Besides the
disk report DFU has a lot of usefull functions and is a must for system adminstrators.

regards Kalle

[Hein van den Heuvel]

Right, sysgen is a good way to force that.

A slightly easier alternative is:

$COPY/CONT/ALLO=50000 NL: ...

Both message have a subtle advantage over the DFU READONLY report.
If the space is not there, then those commands will trigger all the nodes in the cluster to release their pre-allocated, freeblock cache. This will potentially make contiguous space which was not visible.

Hmm... After you delete the file its blocks will probably end up on the LRU list for free block and will thuse be most likely to be used to subsequent allocations.
So you'd better use that contig space soon after determining it is there.

But why do you care? What problem are you trying to solve? There is very little need for purely contiguous files (only directory files). Contiguous best try often works well enough no? What application usage does this have? Or is is just a desire to optimize whether it is needed or not (which I would perfectly understand... I even optimize placing my trashbags in the thrash container :-).

Hein.

[Jan van den Ende]

Sk,

to supplement Hein (where did I get the guts to pretend I can ?)

VMS is MUCH less sensitive to disk and file fragmentation than other OSses.
( There are those that claim defragging is counterproductive: the costs are higher than the gains.)
You can easily optimise your system to care even less:
set SYSGEN ACP_WINDOW to 255 (requires reboot), and ANY fragmented file headers are always read-in completely. (subject to caching and cache aging of course) The most severe impact of fragmented files ("window turns") is now eliminated at the cost of some more file header info kept in memory.
Buy IO performance with some kB of memory!

Hein will probably have to add some notes to this...


hth

Proost.

Have one on me.

jpe

[Robert Gezelter]

Sk Noorul Hassan,

I agree with Hein and Jan.

Huge windows (at times referred to as Cathedral Windows) eliminates the main performance penalty associated with fragmentation, namely window turns.

Allocating a contiguous file as described has the results that have been detailed.

Perhaps the more important question is "Why do you need to check contiguous space?" There are some reasons why this could be needed, but they tend to be rare. OpenVMS, as has been noted, has relatively few situations where contiguous space is an issue (e.g., directory files, and extensions of the Index File). In both cases, pre allocation of sufficient sizes is easily done and certainly appropriate.

I hope that the above is helpful.

- Bob Gezelter, http://www.rlgsc.com

[Wim Van den Wyngaert]

"Who needs contig space ?"

When you install patches you might need it. Certain exe files are contiguous (e.g. in sys$loadable_images:PSDCTIMER.exe). Why ?

And of course, you never know what applications are exactly doing/asking.

In any case, I check every hour if contiguous space is present on almost every disk. Just to be sure. And because freespace counters sometimes lie.

Wim

[Sk Noorul Hassan]

Hi all,

Finally we loaded Unicenter tool to monitor the contiguous space in disks. Thanks for suggestions.