[Info-vax] Empty blocks in FILES-11 directory files.

[Johnny Billquist]

On 2022-09-24 19:42, abrsvc wrote:
> On Saturday, September 24, 2022 at 1:31:31 PM UTC-4, Mark Daniel wrote:
>> It seems as if directory blocks containing zero file entry records, and
>> subrecords, tend to accumulate in active directories. Can only assume
>> these arise when multi-multi-multiversion consecutive file names are
>> deleted from the directory. Recently encountered 32 consecutive empty
>> blocks at which my code sanity checked.
>>
>> Quick solution; create an equivalent directory and copy from the
>> original to the new. Problem Solvered. Assume a backup-restore would
>> accomplish similar, etc.
>>
>> Questions:
>>
>> Is this expected directory file behaviour?
>>
>> Do these empty blocks continue to accumulate, only to be reused should
>> in-order file names be created?
>>
>> Do extensive empty directory blocks represent tangible overhead to the
>> (persumably) XQP? (My code sanity checked at 32 but who knows exactly
>> how many there really were.)
>>
>> Are their tools to measure such directory file efficiency (shall we say)
>> and to "compress" such files (apart from backup-restore).
>>
>> TIA, Mark.
>>
>> PS. I do recall descriptions of the FILES-11 directory internals being
>> very simple-minded and inefficient.
>>
>> PPS. My entire technical hard-copy collection, including such VMS tomes
>> as McCoy's File System Internals, in a fit of pique, went into a recycle
>> bin some years ago. Moral of the story; don't let line-management get
>> under your skin.
>>
>> -- 
>> Anyone, who using social-media, forms an opinion regarding anything
>> other than the relative cuteness of this or that puppy-dog, needs
>> seriously to examine their critical thinking.
> 
> It is my understanding that empty blocks will cause a shuffle of the entries that follow filling the block*.  Empty blocks should only exist at the end of the directory file.  Since directories are/must be contiguous, these trailing blocks will be used as space is required.  I don't believe that the extra allocated blocks is a performance hit.
> 
> * This is why deleting large numbers of files in reverse order is so much faster as all that shuffling is avoided.

What? Does directory files really need to be contiguous? I would be very 
surprised by that. But directories in VMS are always sorted. I can see 
that a delete might leave an empty space, but chances are high that when 
you enter something new, it would need resorting and shuffling the 
directory, so I wouldn't expect you to constantly see large empty spaces 
in a directory under VMS.
Or ODS-2 I should say. Under ODS-1, you can have large empty chunks in a 
directory, since it does not keep files sorted in the first place. ODS-1 
also definitely do not require directories to be contiguous.

I would suspect ODS-5 to be rather similar to ODS-2 here, but don't know 
for sure.

   Johnny