[Info-vax] Byte range locking

[David Froble]

John E. Malmberg wrote:

Hi John ...

> On 11/22/2016 10:50 AM, David Froble wrote:
>>
>> Ok, likely I'm missing something, but let me ask, what's so difficult?
>>
>> Get a request for a string of bytes that for example spans disk blocks
>> 2367 to 2714, with the first and last blocks possibly (likely) only
>> partly required.  So what?  Get locks for all the blocks, allocate a
>> buffer, read the range of blocks, possibly make changes, and write the
>> buffer back to disk.  If cache is in use, it should just work.  Yes,
>> that's a lot of locks to successfully acquire.  But no mods to the DLM.
> 
> RMS can only take out locks for existing blocks.

We ain't talking about RMS.

> Many of the fcntl()/flock() requests are for byte ranges that do not exist.

A lock is on a "resource".  The DLM doesn't know anything about files, existing 
or otherwise.

> Also many of the files are stream or an undefined record format, so no 
> record IDs to use.

Not sure why this should matter?

> With out using RMS locks, the application needs SYSLCK to use the DLM.

And I have some UWSS routines, just for that.  One might call them "Wizard Magic".

> Now setting up byte rangs locks has a lot of overhead, so some platorms 
> have implemented Opportunistic locking.

> The simple explanation for them is that the lock request first attempts 
> to do an exclusive lock on the file.  If it can get that, it just tracks 
> the byte-range locks internally, but does not actually take them out.
> 
> If no other application ever tries to lock that file, then a lot of 
> overhead is skipped.

Yeah, I never understood that.  Either there is a lock, or there isn't.  But 
that makes sense.


> When a second application tries to take out a byte-range lock, the first 
> application is notified and it then takes out the byte-range locks and 
> releases the exclusive lock.  This is a lot of overhead as buffers need 
> to be flushed and distributed caches may be involved on served files.

I'd ask, why not just wait until the file lock is released?  Lot less work.

> I do not know if I can use a user written blocking AST with an RMS 
> granted lock.  If I could do that, then it should be possible to write 
> an byte range locking simulation what would work completely in user mode 
> code.

See above for UWSS ....

Let me know if you want some code to look at ....