[Info-vax] DIRCACHE hit rate.

[Jan-Erik Söderholm]

Richard B. Gilbert wrote:
> Jan-Erik Söderholm wrote:
>> Hi.
>> I have an 7.3-2 Alpha system where I think that
>> the hit rate on the DIR-cache is way to low.
>>
>> DS20 with 9GB disks in BA356 shelfs.
>>
>> Here is what it typicaly looks like :
>>
>>                        CUR     AVE   MIN      MAX
>>  Dir Data (Hit %)     3.00    3.20  0.00    30.00
>>      (Attempt Rate) 312.00  548.90  0.00  2460.00
>>
>>
>> The ACP_DIRCACHE is at the moment at 4000 (blocks)
>> but I have raised it to 10000 without any measurable
>> improvment.
>>
>> Yes, there are a few DIR's that are "large", but not
>> *that* large, maybe 5-10.000 files and a few DIR files
>> of 1-2000 blocks. Another thing is that all files
>> are timestamped in the filename, so there are
>> normaly only ;1 files.
>>
> 5-10,000 files is an extremely large directory in VMS!!
> 
> Your directories should be a MAXIMUM of 100 blocks in size if you want 
> to get decent performance!

I do not "buy" either of those statements.  A max of 100 blocks
for a DIR file is a bit of a joke.

> 
>> My batch jobs create a number of temp files during
>> processing and I've got a feeling that this is
>> slowing them down a bit.
> 
> If the numbers of temp files are in the 5,000-10,000 range, you can be 
> quite certain that it's slowing them down a bit.

They are in the range of 5-10 files *per batch job*, but for
different reasons I want to keep a few days of files which
represents a couple of 1000's of batch jobs.

>>
>> Now, what I'd like to ask, is if anyone knows if
>> this part of VMS has had any major improvments in
>> the 8.x versions ? We are currently thinking of
>> upgrading anyway, and it would be nice to know
>> if these hit rates would improve simply by upgrading.
>>
> 
> I haven't heard of such.  This has been a known "limitation" in VMS for 
> at least twenty years now.

Well, I *do* know that the directory size and parformance was a major
issue way back, but, if I'm not misstaken, the 128 block limit (where
DIR caching was completly shut of) was raised or removed many years
ago (In 7.2, if I'm not wrong). That's one of the reasons I find the
low hit rate on my systems a bit weird.

Anyway, I have another 8.2 system here where I can run a few
simulations...

Jan-Erik.


> 
> Try redesigning the application in a more "VMS friendly" way.
> 
> Or use <obligatory retching noises> Unix.