A good understanding of the MARS efficiency issues is essential especially for users that are interested in downloading large amounts of data.

The aim of this page is to help users to improve their MARS requests performance focusing in CMA reforecast MARS requests.

First you need to understand how the S2S data is organised in MARS.

In general it is organised, as a huge tree, with the indentation below, showing different levels down that tree:

• centre (ECMWF, NCEP, JMA, ...)
  • realtime or reforecast
    •  type of data (control forecast or perturbed forecast)
      • type of level (single level or pressure level or potential  temperature)
        • dates (2015-01-01 or 2015-01-05 or 2015-01-08, ...)
          •  time-steps
            • members (for perturbed forecast)
              • levels (for pl or pt)
                • parameters

The idea is to have in the same tape file, all time-steps, all members, all parameters for a type of level, a type, a date

Following the previous paragraph the natural way to group requests would be:
all parameters, all levels, all members, all time-steps for 1 date.

Note the following:

1. 'all' means 'all' that the user wants. It doesn't have to be all parameters.
2. If a user is interested only on z500,  he may request more dates in one go, since the overall request will not be so big.

Taking under consideration what has been explained above,  if you need to loop in several CMA MARS request, the best approach would be the following:

• date
  
  • hindcast date
    • number (EPS only)
      • level
        • parameter (inner loop)

for date in dates-list

     for hindcast in hindcast-dates-list
          your-request (includes the levels, parameters, steps etc)

for each year from 2010 to 2014

    for months April, June
        for each hindcast date
            your-request
        

cf and pf are stored separately. 

For pf is not efficient to do:
        HDATE = 20040101/20040106/20040111/20040116/20040121/20040126/20040201/20040206/20040211/20040216/20040221/20040226/20040301/20040306/20040311/20040316/20040321/20040326/20040401/20040406/20040411/20040416/20040421/20040426/20040501/20040506/20040511/20040516/20040521/20040526/20040601/20040606/20040611/20040616/20040621/20040626/20040701/20040706/20040711/20040716/20040721/20040726/20040801/20040806/20040811/20040816/20040821/20040826/20040901/20040906/20040911/20040916/20040921/20040926/20041001/20041006/20041011/20041016/20041021/20041026/20041101/20041106/20041111/20041116/20041121/20041126/20041201/20041206/20041211/20041216/20041221/20041226,
        NUMBER = 1,

but it is more efficient to group all members in one go for fewer dates:
        HDATE = 20040101/20040106/20040111/20040116/20040121/20040126,
        NUMBER = 1/to/50
This implies less positioning and more contiguous reads.

We could look at increasing the 10GB limit. We have more hardware and are in a better position to handle bigger chunks.


For the 3 allowed streams, I would extract different type/levtype in different streams. For instance:
a) 1 stream: pf/sfc
b) 1 stream: pf/pl
c) 1 stream: cf/sfc and when it finishes,cf/pl
and I would put 2 requests for each stream above. Once a request finishes and starts the downloading, the next request will kick in, and it will find in many cases that the tape volume is still in the tape drive, which will save in avg 2 minutes for a tape mount.