The amount of data which needs to be written to tape can be estimated on the basis of the projected interaction and trigger reduction rates. Table 3.18 summarises the data rates at the different stages of trigger and data acquisition. The tape writing has to cope with a data rate of 30 MB/s which leads to a total amount of 200 TB for one year of running.
Table 3.18: COMPASS Data Rates
To minimise the amount of tape handling and I/O overhead the data recording will be done on high density/speed tape media. A good candidate as tape device is the SONY 19 mm ID-1 tape with a projected performance of 64 MB/s I/O speed and medium capacity of about 200 GB or higher. Experience with similar devices was gained already at NA49. With this type of tape medium the data of one year running will fit on about 1000 cartridges. Tape media with better capacity/price ratio like the DLT 7000 (30 - 50 GB or higher in the year 1999) or the IBM 3494 will also be considered.
The data rate of 30 MB/s can be transferred over long distances (few km) already with present day technology (dedicated multiple FDDI or HIPPI connection). Therefore a central data recording for COMPASS in the CERN computer centre is considered to be favourable for reasons of an efficient use of the I/O infrastructure. With the high capacity tape media local tape recording and mounting at the experiment site however is still feasible.