We have tested operation of MIDAS using a 10GigE network connection. Using a dummy frontend
generating fake data, we can record MIDAS data to disk at at least 700 Mbytes/sec as reported by
the MIDAS status page.
Two configurations were tested, both run at at least 700Mbytes/sec sustained:
1) MIDAS mhttpd, mserver, mlogger running on the disk server machine (mlogger writes to local
disk), frontend running on remote machine (10GigE mserver connection).
2) MIDAS mhttpd, mserver, mlogger, frontend running on remote machine (mlogger writes data to
an NFS-mounted disk over a 10GigE connection).
In addition, for configuration (2), I simulated online analysis reading fresh midas files at the same
time as MIDAS writes new data. The resulting observation is that Linux seems to be giving main
priority to disk write traffic (700 Mbytes/sec) with the remaining disk bandwidth given to read traffic
(50-100Mbytes/sec). In other words, when running online data analysis on fresh data files, mlogger
continues to run at full speed (analysis does not slow down data taking).
A few problems with MIDAS were observed during this test:
a) mlogger data compression using gzip-1 has to be turned off (limits data rate to about
200Mbytes/sec). We plan to implement high speed LZO/LZ4 data compression that we expect to
keep up with a 10GigE network interface.
b) CPU use by mserver and mlogger is rather high (about 40% CPU)
c) when writing to the NFS disk, mlogger has a pause of 1-2 seconds when closing and reopening
subrun data files. To avoid a interruption in data taking, the SYSTEM event buffer has to be big
enough to ride through this pause, but stock MIDAS limits the maximum size of event buffer to 1GB
(too small), this can be easily increased to 2GB (almost big enough) and with some more work it can
be increased to 4GB, but no more because the buffer length is a 32-bit integer.
d) when writing to the NFS disk, we also see periodic 3-5 second interruptions ("write operation took
5123 ms") and we had one death of mlogger by a timeout of 60 sec.
Details of the hardware:
1) the disk server machine CPU is 3.4GHz Intel i7-4770, mobo is ASUS Z87 WS (10 SATA, 2xGigE),
RAM is 32GB DDR3-1600.
2) disk array is 8x4TB Seagate ST4000VN000-1H4168 NAS disks RAID0 (striped) configuration, raw
data read/write rate is around 1 GByte/sec, disks are directly attached to mobo (no raid card), linux
3) the frontend machine CPU is 3.7GHz Intel i7-4820, mobo is ASUS P9X79 WS, RAM is 32GB DDR3-
4) 10GigE network is Solarflare Communications SFC9120 (both machines) with a cross-over fiber
cable (direct connection,no switches)
5) OS is up-to-date SL6.5 (both machines)