Thursday, 18 March 2010

Tape write performance and check_written_file

To me this was quite a discovery. As usually happens, we already had this information since several months in our mailboxes. FNAL folks told us about this Enstore parameter but we did not pay much attention at that time. Another effect of the "too much information to swallow daily" syndrome (at least for me).
Anyway, there is this funny parameter in Enstore called "check_written_file" which tells Enstore whether to check files were correctly written to tape... by reading them back! So, quite an expensive check, indeed.
Bottom line is that we had it set up at 10 without really realizing. On average, one every 10 files written was read back for checking. A bit too much, isn't it?
Last tuesday 16th in the evening this parameter was increased by a factor of at least 50.
The good news is that the ATLAS performance we report to SLS clearly shows a 30% improvement in the expected moment (top plot). Good!
The not so good news is that the same plot for CMS (bottom plot) does not show any hint of improvement... one could even see a degradation! We believe (hope!) this is due to the fact that CMS is not writting many files in one go these days, so it is dominated by tape mounts.
Will keep an eye on this, but to me it looks like we saved some Euros in tape drive throghput this week ;-)