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Hi,
	I am trying to patch Xrootd for a specific use case, for a site with GPFS + TSM backend.

The system is somehow different from a standard disk + tape, since GPFS reports (via posix) the files always present, even if they are tape only.
In principle, one could forget and let GPFS do the magic: if a fopen is issued to GPFS for a file only on tape, recall is automatically triggered etc …

BUT: the tape system cannot really afford this kind of random recalls, and there is a specific interface to queue recalls and combine them in a smart way (for example, do all the recalls in the same cassette etc).

So I am trying to patch xrootd for this (I was already successful in instrumenting cmsd not to publicize files just on tape…)

Main idea is to use a standard stagein technique, via

frm.xfr.copycmd out /bin/true 0  
frm.xfr.copycmd in stats noalloc /opt/xrootd/bin/copy.sh $SRC $DST
frm.xfr.copymax 200 

where the script enqueues the recall to the proper mechanism.

The patch is needed since xrootd uses frm interface when the file is missing, and this is never the case for a tape only file since GPFS always shows all the files …

So my patch would be to simply tell xrootd that a file on GPFS, but with stat buf.blocks == 0, should be considered as absent, and hence trigger frm.

I was optimistic that a simple change like checking the # of blocks in XrdOssApi:Open_ufs would be sufficient, like

    struct stat buf;
    if (!stat(path,&buf)) {
      if ((buf.st_mode & S_IFMT) == S_IFREG) {
        if (buf.st_size>0 && buf.st_blocks==0){
          // i want to kill this one, I return not existing
        std::cerr<<" APPLYING MY FIX!!!!!!!!!!"<<std::endl;
          return -ENOENT;
        }
      }
    }

and indeed it works, in the sense that the file is recognized as present BUT on disk only, and hence the same error code as for missing files is returned (-ENOENT).

Problem is that, later, no recall command is issued for the file, and an open is issued continuously with 1 sec delay. So probably what I did is not enough. I will file a request for enhancement (as I already did for the cmsd part), but in case someone sees a trivial cause for this behavior, I would be happy to test a solution.

thanks

tommaso




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