0/ rerun this test !!!
1/ is there cache on the scsi controler ?
2/ xfs data cached at mount ? (I don't believe so)
3/ "hdparm -t" is on crack.
4/ du reports a disk usage way ahead of files' sizes total (don't know
xfs enough to estimate this propability) and tar won't read the whole
"du -sk" data.
2/ "time mount /xfs" could help (if mount + tar times are below
expected, this case can be eliminated).
3/ ask hdparm's maintener.
4/ tar, check tar size.
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