I've replaced my backup server and I switched from a Debian Stretch to a CentOS7. I have a SuperLoader 3 LTO4 SCSI on an old adaptec SCSI 160. At first, I had to use the CentOSplus kernel which add the old SCSI aic7xxx module to have my hardware detected.

It's working, but I have a top speed to 2MB/s while I do a "btape speed" test.

I've also tried with another SCSI controller 320; this one is natively supported by the CentOS kernel. Same issue; I tried to boot Debian on the same hardware and at this moment, I get the max speed.

Right now, I'm pretty sure it's a kernel issue. Do you know if there're any settings to avoid this issue?

Edit 17/04/18 : On tapestat i see i have a 99% write time wait, is it why it's so slow ? Any idea ? I'm starting to lose hope :(

  • 1
    May or may not be related to your problem, but worth a try: I recently had I/O throughput issues, and it turned out that clearing the I/O cache with echo 3 > /proc/sys/vm/drop_caches solves at (at least for some time). I suspect some multicore concurrency problem related to caches, but haven't tracked it down yet. – dirkt Mar 29 '18 at 13:58
  • Nope, not better, maybe worst, i have 600Kb/s with the last test drop_caches set at 3 – makz Apr 3 '18 at 6:54
  • @makz Check if the drive buffer has been turned off for some reason. Try mt drvbuffer 1 to turn it back on. I had this problem, too. It is also possible that your SCSI controller doesn't like tape drives, but I see you have already tried a different one. – FUZxxl Apr 3 '18 at 14:55
  • @FUZxxl not better, right now it's the same SCSI card i used in the other server and as i said, if i boot a debian on the same hardware, i have full speed. I'm sure it's a CentOS issue. – makz Apr 4 '18 at 12:55
  • @makz Hm... that's a possibility. No idea what the reason is. – FUZxxl Apr 4 '18 at 13:00

Sooo, after a discussion with another system engineer. We figured it could be the meltdown patch which is in fault here.

Because it's a old pcix controller, the driver structure may trigger the KPI and slow down/limit the throughput.

I tried by disabling the pti on the current kernel but no improvement, maybe there's other patchs we can't disable.

The best option but i can't is to install a kernel pre-meltdown to see if it help.

but i decided to go on a pci express scsi controller and it fixed my issue.

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