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V-San tape performance questions

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V-San tape performance questions

Postby perdrix » Fri Apr 14, 2017 3:46 pm

I'm running Starwind 8. I have an LTO-4 tape attached to an Adaptec 29320A controller (U320) which allows a maximum buffer of 64kB.

I connecting over iSCSI and taking a backup like this:

Code: Select all
mt-st -f /dev/st0l setblk 65536
dd bs=16M -if /dev/sda | mbuffer -s 65536 -t -m 50% -P 80 -o /dev/st0l

The best I'm seeing for throughput is 30MB/s when I'm pretty sure I should be getting more the 60MB/s

The WIndows Resource Monitor tool is showing network usage at about 27%. I'd hope to see this saturating the LAN.

I thought of using jumbo frames but the network card doesn't support them :(

Is there anything I can do at either the Linux end or the Windows Starwind server end to improve throughput?

Strangely reading the output tape after completing the backup at the Windows end using HP LTT seems to limit to a read rate of 32MB/s which doesn't seem "right" given that the drive can do a good 120MB/s (and I've seen that when backing up that Windows system. (There's probably a clue in there somewhere).

Another clue may be that backing up another Windows system over iSCSI using a Windows backup package does manage higher throughput.

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Re: V-San tape performance questions

Postby anton (staff) » Wed Apr 26, 2017 6:45 am

Tape Redirector by its nature uses SPTI which is SCSI Pass-Thru Interface and it's 100% serial. This means we can't send one SCSI command before other one had completed, and this thing alone affects performance dramatically.

There are two ways to go:

1) Wait for StarWind to replace STPI -> SPTD (special driver stack on Windows which is parallel so we can send for execution one command before previous one had completed). <-- I don't know when it's going to happen because it's a very low priority for us...

2) Use VTL with an ability to offload virtual tapes (technically - images) from disk to tape (in this case you'll use tape backup software in a loopback) or public cloud (Amazon AWS or Microsoft Azure). ... pe-library

Good luck! :)
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