Ultrium 960 pauses every 10 GB or so

[jimkberry]

I have an Ultrium 960 that I'm using for a data archival app ( Linux 2.6.16.27, SanBlaze Scsi adapter, Fusion MTP drivers)

It works well enough and I am easily able to sustain data rates up around 73MB/sec of incompressible data, but I am seeing a "pause" of approximately 5 sec every 120 sec or so. I initially thought it might be related to periodic stream starvation, but am pretty sure I've ruled that out.

So I wrote a little test app that does nothing but write data to the drive as fast as it can, all the while checking the data rate and letting me know when a write() takes an anomalously long time.

Without going into excruciating detail (all I'm really looking for to start with is to see if anyone has ever seen similar behavior) suffice it to say that it appears that the "120 sec" isn't the issue, and that instead what I am seeing is a 5 sec pause every 10 GB or so - the exact size is not exactly repeatable, but always real close to 9250 MB. It _always_ happens, though.

So...

Has anyone ever run across anything of this sort with an LTO drive?

Thanks,

-jim berry

[Cass Witkowski · Accepted solution]

I don't have information on how many tracks the tape have but you may be seeing this pause as the tape switches directions as the tape drive writes on a track in one direction and then switches directions and writes on another track in the reverse directon.

[Richard Bickers]

I can confirm that it's end of wrap behaviour that you're seeing. The streaming rate of these drives is 80MB/S for uncompressed data and the amount of data you get per wrap is in the region of 9.25GB (that's the 2^9 definition of GB). It all lines up with what you're seeing.

B.t.w. L&TT also offers performance tests for measuring data rates between the host and the drive - though it doesn't look for gaps like your app does. Just thought I'd mention it in case you weren't aware of it.

http://www.hp.com/support/tapetools

[jimkberry]

Wow. That's pretty cool.

Let's see - according to http://en.wikipedia.org/wiki/Linear_Tape-Open an LTO-3 drive has 704 tracks and 16 heads, so it makes 44 passes to fill the tape.

400 GB / 44 = 9.09 GB = 9309 MB per pass.

Yep. I'll bet that's it.

So this suggests that the actual theoretical maximum sustained data rate for the drive is not, in fact, 80 MB/sec but is actually more like 77 MB/sec. I've always wondered why I couldn't get closer to 80.

More importantly, though, a pause of that length really has to be dealt with by the data collection/archival application. I mean, it's chugging along happily spitting data out at a pretty steady rate and assuming that it can do collection steadily at more-or-less that rate, and then there's this pause for 4-ish seconds. That's long enough to get a backlog of better than 300MB, which could easily be larger than a buffering scheme has accounted for if it doesn't expect the "long pause".

Oh - regarding LTT: I actually managed to get it installed on my machine even though it's not a supported OS (it's Linux From Scratch and doesn't know about stuff like RPMs) and it has always said that my max data rate was around 76 MB/sec, which jibes well with all of this.

Thanks a lot,
-jim berry