On 21 jan 2010, at 00.20, Al Hopper wrote: > I remember for about 5 years ago (before LT0-4 days) that streaming > tape drives would go to great lengths to ensure that the drive kept > streaming - because it took so much time to stop, backup and stream > again. And one way the drive firmware accomplished that was to write > blocks of zeros when there was no data available. This also occurred > when the backup source was sending a bunch of small files, which took > longer to stream and did'nt produce enough data to keep the drive > writing useful data. And if you had the tape hardware setup to do > compression, then, assuming a normal 2:1 compression ratio, you'd need > to source 240Mb/Sec in order to keep the tape writing 120Mb/Sec. The > net result was the consumption of a lot more tape than a > back-of-the-napkin calculation told you was required. > > Obviously at higher compression ratios or with the higher stream data > write rates you quote below - this problem becomes more troublesome. > So I agree with your conclusion: "The only way to realistically feed > that is from disk."
Yes! Modern LTO drives can typically vary their speed about a factor four or so, so even if you can't keep up with the tape drive maximum speed, it will typically work pretty good anyway. If you can't keep up even then, it will have to stop, back up a bit, and restart, which will be _very_ slow. Having a disk system deliver data at 240 MB/s at the same time as you are writing to it can be a bit of a challenge. I haven't seen drives that fill out with zeros. Sounds like an ugly solution, but maybe it could be useful in some strange case. /ragge s _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
