Questions[tape](server)

As I understand it, LTO tapes write data in "wraps", where the first wrap unspools the tape into the drive, and the second wrap spools it back into the cartridge. This process is repeated a number of times, with the idea being that once the end of the tape has been reached, all the tape will be back in the cartridge and it can be ejected with little rewinding.

However I have noticed that when you get to the end of a tape, the drive sounds like it is about half way through the final wrap, and so the drive spends some time rewinding the tape before ejecting it, even though it has reported that the end of the tape has been reached.

Is this because there is some reserved capacity on the tape, to allow for things like rewriting failed blocks or to skip over bad sections of tape without reducing total capacity? Or is there some other reason for this apparently early finish of the tape?

How can I determine how much space is remaining on an LTO tape?

If I seek to the end and check the status, I can see I am on file number 17, but what I do not know is how large each file was.

$ mt -f /dev/nst1 eod
$ mt -f /dev/nst1 status
SCSI 2 tape drive:
File number=17, block number=0, partition=0.
Tape block size 0 bytes. Density code 0x44 (LTO-3).
Soft error count since last status=0
General status bits on (81010000):
 EOF ONLINE IM_REP_EN

Short of reading the entire strip, how can I determine the used / free capacity of the tape?

I have an Ultrium 448 tape drive, LTO-2 tapes (200/400GB) and I'm using HP Data Protector as the backup client. The system is set to format and overwrite all tapes but I never manage to get more that ~150GB on a tape before it asks for another tape to be inserted.

Any ideas why this may be happening?

Over a period of three weeks, I experienced six complete failures of LTO-1 and LTO-2 tape drives at client sites. Some had failed mechanisms. Others lost the ability to write reliably. These were HP Ultrium 232, 448 and 460 drives. Most of these units were deployed between 2006 and 2008, so the timing of the failures is right. The capacities (400GB) are correct for the applications. I replaced a couple of drives with equivalent devices, just for consistency. One server also had a SCSI HBA failure, further delaying recovery. At that point, the customer asked if there were any hard disk solutions available that would be better (or less-finicky) than tape.

As I began to look for replacements, I discovered that the RDX removable disk storage technology has been adopted by the major server manufacturers (HP, Dell, IBM). From my perspective, it looks like docked 2.5" SATA disks connected either internally or externally via USB2 in capacities up-to 1TB. Since these are actual disks, it seems like recovery and seek time would be reasonable. But I have a few questions about the technology in practice.

• Does anyone here use these drives with success? Is there anything to watch out for?
• What differentiates RDX from straight external USB disks?
• One of the advantages to tape in my application is that the drives have hardware compression. This helps immensely for the highly-compressive datasets I have to backup on Linux systems. Am I correct in assuming RDX relies on software compression?
• Since these are physical disks, are there any mountpoint issues in Linux or Windows? One of the nice things about tape is that its not a mounted filesystem and typically isn't affected by viruses, rootkits, system crashes, etc.
• In addition, I watched a primer on using RDX with Cactus Lone-Tar, and cringed when I saw them using a mkfs command to create a filesystem on the RDX drive at /dev/sda. Is there any chance of device renaming/reordering (from adding a SCSI controller, inserting a USB key, etc.), or will the docking unit persist at a particular device name as you swap drives?
• Are the backup speeds of 30 megabytes/second accurate?

I'm curious, as this could be an interesting alternative. The series of tape drive failures came at a time where it makes sense to reevaluate other options before moving forward.

We've backed up our data on LTO tapes for years and it's a real comfort to know we have everything on tape. A sister project and one of our data providers have both moved to 100% disk storage because the cost of disk has dropped so much. When we propose systems to potential customers these days we tend to downplay or not mention our use of tape systems for data storage since it might seem outdated.

I feel more comfortable with having data saved in two separate formats: disks and tape. In addition, once data is securely written to tape, I feel (perhaps naively) that it's been permanently saved. Not having to rely on a RAID controller to be able to read back data is another plus for me.

Do you see a place for tape backup these days?