Streaming LTO-5
14. Aug, 2009 
4 Comments
Chris Mellor (twitter:@Chris_Mellor) recently posted an article over at The Register about LTO-5 entitled Is LTO-5 the last harrah for tape?. He makes an interesting point about the future of LTO and whether LTO-5 will be the last generation of the technology. Most of the comments on the article disagree with Chris’s opinion.
I believe that there is another major issue with LTO-5 that must be addressed. The challenge with LTO (and most other tape technologies) is its limited ability to throttle performance. Users must carefully manage their environment to ensure that they stream their drives or else backup performance will decline dramatically. As drives become faster, the challenge of optimizing your environment for the technology becomes more difficult. You can read more about this in my blog post entitled The Fallacy of Faster Tape.
Backup software ISVs have recognized physical tape’s limitations and have developed technologies to improve backup speeds; a classic example is multiplexing. Customers can multiplex their backups to improve ingest performance; however, the result is a degradation in restore speed. The key takeaway is that faster drives bring new infrastructure challenges that must be addressed. To illustrate this point numerically, I updated the chart from The Fallacy of Faster Tape with estimated LTO-5 performance.
Click for larger image
The chart shows that the minimum usable performance for LTO-5 is between 62 and 90 MB/sec and the required streaming speed increases linearly with the number of drives. The streaming requirement is the crux of the issue and creates a major management and infrastructure challenge.
The best solution to this problem is to introduce disk as a backup target. Disk provides infinitely variable ingest speeds and will not suffer the shoe-shining penalties of tape. However, if tape copies are required, you must carefully review the restore performance of your disk device because shoe-shining is still a possibility if the disk system cannot restore data faster than 60 MB/sec. This issue is particularly relevant in the context of deduplication since the processing of rebuilding the deduplicated data can impact restore performance. I wrote about this problem in another blog post. SEPATON has architected the S2100-ES2 and DeltaStor to provide the fastest backup and recovery performance to address these requirements.
In summary, LTO-5 shows the LTO consortium’s commitment to increase tape density and performance. The real world performance improvements experience by end users transitioning to LTO-5 will vary widely. This has always been the case with tape technology, but the added streaming requirements of LTO-5 increases the challenge. However, physical tape still has a place in many enterprises as a deep archival medium and LTO-5 is well suited for this role.
You raise good points here … much of the design of tape over the last ten years has been to massively increase speed and capacity, but there’s a growing and important need for tape technology to grow into the lower end of the market place that needs the capacity but can’t feed the speed. This is something I address in a few different pieces on my blog as well.
Preston,
Thank you for your comment. We are in complete agreement and your point about the lower end of the market is a great one. If the big guys with high performance infrastructures and large IT staffs cannot stream drives, where does that leave smaller environments? Clearly in a tough spot. I think that this is why smaller environments have been the most aggressive adopters of deduplication technology.
FWIW, most LTO drives have the ability to slow down to match data speeds below optimium and thus avoid shoeshining. There is still a minimum speed they can run at but it’s not all doom and gloom if your source can’t keep up with drive maximum speed.
Additionally: many backup programs use blocking factors which are too small for LTO (65kb or so). This needs to be cranked up to get best speed out of drives.
Hi,
Thank you for your comment. Your point about throttling is absolutely right. The challenge is that there is still a minimum threshold and as drive speeds increase, this threshold gets higher. The other challenge is that hardware compression works against you since it reduces the amount of data written to tape and thus requires more bandwidth. (e.g. 2x compression reduces the effective bandwidth of data written to tape by 1/2.)
Managing tape is not all doom and gloom, but it certainly needs to be managed carefully and it is extremely difficult to achieve the state performance specifications. It often requires a trade-off between fastest backup (using lots of multiplexing) and fast restore speeds (limited multiplexing). In today’s data centers where data is more critical than ever, the choice of fast backup or fast restore is a difficult one to make.