Re: TSM and LTO Throughput

This is an account of our experience with TSM and LTO tape drives from October 2001. I posted some of it on the list back then.

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I recently was able to experiment on systems with beefy machines, EMC disk, LTO library, and Fibre. Despite all this hardware, and despite all efforts, I was unable to get reasonable performance with TSM backing up the file systems to LTO drives. I tried backing up over the SAN with the Managed System for SAN client (Storage Agent) and with the standard client to a local TSM server via shared memory. As the attached table shows, backup rates for lots of small files is abysmal, 3 MB/sec or less. The backup rate for larger files was better, but still far below expectations.

Tivoli says that these rates are NORMAL.

Our analysis of this is that the TSM client is unable to send data at a rate fast enough to keep the LTO drives streaming. This is catastrophic for the LTO. LTOs are not designed to be speedy when they have to stop, backup, and index forward, as they have to do when not streaming. To stream an LTO drive, its tape heads must be fed at 15 MB/sec. With drive compression turned on, this means that the entire system needs to pass data at up to 75 MB/sec to EACH drive. This is hard to do with most systems, getting disk systems to move data at that speed is difficult. In addition, there is per file overhead required for the file system and TSM database. It likely to be a rare circumstance that will allow LTOs to stream with drive compression turned on.

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The lesson is, with LTO-1 technology, the entire data path must support the streaming speed of the tape drive time the compression ratio or the drives are MUCH slower. The objects per second rate needs to be low also to prevent gaps in the streaming of data. I have found few disk systems which can keep up with a streaming LTO, and then only with large objects (few objects per second). TSM and the drive microcode have improved, but upgrading to LTO-2 is the best solution. LTO-2 drives are variable speed and will throttle down to match your data rate.

Orville L. Lantto
Datatrend Technologies, Inc. (http://www.datatrend.com)
IBM Premier Business Partner
121 Cheshire Lane, Suite 700
Minnetonka, MN 55305
Email: Orville.Lantto AT datatrend DOT com

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