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HP LTO2 drives can stream from 10-30 MB/sec and IBM drives at 18, 20 and 35 MB/sec. So if he's seeing 6 MB/sec per drive, NONE of these drives are streaming, which adds to the poor performance. When running w/o MSEO and getting 60 MB/sec per drive, that’s pretty near double the native transfer rate, so there’s probably around 2:1 compression.
From our estimates, it takes about 87 clock cycles (again this is an ESTIMATE) to compress and encrypt each byte of data on a UNIX machine. The media server has roughly 4.2 GHz of processing power. Say we remove 10% for processing other than MSEO and we have roughly 3.8 GHz. That means you can compress and encrypt about 44 MB/sec of data going through the media server.
With the HP LTO2 drives, you need 20 MB/sec through the media server for the drive to stream and with an IBM LTO2 drive you need 36 MB/sec (assuming a 2:1 compression ratio). Because the media server should be able to handle around 44 MB/sec, if you connect one HP LTO4 drive you should get about 44 MB/sec with MSEO. With an IBM drive, it should stream at 20 MB/sec, which means 40 MB/sec through the media server with the 2:1 encryption. Bottom line, connect a single IBM LTO2 tape drive or a maximum of two HP LTO2 tape drives (each operating at a slower speed but still streaming) for optimal performance to make certain the drives stream. If one uses IBM drives and connects more than one drive (it doesn't say how many are connected), all the drives will shoe shine and performance will drop off dramatically, which is what could certainly be happening to get the 6 MB/sec.
An HP LTO4 drive requires 40 MB/sec to maintain streaming. Because MSEO is compressing the data (unless you want to turn off compression and use twice as much tape), MSEO needs to process 40 MB/sec x compression ratio. At 2:1 compression, you’d need 80 MB/sec through the media server, which is a whole lot more than the 44 MB/sec, which the media server can handle. And that’s attaching a SINGLE LTO4 tape drive. If you attach two drives, you need to move 160 MB/sec through the media server. By the way, an IBM LTO4 drive will stream at 30 MB/sec. but you’d still need 60 MB/sec through the media server, which is more than the media server can handle. Bottom line: You won’t be able to keep a single LTO4 drive streaming with the existing media server and by adding additional drives it makes it even worse.
NetBackup 6.5.2 added a Key Management Service (part of the Enterprise Server and Server) that runs on the master and manages keys for LTO4 and IBM TS1120/30 tape drive encryption (tape drives that support the SCSI T10 encryption standard). This allows the drive to run at full speed while compressing and encrypting the data. You should see the same throughput as w/o MSEO (HP demonstrated this at the Symantec VISION conference last June). However, given the drive can run 120-140 MB/sec native (HP vs. IBM max speed) and with 2:1 compression it should be able to double that speed, you may not need to connect as many tape drives to a media server as you might be thinking of doing. To get that high of data rate to a tape drive may require extensive multiplexing, which could slow down restores. You need to consider the trade-off between multiplexing and the number or drives you use, but you always want to make sure you keep the drives streaming.
Don Peterson
Principal Product Manager, NetBackup
Data Protection Group
Symantec Corporation
www.symantec.com
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Office: 651-746-7236
Email: don_peterson AT symantec DOT com
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