[Veritas-bu] Encryption - LTO-4 or Media Server

[Don Peterson]

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
_________________________________

Office: 651-746-7236
Email: don_peterson at symantec.com


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