At 18:48 +0800 3/1/01, Rob Findlay wrote:
>Hi Backup People,
>I have set up one of my clients with an Ecrix VXA tape drive using the above
>media. It was my understanding that with the hardware compression built into
>the drive turned on I would get the full 66 Gig capacity. My client has just
>rung me to say that Retrospect is requesting a new tape & a quick inspection
>(over the phone via my client) of the original tape in the scheduled backup
>set revealed that it has used 33 Gig. I will have to go & investigate this
>but could someone who knows please tell me if I need to have software
>compression turned on to get 66 Gig out of the tape or is there something
>else making Retrospect ask for another tape.

For maximum capacity...

1.  make sure VXA drive has the latest firmware
2.  make sure VXA drive has hardware compression turned on
3.  make sure VXA drive favors capacity over speed

There are 2 settings, when configuring the VXA drive, that that 
affect tape capacity...

     capacity                 [ device] [capacity] [y or n]
     set_compression          [ device] [set_compression] [y or n]

I believe that the factory defaults are:
    set_compression  y
    capacity         n

At 11:42 -0600 3/1/01, Douglas K Wyman wrote:
>Streaming tape drives (virtually all modern drives including Exabyte,
>DDS-DAT, AIT, VXA, DLT, LTO etc) have a real-time requirement for
>data flow. If the backups system is not able to keep the tape drive buffer
>above its low water mark, the drive will start writing extended record
>gaps (tape with no user data; the lexicon and semantics vary from
>vendor to vendor) in order to avoid stopping the tape motion while
>waiting for data. Obviously this consumes more tape than if all data
records were written contiguously without record gaps.


My understanding is that the VXA drive does packet writes and has a 
variable speed writing mechanism and thus has much less of a problem 
with inter-record gaps (gaps of about 100k per pause with capacity 
turned on with about a 10% loss in speed)

(I'll Bcc: Ecrix tech support on this just in case I have the info wrong.)
  [EMAIL PROTECTED]      Ben Liberman

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