Veritas-bu

[Veritas-bu] Considering moving to NetBackup

2003-01-27 14:14:35
Subject: [Veritas-bu] Considering moving to NetBackup
From: Fabbro.Andrew AT cnf DOT com (Fabbro, Andrew P)
Date: Mon, 27 Jan 2003 11:14:35 -0800
>When using MPX, you're splitting the stream across multiple drives. Did you
have multiple drives available (and occupied) 
>by the MPX'ed tapes that were being copied to the non-MPX offsite?

I don't think that's correct.  MPX allows you to combine multiple data
streams onto one drive.

So if you had:

        client1 with 2 streams (i.e., two filesystems, configured as
separate streams)
        client2 with 3 streams
        client3 with 3 streams

and an MPX of 6, one drive would handle all streams from client1 and
client2, and one stream from client3.  The other 2 streams from client3
would go to other drives (or wait if there were no other drives available).
On the tape, you would see blocks from all 6 streams mixed together (2 from
this stream, 3 from that one, 1 from this one, etc.)

The purpose of multi-plexing is to keep modern drives streaming.  If a drive
receives sufficient data that doesn't have to pause and reposition (aka
"shoe-shining"), the write speed is greatly improved.  When a vendor quotes
you a write speed, the assumption is that the drive is streaming (and sneaky
vendors always assume highly compressable data, but that's another post ;)

NetBackup CAN split a backup that consists of separate streams to multiple
drives (one stream to this drive, one stream to that drive).  But there is
nothing that splits a single stream over multiple drives.

--
 Drew Fabbro [fabbro.andrew AT cnf DOT com]
 Unix Systems Group
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 Cell: 503-701-0369