News Group Posts

tape write performance

Consider that any backup software has to first find the files it has to backup. If you time a "find / > /dev/null" it often not insignificant.

As Tom points out below, backing up small files really cuts performance because every one of them needs its own header even if the file itself is empty!

Newsgroups: comp.unix.sco.misc
From: tom@microlite.com (D. Thomas Podnar)
Subject: Re: scsi bus bottleneck??????
Date: Sat, 6 May 2000 14:20:30 GMT
Message-ID: <200005061420.KAA24770@mlite.microlite.com> 

: Jeff Liebermann <jeffl@comix.santa-cruz.ca.us> wrote:
: 
: On Fri, 5 May 2000 20:59:12 -0400, "Michel Donais" <donais@gsig-net.qc.ca>
: wrote:
: 
: >The System is operated under Sco openserver 5.0.4p
: >Main board: Gigabyte GA-6BXU   440xBX-ATX
: >Processor   : Pentium III 450mhz
: >This system have an onboard  Ultra2 SCSI - LVD
: > interface and an Ultrawide SCSI interface.
: >Scsi Chipset is an Adaptec AIC-7890P
: >On the Ultra2 - LVD I have a Quantum 9.1gig Atlas 10k
: >and a tape drive Tandberg SLR-100
: 
: Oh-oh...
why Oh-oh?
: 
: >The speed of the tape should be 18gig/hr uncompressed
: >and 36 gig/hr compressed.
: 
: My guess is that you should get about 15MBytes/min uncompressed, and
: 80MBytes/min compressed.  

But he says down below that he's getting 3233k/sec which is 189MB/min.

: >The hardware tape command is set to compress/uncompress.
: 
: >The best speed I got compressed is near 3233k/sec around
: >12gig hour.  The speed is nearly the same as if I set Tape command
: >to  plain uncompress backup.
: 
: >Is there a way to clear or avoid what I think is a bottleneck on the
: >ultra2-lvd Scsi bus.
: 
: Yep.  Do NOT share the SCSI Ultra2 LVD bus with the DAT drive.  Get a
: seperate SCSI bus or board.  Mixing SCSI technologies on a single bus will
: function by switching it to the radically reduced single ended mode and will
: not deliver optimimum performance.

1) The Tandberg HAS an LVD interface. He is not mixing SCSI technologies.

2) The Tandberg SLR-100 is NOT a DAT drive. It is an SLR (QIC) drive.

3) The Tandberg has an 8MB buffer which is quite a bit bigger than many
   other tape drives.

: SCSI Trade Association FAQ  (See Question #1 and #4).
:   http://www.scsita.org/experts/SCSI_ExpertFAQ.html
: -- 
: Jeff Liebermann  150 Felker St #D  Santa Cruz CA 95060
: (831)421-6491 pgr (831)426-1240 fax (831)336-2558 home
: http://www.cruzio.com/~jeffl   WB6SSY
: jeffl@comix.santa-cruz.ca.us   jeffl@cruzio.com

Michel,

The biggest question may be how much data you are really writing.

On a Unix system, just installing the base operating system leaves files
all over the place. During a normal backup there is a lot of overhead,
scanning the directory track, getting the data, writing small things like
device nodes, etc. which keeps your backup software pipeline from
staying full all the time, and the files are reasonable small.
Plus there is a lot of startup overhead when you begin a backup that
takes up some of that time and skews the number slightly.

Double buffering backup software can help, as can the fact that
most double buffering can be tuned for increased performance.

189MB/min is quite acceptable on a sysem with many or mostly small files.
Lots of people on this group would kill for that.

If you had a lot of 500MB and larger databases, where the OS could
transfer the data to the drive faster, you'd probably see better
performance.

Using one of the crash recovery products to do a backup, re-create
your filesystems, and do a restore can also help improve performance,
as all the files will be laid back down on the hard drive in reasonably
contiguous fashion, and the OS will be able to back them up faster.

Tom
---
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