Performance Tuning for (cheap) Backups

Introduction

There are a variety of reasons to try to improve the speed of your backups:

Limit the interruption to users by decreasing the backup window
Reduce tape drive wear by streaming data to tape, for tape drives that support streaming (i.e. DLT)

This page is about performance tuning cheap/free software. You should probably try Storage Mountain or similar for info about performance tuning commercial software like Veritas NetBackup or Legato NetWorker.

Tunables

My current environment consists of a Linux server with a SCSI attached DLT 8000 drive (in an Adic FastStor 22 changer). Backup software is GNU tar and BRU. With this set of hardware and software, the following options are available for tuning:

Tape drive hardware compression enabled/disabled
Linux kernel SCSI Tape (st) driver buffer
Backup software buffer
Backup software compression

Drive compression

Linux defaults to enabling drive compression. Drive compression can be disabled by running mt -f /dev/tape-device compression 0. See the mt(1) man page for more info.

st driver buffer

The st drive defaults to a 32k buffer. Presuming you are loading the st driver as a kernel module, you can adjust the buffer by adding a line to /etc/modules.conf like the following:

options st buffer_kbs=128

After making the change and making sure the module is reloaded (either by rebooting or by doing an rmmod/insmod), you should be able to run dmesg and see a line like the following with the increased bufsize:

st: Version 20010812, bufsize 131072, wrt 129024, max init. bufs 4, s/g segs 16

See drivers/scsi/README.st in the kernel source tree for more info.

Backup software buffer

GNU tar defaults to a 10k buffer. This can be increased via the -b command line option, which is in units of 512 bytes. I.e. specifying tar cbf 64 /dev/tape-device . tells tar to use a 32k buffer. See the tar(1) man page for more info.

The most recent version of BRU version 16 defaults to a 32k buffer. This can be adjusted in /etc/brutab.

Backup software compression

Neither GNU tar nor BRU defaults to using software compression. GNU tar can use gzip, bzip2 or any other compression program you want with the appropriate command line options. gzip is the most common and is enabled by adding the -z command line option. BRU software compression can be enabled with the -Z command line option.

Findings

I tested various settings for all of these options on a variety of different filesystems. What I found was the following:

Turning drive compression off slowed things down significantly
Increasing the kernel buffer from 32k to 128k gave a slight improvement
Increasing the kernel buffer from 128k to 512k gave no change
Increasing the tar buffer from 10k to 32k gave a big improvement
Increasing the tar buffer from 32k to 128k gave a slight improvement
Increasing the tar buffer from 128k to 512k gave no change
Increasing the BRU buffer from 32k to 128k gave a slight improvement
Enabling software compression slowed things down significantly

Summary

The resulting set of options that resulting in the best performance with our setup was:

Left drive compression enabled
Increased st buffer to 128k
Increased software buffer (both GNU tar and BRU) to 128k
Left software compression disabled

Unfortunately I was not able to make our DLT 8000 stream even with the best of all of these options enabled. (Tested by sticking my ear up next to the drive and listening. :) However, it did significantly improve backup performance over the default configuration.

Home

jheiss at aput.net

$Id: perftune_backups.shtml,v 1.2 2005/04/18 18:18:08 jheiss Exp $