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Issue:
======
When database in forced writes mode grows rapidly, filesystem
disk operations produce a serious overhead, making performance
up to 3 times lower compared with turned off forced writes.
Scope:
======
Mainly affects Linux, because Linux misses appropriate system
call to make database file grow efficiently.
Document author:
=================
Alex Peshkov (peshkoff@mail.ru)
Document date: 2007/11/21
==============
To make firebird have better performance under such circumstances
you may place your database not in a regular file on some
filesystem, but on raw device. Any type of block device is
supported.
For example:
gbak -c my.fbk /dev/sda7
will restore your database on the third logical of extended
partition of your SCSI(SATA) disk0.
Known issue:
To be able to do physical (using nbackup utility) copy of
database you MUST specify explicit name of difference file:
# isql /dev/sda7
SQL> alter database add difference file '/tmp/dev_sda7';
This is required because default location of difference file
will be in /dev, which is surely not what you need. It's also
better to know how many blocks on block device are actually
occupied (or you will have to copy all data on raw device,
which can make size of your copy abnormally large). To obtain
real size of database, you should use '-S' switch of nbackup:
# nbackup -s -l /dev/sda7
77173
Where 77173 is a number of pages, occupied by database. Take
care - this is database's page size, not disk physical block
size! If unsure, use
# gstat -h /dev/sda7
Database "/dev/sda7"
Database header page information:
Flags 0
Checksum 12345
Generation 43
Page size 4096 <== that's what you need
ODS version 11.1
. . . . . . .
You may use nbackup output directly in a script, performing
database backup:
# DbFile=/dev/sda7
# DbSize=`nbackup -L $DbFile -S` || exit 1
# dd if=$DbFile ibs=4k count=$DbSize | # compress and record DVD
# nbackup -N $DbFile
Or perform physical backup using nbackup:
# nbackup -B 0 /dev/sda7 /tmp/lvl.0
In all other aspects raw devices do not have known specific
in use.
Tip: it's good idea to have raw devices in databases.conf - in
case of HW reconfiguration of your server you will not need to
change connection strings.