Added readme for raw devices

doc/README.raw_devices

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+Issue: 
+======
+ When database in forced writes mode grows rapidly, file system 
+ 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 circumsatnces
+ you may place your database not in a regular file on some 
+ filesystem, but on raw device. Any type of block devices 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 
+ acpects of use.
+ Tip: it's good idea to have raw devices in aliases.conf - in 
+ case of HW reconfiguration of your server you will not need to
+ change connection strings.