Relation between sysdevices, sysdatabases and sysusages
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1. Virtual Page Number: Virtual Page number is unique across the server, always in 2k size. Does not depend upon server page size.
2. Logial Page number : Unique across a database. Depends upon the page size of server ( 2k, 4k, 8k and 16k)
3. vdevno from sysdevice as vedno(low/2power24)&255
4. The high Order four bits of sysusage.vstart stores the virtual device number and can be calculated as vdevno=(vstart/2pow24)&255
sysdevices.low : Starting vitual page for a devices and generally calculated as vedvno*2power24
sysdevices.high : Last vitual page of the device allocation
Device Size: (sysdevices.high – sysdevices.low+1)
sysdatabase.dbid : Database Id for a database.
sysusages.vstart : is the vitual page number from where the database fragment starts
sysusages.lstart : Is the start of the logical page number
sysusages.size : Number of contiguous database (logical) pages
sysusages.dbid : Database id for fragment.
vstart of a database fragment is lying between the low and high of device used.
Version 15.x
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1. Virtual Page Number: Virtual Page number is unique across the device, always in 2k size. It does not depend upon server page size.
2. Logial Page number : Unique across a database.
3. Now, device identification number is stored in the vdevno column and no longer as part of the high or low column.
and also no longer as part of the sysusage.vstart column.
sysdevices.low : Generally Zero , block offset of virtual page in 2K page if there is offset with vstart parameter of disk init.
sysdevices.high : Last virtual page of the device allocation
sysdevices.vdevno : Virtual device number for a device (for master device =0 and maximum value is defined by number of devices config parameter)
Device Size : (sysdevices.high – sysdevices.low+1)
sysdatabase.dbid : Database Id for a database.
sysusages.vstart : is the virtual page number from where the database fragment starts in a device specified by vdevno
sysusages.lstart : Is the start of the logical page number of the database.
sysusages.size : Number of contiguous database (logical) pages
sysusages.dbid : Database id for fragment.
sysuages.vdevno : Vdevno of device on which database fragment is created.
These global variable always represents as
@@maxpagesize – logical page size
@@pagesize – virtual page size
Source : sybooks
Check the Sybase Wiki @ sybasewiki.com
Anurag has more than 6+ years of experience in Sybase Database Development .His Area of expertise includes Performance, Query Optimization, Cost Optimization, TSQL Development. He is also involved in Consultancy to Financial Firms for Database Implementation and Maintenance. He has supported many Global Financial firms.
nice document..