Shared Memory Segment Emergency Recovery

Sometimes when a process gets killed or crashes unexpectedly a database shared memory segment may become locked and inaccessible by other processes (either processes that are already running or newly launched processes) yet there may be a need to rescue valuable data remaining in this in-memory database.

The shdump utility can be used to create an emergency backup for such a segment.

• shdump should be used as "a last resort" only, when all other options are exhausted and right before the final cleanup/restart of "frozen" application(s). This tool should never be combined with regular database operations. (Please refer to Backup and restore if you need to backup data routinely.)
• It is not guaranteed that data will be saved completely or that all transactions will be completed.
• On the Windows platform at least one process holding the affected shared memory segment should remain alive (even if stuck/non-operational).

Rescue procedure overview

1. Dump the database segment into a file with shdump.
2. Create a temporary database and load the file into it, then make a regular backup from this temporary database. This step can be performed on a different machine, so that the affected shared memory segment may be left in memory.
3. Restore the backup into a regular eXtremeDB instance, check the data and use it normally.

Usage

 
    shdump [-dr] [-s size] [-c max_connections] [-a address] [-p page_size] [ -n dbname] segment_name);
     

• size is the size of the memory segment in bytes. Suffix-modifiers 'k', 'm', 'g' meaning kilobytes, megabytes and gigabytes can be appended.
• address is an address to which to restore segment (in HEX format: 0xHHHHH)
• page_size is database page size. Default value is 512 bytes. Used only when restoring database
• dbname is the name of resulting database. Used only for restore

Example

1. Dump step

    
       shdump  -s 128M segmentName
   						 

   This will create file segmentName.dmp with size of 128Mb and save the contents of the shared memory segment named "segmentName".

2. Backup step

    
       shdump -a 0x20000000 -r -p 4096 -n kbss_stds -s 128m -c 256 segmentName
   						 

   This command will load the segment dump file "segmentName.dmp" into memory, open an IM database using that segment and create backupdb.bak file with the regular eXtremeDB backup.

   This file is suitable for loading by xsql with the -image option or with the mco_db_load() eXtremeDB C API call.

3. Optional XSQL check step

   Assume that database parameters are described in the corresponding xsql.cfg configuration file:

    
       xsql  -c xsql.cfg -image backupdb.bak