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From Suresh Thalamati <tsur...@Source-Zone.org>
Subject partial out of order write problem in log i/o
Date Thu, 02 Dec 2004 23:29:15 GMT
 
Hi all,

<>I am looking into  incomplete log record write that occurs because of
an out of order partial writes gets recognized as complete during
recovery if the first sector and last sector happens to get written.
 Current system recognizes incompletely written log records by checking
the length of the record that is stored in the beginning and end.
 Format the log records are written to disk is:

  +----------+-------------+------------------+

  | length     |  LOG RECORD |    length   |

  +----------+-------------+------------------+

<> 
This mechanism works fine if sectors are written in sequential manner or
log record size is less than 2 sectors. I  believe on SCSI types disks
order is not necessarily sequential, SCSI disk drives may sometimes do a
reordering of the sectors to optimize the performance.  If a log record
that spans multiple disk sectors is being written to SCISI type of
devices,  it is possible that first and last sector written before the
crash; If this occurs recovery system will incorrectly  interpret the
log records was completely written and replay the record. This could
lead to recovery errors or data corruption.

This problem also will not occur if a disk drive has write cache with a
battery backup which will make sure I/O request will complete.

Any comments?

<>Thanks
-suresh

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