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Date: Mon, 13 Dec 2004 10:42:59 -0800 (PST)
From: "Suresh Thalamati (JIRA)" <derby-dev@db.apache.org>
To: derby-dev@db.apache.org
Subject: [jira] Assigned: (DERBY-96) partial log record writes that occur
 because of out-of order writes need to be handled by recovery.
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     [ http://nagoya.apache.org/jira/browse/DERBY-96?page=history ]

Suresh Thalamati reassigned DERBY-96:
-------------------------------------

    Assign To: Suresh Thalamati

> partial log record writes that occur because of out-of order writes need to be handled by recovery.
> ---------------------------------------------------------------------------------------------------
>
>          Key: DERBY-96
>          URL: http://nagoya.apache.org/jira/browse/DERBY-96
>      Project: Derby
>         Type: New Feature
>   Components: Store
>     Versions: 10.0.2.1
>     Reporter: Suresh Thalamati
>     Assignee: Suresh Thalamati

>
> 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.

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