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From "Suresh Thalamati (JIRA)" <derby-...@db.apache.org>
Subject [jira] Commented: (DERBY-96) partial log record writes that occur because of out-of order writes need to be handled by recovery.
Date Thu, 09 Dec 2004 23:38:16 GMT
     [ http://nagoya.apache.org/jira/browse/DERBY-96?page=comments#action_56482 ]
Suresh Thalamati commented on DERBY-96:

Some thoughts on how this problem could be solved:

To identify the partial writes, some form of checksum has to be added to the log data written
to the file. On recovery using the checksum information partial written log records could
be identified and thrown away.  Checksum information has to be included 
with the log data before it is written to the disk. Now the issue is when do we calculate
the checksum and write to the disk. 

Following are some logical points when the checksum can be calculated and written along with
log informaton:

1)Calculate the checksum for each log records and store the information as part of log record
data structure.  Disadvantage of this approach, storing checksum with each log records could
be expensive with respect to the amount of space and time spent to calculate. 

2)Calculate checksum for group of log records in the log buffers before writing the buffer
to the disk and also write an addition log records that will have the checksum information
and the length of the data. This log records (LogCheckSum) will be prefixes to the log buffer.
The reason  checksum log records are to be written in the beginning  is it is easier to find
to how much data has to be read during recovery to verify the checksum. 

    Log data is written only when log buffers is full or make sure WAL protocol is not violated.
 Size of the data that is part of the checksum can potentially be 32K or whatever log buffer
size is. Overhead with this approach is less compared to the first approach. 

3)Block-based log i/0: Idea is to group log record data into 4k/8K pages with a checksum on
each page. During recovery checksum will be recalculated for  each 
and match it one on the disk, if checksum does not match it is possibly as partial write.

This approach is liked to have more overhead compared to the second one.  But this approach
also has the benefit of making log writes aligned.  Not sure yet whether there is any performance
by doing so. (Please see aligned Vs Non-Aligned e-mail thread on derby list). 

I should also bring to the attention this approach will likely require more changes  than
1 & 2 , reasons for that are :

a)Current system assumed LSN to file offset. If  the data is written in page format , that
will no longer be true. 
b)To strict to WAL protocol , it may be required that an unfilled page needs to be written.
 If this unfilled page happened to have a COMMITTED log records it can not be simply rewritten;
If the rewrite is incomplete log records with committed information will be thrown away. 
To avoid this issue,  log pages can not be written , which could lead to of unused space in
the log file or implement safe-write mechanism(ping-pong algorithm). 

Irrespective of what approach is used to solve this problem, I believe new type of information
(checksum) has to be written to the disk, which will not be understood by Old versions.  

Any comments/suggestion ?


> 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:
>     Reporter: 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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