Jerry Stuckle <jstucklex@attglobal.net> wrote:
> alf wrote:
>>
>> Not sure I agree. ext3 enables a quick recovery because there is a
>> trxlog of the file system itself. In ext2 you can lose files. So there
>> is a small step froward.
>
> So? If the file itself is corrupted, all it will do is recover a
> corrupted file. What's the gain there?
The gain is, that you have a chance to recover at all. With no files,
there is *no* way to recover.
However, thats not a real problem. MySQL never touches the datafile
itself once it is created. Only exception: REPAIR TABLE. This will
recreate the datafile (as new file with extension .TMD) and then
rename files.
DELETE just marks a record as deleted (1 bit). INSERT writes a new
record at the end of the datafile (or into a hole, if one exists).
UPDATE is done either in place or as INSERT + DELETE.
Most file operations on MyISAM tables are easier, faster and less
risky, if the table uses fixed length records. Then there is no need to
collapse adjacent unused records into one, UPDATE can be done in place,
there will be no fragmentation and such.
The MyISAM engine is quite simple. Data and index are held in separate
files. Data is structured in records. Whenever a record is modified,
it's written to disk immediately (however the operation system might
cache this). MyISAM never touches records without need. So if mysqld
goes down while in normal operation, only those records can be damaged
that were in use by active UPDATE, DELETE or INSERT operations.
There are two exceptions: REPAIR TABLE and OPTIMIZE TABLE. Both
recreate the datafile with new name and then switch by renaming.
There is still no chance to lose *both* files.
Indexes are different, though. Indexes are organized in pages and
heavily cached. You can even instruct mysqld to never flush modified
index pages to disk (except at shutdown or cache restructuring).
However indexes can be rebuilt from scratch, without losing data.
The only thing lost is the time needed for recovery.
HTH, XL
--
Axel Schwenke, Senior Software Developer, MySQL AB
Online User Manual:
http://dev.mysql.com/doc/refman/5.0/en/
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