Re: Linux does not care for data integrity

[Bill Davidsen <davidsen@tmr.com>]

Matthias Andree wrote:
> On Sun, 29 May 2005, Greg Stark wrote:
> 
> 
>>Oracle, Sybase, Postgres, other databases have hard requirements. They
>>guarantee that when they acknowledge a transaction commit the data has been
>>written to non-volatile media and will be recoverable even in the face of a
>>routine power loss.
>>
>>They meet this requirement just fine on SCSI drives (where write caching
>>generally ships disabled) and on any OS where fsync issues a cache flush. If
> 
> 
> I don't know what facts "generally ships disabled" is based on, all of
> the more recent SCSI drives (non SCA type though) I acquired came with
> write cache enabled and some also with queue algorithm modifier set to 1.
> 
> 
>>Worse, if the disk flushes the data to disk out of order it's quite
>>likely the entire database will be corrupted on any simple power
>>outage. I'm not clear whether that's the case for any common drives.
> 
> 
> It's a matter of enforcing write order. In how far such ordering
> constraints are propagated by file systems, VFS layer, down to the
> hardware, is the grand question.
> 
The problem is that in many options required to make that happen in the 
o/s, hardware, and application are going to kill performance. And even 
if you can control order of write, unless you can get write to final 
non-volatile media control you can get a sane database but still lose 
transactions.

If there was a way for the o/s to know when a physical write was done 
other than using flushes to force completion, then overall performance 
could be higher, but individual transaction might have greater latency. 
And the app could use fsync to force order of write as needed. In many 
cases groups of writes can be done in any order as long as they are all 
done before the next logical step takes place.

This would change the meaning of fsync from "force out the data" to 
"wait for the data to be written" in some implementations.

-- 
bill davidsen <davidsen@tmr.com>
   CTO TMR Associates, Inc
   Doing interesting things with small computers since 1979