Re: [Qemu-devel] [RFC] Disk integrity in QEMU

[Anthony Liguori <anthony@codemonkey.ws>]

Laurent Vivier wrote:
> Le jeudi 09 octobre 2008 à 12:00 -0500, Anthony Liguori a écrit :
> [...]
>   
>> So to summarize, I think we should enable O_DSYNC by default to
>> ensure 
>> that guest data integrity is not dependent on the host OS, and that 
>> practically speaking, cache=off is only useful for very specialized 
>> circumstances.  Part of the patch I'll follow up with includes
>> changes 
>> to the man page to document all of this for users.
>>     
>
> perhaps I'm wrong but I think O_DSYNC (in fact O_SYNC for linux) will
> impact host filesystem performance, at least with ext3, because the
> synchronicity is done through the commit of the journal of the whole
> filesystem:
>   

Yes, but this is important because if the journal isn't committed, then 
it's possible that while the data would be on disk, the file system 
metadata is out of sync on disk which could result in the changes to the 
file being lost.

I think that you are in fact correct that the journal write is probably 
unnecessary overhead in a lot of scenarios but Ryan actually has some 
performance data that he should be posting soon that shows that in most 
circumstances, O_DSYNC does pretty well compared to O_DIRECT for write 
so I don't this is a practical concern.

Regards,

Anthony Liguori

> see fs/ext3/file.c:ext3_file_write() (I've removed the comments here) :
>
> ...
>         if (file->f_flags & O_SYNC) {
>  
>                 if (!ext3_should_journal_data(inode))
>                         return ret;
>
>                 goto force_commit;
>         }
>
>
>         if (!IS_SYNC(inode))
>                 return ret;
>
> force_commit:
>         err = ext3_force_commit(inode->i_sb);
>         if (err)
>                 return err;
>         return ret;
> }
>
> Moreover, the real behavior depends on the type of the journaling system
> you use...
>
> Regards,
> Laurent
>