Re: How to flush data to disk reliably?

[Grzegorz Kulewski <kangur@polcom.net>]

Thanks for your fast response!

On Mon, 2 May 2005, Alan Cox wrote:
>> I am asking how to flush the data from these logs to disk. I know of
>> several methods:
>> 1. open with O_SYNC,
>> 2. sync(2),
>> 3. fsync,
>> 4. fdatasync,
>> 5. msync (if they are mmaped).
>>
>> Which of these are best and most reliable for (a/b) and for (IDE/SCSI)?
>
> For scsi the combination of O_SYNC and ext3 or fsync and ext3 should be
> reliable. fdatasync doesn't write back all the metadata so depending on
> the use may not be sufficient. FAT based fs's I believe you need a
> current kernel for full O_SYNC behaviour.

What about other filesystems? Does anybody know anwser for Reiserfs3, 
Reiser4, JFS, XFS and any other popular server filesystems? I assume that 
if log file is some block device (like partition) both O_SYNC and fsync 
will work? What about ext2? What about some strange RAID/DM/NBD 
configurations? (I do not know in advance what our customers will use so I 
need portable method.)


>> What are differences between them?
>
> See the manual pages and/or standard.

I already saw them. But I am asking about current implementation status on 
Linux. For example does fsync differ from fdatasync if file is block 
device? Does O_SYNC always equal "write; fsync" for all not read only 
operations? Is it faster because only one syscall is executed?

Also flushing should be slow (for example 50 flushes/s) because disk seeks 
are slow. So if I need say 200 reliable writes to log per second may I put 
4 independent disks into the server and use them as 4 independent log 
files? But fsync operation blocks. Is there any "submit flush request and 
get some info when done" command or should I use 4 threads / processes?


Thanks in advance,

Grzegorz Kulewski