Slower performance with ext4

[Mark Kirkwood]

Christopher Chan wrote:
>
> Journaling only for metadata is not 'as much journaling as any other 
> canditates.' You cannot say metadata journaling only as equivalent to 
> the data and metadata journaling that is possible with ext3. XFS's 
> journaling only provides filesystem metadata consistency which is why 
> you get files full of NULLs after a crash/power out. MTAs rely on fsync 
> calls and how a filesystem behaves in regards to fsync requests is the 
> real determiner of whether there is a data guarantee or not. XFS does 
> not provide data guarantee. It, at best, provides a metadata guarantee. 
> XFS should not be used for mta queues unless it is in conjunction with 
> hardware raid that has a bbu cache. XFS is best suited for streaming 
> applications where the data loss is tolerated.
>
>   
Sorry, but that is completely incorrect. Applications that use fsync are 
safe with any filesystem - fsync forces the modified buffers to *disk*, 
so all discussions about os and filesystem caching are irrelivant[1].

Cheers

Mark

[1] You do have to consider whether the underlying disk firmware honors 
the fsync request to flush - this is why scsi disks are still often 
preferred for data critical situations. It is only recently with the 
advent of more advanced sata firmware that they too are now reasonably 
usable in those situations (tho you want to leave write barrier support 
enabled then!)