Barriers and journaling filesystems

> There is also some potential for improvement by carefully ordering operations so that
barriers (which are normally implemented as "flush all outstanding operations to media"
requests) do not force the writing of blocks which do not have specific ordering requirements.

SCSI actually provides a convenient mechanism for doing this. You can declare a command as
being "Ordered" which will prevent it from being reordered relative to any other commands
which have also been declared as such. Unfortunately AFAIK ATA provides no such niceties. I
also don't know how widely supported this feature is by the various SCSI HBA drivers (I
suspect not very) but it is an option to explore.