Jonathan Edwards writes: > > On Jan 5, 2007, at 11:10, Anton B. Rang wrote: > > >> DIRECT IO is a set of performance optimisations to circumvent > >> shortcomings of a given filesystem. > > > > Direct I/O as generally understood (i.e. not UFS-specific) is an > > optimization which allows data to be transferred directly between > > user data buffers and disk, without a memory-to-memory copy. > > > > This isn't related to a particular file system. > > > > true .. directio(3) is generally used in the context of *any* given > filesystem to advise it that an application buffer to system buffer > copy may get in the way or add additional overhead (particularly if > the filesystem buffer is doing additional copies.) You can also look > at it as a way of reducing more layers of indirection particularly if > I want the application overhead to be higher than the subsystem > overhead. Programmatically .. less is more.
Direct IO makes good sense when the target disk sectors are set a priori. But in the context of ZFS, would you rather have 10 direct disk I/Os or 10 bcopies and 2 I/O (say that was possible). As for read, I can see that when the load is cached in the disk array and we're running 100% CPU, the extra copy might be noticeable. Is this the situation that longs for DIO ? What % of a system is spent in the copy ? What is the added latency that comes from the copy ? Is DIO the best way to reduce the CPU cost of ZFS ? The current Nevada code base has quite nice performance characteristics (and certainly quirks); there are many further efficiency gains to be reaped from ZFS. I just don't see DIO on top of that list for now. Or at least someone needs to spell out what is ZFS/DIO and how much better it is expected to be (back of the envelope calculation accepted). Reading RAID-Z subblocks on filesystems that have checksum disabled might be interesting. That would avoid some disk seeks. To served the subblocks directly or not is a separate matter; it's a small deal compared to the feature itself. How about disabling the DB checksum (it can't fix the block anyway) and do mirroring ? -r > _______________________________________________ > zfs-discuss mailing list > zfs-discuss@opensolaris.org > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
