Scott Fagg wrote:
Would realtime have any use outside of video-streaming ? What sort of
benefit might it offer other apps, e.g. db servers ?
Originally the realtime subvolume was designed for media streaming
applications. It was coupled with another Irix feature called GRIO
or guaranteed rate I/O. Basically a scheme for reserving disk
bandwidth for specific I/O streams. At this time it only supported
There is code in there now to do buffered I/O as well, I cannot say I
have used it at all.
The basic differences are:
o The realtime device only gets file data I/O, there is no metadata
on the device at all. You need another partition for this. All the
inodes, extent information, freespace maps etc live on this
partition - known as the data subvolume.
o In order to get a file to live on the realtime subvolume, you have
to use a special ioctl call on the file descriptor after it is
opened and before any space is allocated.
o Space is allocated in muliples of the real time extent size, using
a different allocator which uses a binary chop algorithm to parcel
out disk space. i.e. the first inode gets space starting at the
begining of the space, the second starts in the middle, the third
goes 25% of the way into the space etc. It tends to do a good job
of keeping things contiguous.
o The allocator was also intended to be deterministic, so that the time
taken to allocate space should always be about the same.
So if you want big fat files which get layed out in one chunk, and
spindles where no other I/O gets in the way of the data I/O, then
it can be handy. It does sound like something a database might
like to use.