Re: problems with real-time option.

[Steve Lord <lord@sgi.com>]

On Thu, 2003-08-28 at 10:41, K Ramesh wrote:
> Hello,
> 	Thanks Nathan, the version of xfsprogs works just fine. I have yet
> another question on realtime subvolumes: Is XFS realtime subvolume in Linux
> comparable to "raw" device binding that's avaliable in linux now? Both seem to
> have the same alignment constraints. Can someone throw light on this issue?
> 	Say if I expected only to write data in to the disk, does using raw I/O
> or realtime subvolumes increases throughput?
> 
> Regards
> Ramesh
> 

The difference between raw and the realtime subvolume is that with a
raw partition you get no naming, and you get one data stream - the
device. With realtime, you get similar access rates, but you can use
the filesystem to manage naming and placement of multiple streams.
An XFS filesystem with a realtime subvolume still has a data subvolume,
and this is where ALL the metadata lives, there is nothing on the
realtime subvol except for the contents of files. The allocator used
to manage the space is wasteful - it basically binary chops the space
when laying things out, but this makes it very hard to fragment the
files.

You also get to use standard utilities to manage files (at least to
rename them, remove them, list them, etc).

So if you only have one data stream, and one set of data to manage,
then raw will be just as good.

Steve

-- 

Steve Lord                                      voice: +1-651-683-3511
Principal Engineer, Filesystem Software         email: lord@sgi.com