pauses sync'ing between tmpfs and disk on Linux 2.4.x

[Ray Van Dolson]

I've set up a 1GB tmpfs filesystem on a system with a single IDE disk and
2GB's of memory.  I'm storing a large amount of RRD files (~300MB) on the
tmpfs filesystem to make their generation a bit speedier... this part works
great.

However, I want to rsync these files over from time to time to a directory on
the local filesystem (same physical server).  I'm using rsync 2.6.4pre3 and
am hoping to understand a bit better what is happening.

When I run rsync -av /path/to/tmpfs /path/to/diskdir things move along pretty
fast for while, then there's a big pause... sometimes for up to 30+ seconds
where nothing seems to be happening, but all IO on the system ceases (can't do
anything in another xterm).  Then rsync starts moving along again for a while,
and then pauses again... after about three such pauses it finishes the entire
rsync process.

I'm wondering what I can do to speed things up... perhaps whatever processes
that write to the tmpfs filesystem are fighting with rsync... but doesn't
rsync just need read access?

I've also tried with -W (copy whole file) and used a smaller -B value.
Anyways, I'll explain a bit about the curren scenario:

1. Daemon on system receives data from remote devices.
2. Daemon calls rrdtool to write a .rrd file to the tmpfs filesystem.
3. rsync runs periodically (every 5 minutes) to sync up the tmpfs filesystem
   with a directory on the local filesystem.

The tmpfs filesystem is a directory with about 100 directories inside of it,
each containing rrd files.  I'm considering using a script to just rsync one
of these subdirectories at a time over a period of time to "distribute" the
load.

The main issue is that while rsync is running and enters these random 30
second "pauses", no IO can happen and things get really backed up on the
system and it ends up being faster for me to just dump the tmpfs completely
and go back to writing directly to disk as slow as that was...

Any suggestions?

-- 
Ray Van Dolson
Linux/Unix Systems Administrator
DigitalPath Networks