[Sunhelp] Slow writes compared to reads with DiskSuite

[Dale Ghent]

On Thu, 15 Jun 2000, David Rouse wrote:

| Assuming that this was a fair test, I guess then it is normal to have 
| asymmetrical reads v writes with DiskSuite. I'll also buy the 
| differences between the U1 and U250 mirrors because the U1 has a better 
| setup (more SCSI controllers) I also knew that a software RAID wouldn't 
| provide blazing performance. But should the RAID 5 writes be this bad? 
| Worse than the U5! What would a hardware RAID 5 (like the A1000) give 
| us? Has anyone heard when Sun's RAID card would be available for the 
| internal disks of a U250?

You are running into a natural thing when it comes to RAID 5, especially
RAID 5 done in software.

See, on a RAID 5 volume, the DiskSuite RAID5 driver has to calculate
parity for every block written to the volume. This of course takes much
more CPU time than normal, and the write write does not return (ie: is not
fully commited) until the parity is calculated and the data written to
disk.

The speed with mirroring is faster because you're just writing data, and
not having to calculate any parity at all. This is why if you have the
disk space available, a RAID 0+1 solution is more preferential than a
RAID5 one (of course, RAID5 is better for situation where you are
warehousing alot of data and 95% of your time is spent reading from it
than writing to it. You'll have more disk space this way.)

This is where hardware-based RAID5 systems come into play. The typical
RAID box has a cache of some sort (the A1000 you mentioned comes with 24MB
from the factory) which sits between the server and the RAID controller in
the RAID box. The server sends data over the SCSI (or FC-AL) bus to the
RAID box, and into the cache. A processor on the other side of the cache
takes data from the cache, calculates parity, and writes to the disk
volume. The cache essentially acts as a buffer for writes and as a cache
for reads, and the processor removes the overhead of parity calculation
and volume management from the server.


HTH
/dale