[Sunhelp] Slow writes compared to reads with DiskSuite

[Dale Ghent]

On Thu, 15 Jun 2000, Dale Ghent wrote:

| 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.

Let me add to this and say that the reason writes are sped up some much
with a hardware RAID cache is because the cache controller in the RAID
array tells the OS that the write is done as soon as the write hits the
cache. The aformentioned processor in the RAID box takes care of actually
writing the data to the disks behind the scenes (this is why you so
battery backups for RAID boxes with cache... if power were lost to the
array, there could be uncommitted writes still in the cache. The battery
keeps the data alive in the cache until power is restored and the data can
be written to the disks.

/dale