20TB Storage System (fsck????)

[Chuck Swiger]

Piotr KUCHARSKI wrote:
> On Wed, Sep 03, 2003 at 02:37:50PM -0400, Chuck Swiger wrote:
>>I'm wary of the write-performance hit from putting too many drives wide 
>>in a RAID-5 (or -5,0) configuration. 
> 
> How many is "too many"?

At one point, the advice used to be to use between four and seven disks for a 
RAID-5 volume.  For example, the Apple XServe RAID box has 14 bays, but Apple 
seems to recommend configuring it as two 7-drive RAID-5 volumes, rather than a 
single 14-drive-wide RAID-5 volume.

 > Or, rather, what are write-performance penalties
> when using sixteen disks in one hw raid5 set? (With two raid volumes,
> 2TB and 1.75TB available for OS.)

Find yourself a buncha small files-- a CVS repository, or /usr/ports will do, 
and compare write performance to a single drive versus RAID-5.  Basicly, you get 
all of the drives in the RAID-set scribbling away at a fraction of the write 
speed of a single drive, yes?  Three disk transactions per write, versus one?

Also note that all this disk activity requires three times the I/O bandwidth, 
interrupts, and assorted overhead.  If the OP has hardware RAID which is 
designed to support a wide array, OK, but setting up a too-wide a RAID-5 array 
means that things like the system bus may bottleneck performance, rather than 
the drives.

Normally, disk I/O speed is the limiting factor, and your bus and memory are 
sitting around waiting for the DMA to complete (well, being used by the CPU to 
run other processes).  Let's put it this way, things don't go faster when the 
drives are waiting for the bus to become available, rather than vice-versa.  :-)

-- 
-Chuck