Neil Brown wrote:

On Tuesday March 7, raziebe@gmail.com wrote:
 

quoted

Neil.
what is the stripe_cache exacly ?
   

In order to ensure correctness of data, all IO operations on a raid5
pass through the 'stripe cache'  This is a cache of stripes where each
stripe is one page wide across all devices.

e.g. to write a block, we allocate one stripe in the cache to cover
that block, pre-read anything that might be needed, copy in the new
data and update parity, and write out anything that has changed.
 

I can see that you would have to read the old data and parity blocks for 
RAID-5, I assume that's what you mean by "might be needed" and not a 
read of every drive to get the data to rebuild the parity from scratch. 
That would be not only slower, but require complex error recovery on an 
error reading unneeded data.

Similarly to read, we allocate a stripe to cover the block, read in
the requires parts, and copy out of the stripe cache into the
destination.

Requiring all reads to pass through the stripe cache is not strictly
necessary, but it keeps the code a lot easier to manage (fewer special
cases).   Bypassing the cache for simple read requests when the array
is non-degraded is on my list....

It sounds as if you do a memory copy with each read, even if a read to 
user buffer would be possible. Hopefully I'm reading that wrong.

-- 
bill davidsen [off-list ref]
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979