Re: write-behind performance ... or how behind can write-behind write

[Paul Clements <paul.clements@steeleye.com>]

Georgi Alexandrov wrote:

> Generally with the healthy array I'm getting the write performance of
> the SATA disk alone (in terms of requests/sec issued to the disk and
> bytes/sec written). The SATA disk is obviously a bottleneck even with
> the write-behind option set(2).

write-behind can help with two things:

1) overcoming latency (say one disk is on the network -- it may be the 
same speed as the source disk, but it takes longer round-trip for each 
I/O to complete)

2) temporary slowness of a device (say at a peak in I/O) -- the queue 
can temporarily hide the slowness of the secondary disk, but this won't 
last very long -- if writes continue at a pace faster than the disk can 
handle (i.e., the queue gets filled) then the array drops back to 
non-write-behind behavior

> So the questions is How behind can write-behind write? And can we get a
> better performance in a similar setup.

By default, it queues up 256 writes. This can be increased, but I've 
actually seen worse performance in some cases -- not sure why. I haven't 
had the time to dig into it and figure it out.

--
Paul