tuning vfs.zfs.vdev.max_pending and solving the issue of ZFS writes choking read IO

[Bob Friesenhahn]

On Wed, 24 Mar 2010, Dan Naumov wrote:
> Has anyone done any extensive testing of the effects of tuning
> vfs.zfs.vdev.max_pending on this issue? Is there some universally
> recommended value beyond the default 35? Anything else I should be
> looking at?

The vdev.max_pending value is primarily used to tune for SAN/HW-RAID 
LUNs and is used to dial down LUN service time (svc_t) values by 
limiting the number of pending requests.  It is not terribly useful 
for decreasing stalls due to zfs writes.  In order to reduce the 
impact of zfs writes, you want to limit the maximum size of a zfs 
transaction group (TXG).  I don't know what the FreeBSD tunable is for 
this, but under Solaris it is zfs:zfs_write_limit_override.

On a large-memory system, a properly working zfs should not saturate 
the write channel for more than 5 seconds.  Zfs tries to learn the 
write bandwidth so that it can tune the TXG size up to 5 seconds (max) 
worth of writes.  If you have both large memory and fast storage, 
quite a huge amount of data can be written in 5 seconds.  On my 
Solaris system, I found that zfs was quite accurate with its rate 
estimation, but it resulted in four gigabytes of data being written 
per TXG.

Bob
--
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/