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

Dan Nelson dnelson at allantgroup.com
Wed Mar 24 17:55:49 UTC 2010

In the last episode (Mar 24), Bob Friesenhahn said:
> 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.

There isn't a sysctl for it by default, but the following patch will enable
a vfs.zfs.write_limit_override sysctl:

Index: dsl_pool.c
RCS file: /home/ncvs/src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dsl_pool.c,v
retrieving revision
diff -u -p -r1.4.2.1 dsl_pool.c
--- dsl_pool.c	17 Aug 2009 09:55:58 -0000
+++ dsl_pool.c	11 Mar 2010 08:34:27 -0000
@@ -47,6 +47,11 @@ uint64_t zfs_write_limit_inflated = 0;
 uint64_t zfs_write_limit_override = 0;
 extern uint64_t zfs_write_limit_min;
+SYSCTL_QUAD(_vfs_zfs, OID_AUTO, write_limit_override, CTLFLAG_RW,
+	&zfs_write_limit_override, 0,
+	"Force a txg if dirty buffers exceed this value (bytes)");
 kmutex_t zfs_write_limit_lock;
 static pgcnt_t old_physmem = 0;

> 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.

I had similar problems on a 32GB Solaris server at work.  Note that with
compression enabled, the entire system pauses while it compresses the
outgoing block of data.  It's just a fraction of a second, but long enough
for end-users to complain about bad performance in X sessions.  I had to
throttle back to a 256MB write limit size to make the stuttering go away
completely.  It didn't affect write throughput much at all.

	Dan Nelson
	dnelson at allantgroup.com

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