Matt, how about running the same disk benchmark(s), with sync=disabled vs
sync=enabled and the ZIL accelerator in place?


[] On Behalf Of Matt Van Mater
Sent: Monday, October 01, 2012 9:19 AM
Subject: [zfs-discuss] Best way to measure performance of ZIL

Hi all, 

I currently have a OCZ Vertex 4 SSD as a ZIL device and am well aware of
their exaggerated claims of sustained performance.  I was thinking about
getting a DRAM based ZIL accelerator such as Christopher George's DDRDive,
one of the STEC products, etc.  Of course the key question i'm trying to
answer is: is the price premium worth it?  
--- What is the (average/min/max) latency of my current ZIL device?
--- How how will improving ZIL latency improve performance of my pool that
is used as a NFS share to ESXi hosts which forces sync writes only (i.e will
it be noticeable in an end-to-end context)?

I've been looking around and haven't found a succinct way of measuring the
latency of an individual device when used as a ZIL in a zpool.  I am
experienced using iometer to measure individual devices, but as always it
isn't easy to decompose that benchmark to determine where the bottlenecks
occur.  It is possible to run multiple tests with multiple hardware
configurations and compare iometer results, but i'm trying to avoid having
to buy the ZIL accelerator just to "see" what the impact would be.  I'd hate
to buy an expensive device just to find out that NFS is the main latency
bottleneck all along and the ZIL is inconsequential.  

The only thing I can think of is to create a zpool consisting of a single
OCZ Vertex 4 SSD, sharing it as NFS and running iometer on a VM and see how
it performs in a "real world" use-case... but that doesn't necessarily
isolate how well it performs as a ZIL.

Another thing I found is Brendan Gregg's latencytop.d dtrace script
but the examples I've seen don't seem to isolate an individual disk.  Are
there any other useful scripts, commands or resources I should be aware of?

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