Re: Why is NCQ enabled by default by libata? (2.6.20)
From: Justin Piszcz
Date: Tue Mar 27 2007 - 06:30:57 EST
On Tue, 27 Mar 2007, Tejun Heo wrote:
Justin Piszcz wrote:
Checking the benchmarks on various hardware websites, anandtech,
hothardware and others, they generally all come to the same conclusion if
there is only 1 thread using I/O (single user system) then NCQ off is the
best.
Are they testing using Linux? I/O performance is highly dependent on
workload and scheduling, so result on windows wouldn't be very useful.
Posting some links here would be nice.
I see 30-50MB/s faster speeds with NCQ turned off on two different SW
RAID5s.
You're testing raptors, right? If the performance drop is that drastic and
consistent over different workloads, we'll have to disable NCQ for raptors.
I'm not sure about other drives. Care to perform tests over more popular
ones (e.g. recent seagates or 7200rpm wds)?
--
tejun
You are correct, it definitely depends upon the workload, and a lot of the
benchmarks do use Windows; however, I will have to check later, I recall
finding a few that did test under Linux.
For a plain untar with lots of small files, the benefit is not as big as
sequential reads/writes of big files; however, there is still an
improvement:
Raid5 Quad 150 Raptor (NCQ)
# time sh -c 'tar xf linux-2.6.20.tar; sync'
real 0m21.721s
user 0m0.174s
sys 0m1.541s
Raid5 Quad 150 Raptor (NO NCQ)
# time sh -c 'tar xf linux-2.6.20.tar; sync'
real 0m16.761s
user 0m0.195s
sys 0m1.361s
Raid5 Six 400GB Sata Drives (NO NCQ)
# time sh -c 'tar xf linux-2.6.20.tar; sync'
real 0m54.844s
user 0m0.189s
sys 0m1.432s
Raid5 Six 400GB Sata Drives (NCQ)
# time sh -c 'tar xf linux-2.6.20.tar; sync'
real 1m7.322s
user 0m0.194s
sys 0m1.492s
Justin.
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