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Summary

I'm optimizing a system in which synchronous disk writes are a key factor in performance. My current drives are too slow. I have a single Intel 520 drive that performs well enough (<1ms), but I need five drives. I bought five Intel 530 SSDs in hopes they would perform like the Intel 520, but instead they take 10ms to write a single byte to disk under various versions of Linux. Curiously, if I connect the Intel 530 drives over a USB-to-SATA adapter instead of using SATA directly, they're much faster (~200us per write). What's wrong?

Microbenchmark

Here's my microbenchmark:

I normally run this on ext4. (I've tried it on a raw device as well; see negative results section below).

Machines

These are the machines I've tried. The Intel 530 over SATA takes 10ms per write on each of these, and other SSDs take <1ms per write on each of these.

name	type	description	model	distro	kernel	SATA	USB	Purchased
rc66	server	This is the machine type I'd like to use (we have 80 of them).	Supermicro xx	RHEL6	2.6.32	3gbps	2.0	2011
rcmonster	server	This is a newer, beefier machine with Intel 520 drives, but also stuck on RHEL6.	Intel xx	RHEL6	2.6.32	6gbps		2013
flygecko	desktop	This is a newer desktop machine with a newer version of Linux.	Supermicro xx?	Arch	3.12			2013
x1	laptop	This is a laptop with a newer version of Linux.	Lenovo Thinkpad X1 Carbon	Debian Jessie	3.12	mSATA 6gbps	3.0	2012

Disks

These are the disks I've tried:

model	qty	performance
Crucial M4	160
Intel 520	2
Intel ?? on flygecko	1
Intel 530	5
Intel 530 attached over USB-to-SATA adapter	1
Cheap USB thumb drive	1

Negative Results

The first thing I did was update from the DC22 firmware to the current DC33 firmware. No effect.
I tried a machine with 6gbps SATA (rcmonster). No effect.
I tried a machine with a newer kernel (flygecko). No effect.
I tried disabling APM power saving with hdparm -B. No effect.
Did I try different NCQ sizes? Doesn't work on some machines.
I tried changing the I/O scheduler. This shouldn't have an effect since there's only one I/O outstanding at a time. No effect.
I tried running the benchmark on the raw block device rather than an ext4 partition. This helped but only slightly, reducing latency per write from 10ms to about 9ms.

Additional Questions

One of the differences between the Intel 520 and Intel 530 is that the 530 does more aggressive power saving. Is there a way to turn that off?
Are the software paths under Linux for the Intel 520 and the Intel 530 identical?
Would Intel be willing to trade my five 530s in exchange for five 520s?

blktrace measurements

btt