[PATCH 0/4] dm-latency: Introduction

[Coly Li <i@coly.li>]

From: Coly Li <bosong.ly@alibaba-inc.com>

Dm-latency patch set is an effort to measure hard disk I/O latency on
top of device mapper layer. The original motivation of I/O latency
measurement was to predict hard disk failure by machine learning method,
I/O latency information was one of the inputs sent to machine learning
model.

This patch set was written in Aug~Sep 2013, I deployed it on many
servers of Alibaba cloud infrastructure. After running for weeks, some
interesting data about hard disk I/O latency was observed. In 2013, I
gave a talk on OpenSuSE Conference about this topic
(http://blog.coly.li/docs/osc13-coly.pdf).

When generating time stamp for I/O request, clock source is a global
unique resource which is protected by spin-locks. Dm-latency was tested
on SAS/SATA hard disk and SATA SSD, it worked well as expected. Running
dm-latency on PCI-e or NVMe SSD should work (I didn't test), but there
will be spin-lock scalability issue, when accessing clock source for
time stamping.

Dm-latency is good for I/O latency measurement to hard disk based
storage, no matter local or distributed storage via network. For PCI-e
or NVMe SSD, I suggest people to look for device provided statistic
information, if there is.

The code is very simple, there is no resource allocation/destory, no
spin_lock/spin_unlock. The patch set gets merged into Alibaba kernel
more than 1 year, no bug reported in last 12 months.

This patch set has 4 patches,
- [PATCH 1/4] dm-latency: move struct mapped_device from dm.c to dm.h
- [PATCH 2/4] dm-latency: add I/O latency measurement in device mapper
- [PATCH 3/4] dm-latency: add sysfs interface
- [PATCH 4/4] dm-latency: add reset function to dm-latency in sysfs
interface
All these patches are rebased on Linux 4.0-rc1.

Today Laurence Oberman from Redhat sent me an email asking whether this
patch set is upstream merged, because he is thinking of pulling this
patch set into their kernel. I'd like to maintain this patch set, hope
it could be merged.

Thanks in advance.

Coly Li