Getting The Most Out Of Your Flash/SSDs

Getting The Most Out
Of Your Flash/SSDs
Young Paik
Technical Marketing Director
young@aerospike.com

Aerospike aer . o . spike [air-oh- spahyk]
noun, 1. tip of a rocket that enhances speed and stability

Introduction
Flash/SSDs (used interchangeably) are still
relatively new.
Getting the most out of them requires a good
understanding of how they work and how
Aerospike uses them.

© 2014 Aerospike. All rights reserved. Confidential

Pg. 2

Agenda









SSDs vs. Rotational Drives
What Aerospike Does To Make The Most of SSDs
The Factors That Most Improve The Performance of
SSDs
Testing SSDs
More on Testing SSDs
Even more on Testing SSDs
Final Preparations For Your Drives


Pg. 3

Differences Matter
Some will tell you that their databases will work
on SSDs and that no changes are necessary.
There are differences between SSDs and
rotational drives that are important. You must do
more than simply swap out your old drive and put
in an SSD to get the best performance.


Pg. 5

Comparing Old and New
There are differences between rotational and SSD disks that are independent of the
database you are using.
Characteristic

Rotational

SSD

Notes

Random read

Poor

Excellent

This is where SSDs shine the most. With no moving
parts, SSDs are clearly the choice for random reads.

Random write

Poor

Good

Similar to reads, but SSDs are not quite as fast with
random writes as they are with reads.

Sequential write

Good

Excellent

Rotational drives narrow the gap here. While they
are close in pure write performance, any reads
during these writes will require the movement of the
heads on rotational drives.

Rewritability
(durability)

Excellent

Poor

This is where SSDs are the weakest. NAND (Flash)
chips have limits to how many times you can write to
the same area. Databases must take this into account
to avoid “hotspots.” Databases that do not are
relying on the operating systems (i.e. the TRIM
command) to alleviate these issues. Aerospike
manages this differently.


Pg. 6

What Aerospike
Does To Make The
Most Of SSDs

Techniques
In order to make the best use of SSDs, Aerospike has
designed an architecture that does the following:
Uses raw disk

Aerospike does not use a file system, which would only slow
down the database.

Writes in large blocks

Rather than trying to write many smaller items, it is much
more efficient to write a few large ones. Aerospike uses
black sizes that are integral multiple of 128 KB.

Reads in small blocks

Reads are done in 512 byte data segments.

Handles
defragmentation on a
regular basis

All databases must delete data. This creates fragmentation
of the data on disk, which makes it harder to use efficiently.
Aerospike does this through a continual process called
defragmentation. This means you do not need the TRIM
command used on most operating systems.

Works with vendors

Aerospike works closely with SSD manufacturers to test
hardware and provide feedback for the best performance.


Pg. 8

Accessing An Object In Aerospike
Writing A New Standard Data Type Record With SSDs

Client

Master Node

DRAM (Index)

SSD (DATA)
1) Client finds Master Node from
partition map.
2) Client makes write request to
Master Node.
3) Master Node make an entry indo
index (in DRAM) and queues write
in temporary write buffer.
4) Master Node coordinates write
with replica nodes (not shown).
5) Master Node returns success to
client.
6) Master Node asynchronously writes
data in blocks.
7) Index in DRAM points to location
on SSD.

Asynchronous write

Block size (128 KB by default)


Pg. 9

Defragmentation In Aerospike
How Space Is Freed Up
SSD (DATA)

Aerospike writes the data in large data
blocks.

1
2
3
4
5
6
7
8



Pg. 10

SSD (DATA)

As new data is added to the disk, new
blocks will be continually written to
the SSD.

1
2
3
4
5
6
7
8



Pg. 11

SSD (DATA)

Over time, some records will be
deleted or updated, resulting in
fragmented usage on the flash/SSD
disk. This unused space must be freed
up.

1
2
3
4
5
6
7
8

Block size


Pg. 12

SSD (DATA)

Some databases use a nightly process
called “compaction,” which is an
intensive process. Aerospike runs a
regular process (every few minutes) that
looks for blocks below some level of use
(called the high watermark).

1
2
3
4
5
6
7

In this example, if the high watermark is
50%, blocks 1 and 3 to the left are below
50% occupied. The defragmenter will
take the data in these blocks and merge
then into another block.

8

Block size


Pg. 13

SSD (DATA)

The defragmenter will get write the new
block (block 7) and clear up blocks 1 and
3 for new writes.

1
2
3
4

Because this runs constantly, there is no
special time where the performance of
the database is bad.

5
6
7
8

This algorithm operates best when the
SSD is less than 50% occupied. As disk use
grows above this, the performance of the
defragmenter will decrease.

Block size


Pg. 14

Aerospike Certification Tool (ACT) for SSDs
■ Industry Standard Flash (SSD / PCI-E) Benchmark
■ Open Source Tool used by Flash Vendors to certify drives

The Factors That
Most Improve The
Performance of SSDs

How To Prepare Your System
➤ Select




the correct hardware

SSD
Disk Controller

➤ Configure

the hardware
➤ Configure Aerospike


Pg. 17

Most Important Factors for SSD Performance
Factor

Importance
(rough)

Notes

Interface
(SATA v. PCIe)

Very High

One of the most critical choices is the use of interface. Today, the
difference in price and layout is huge, so is quite easy for customers to
make. If the very low latency is absolutely required, use PCIe. Costs are
2x-5x what they would be on SATA.

Consumer v. Enterprise

Very High

A few years ago the difference between these types was small, but
today very few consumer rated drives pass Aerospike certification.

Make/model

Very High

Differences in specific models from the same maker can be very large.
In some cases, the manufacturer may have quietly made changes to the
hardware and firmware, but not changed the model number.

Disk controller (RAID,
HBA)

Very High

Aerospike prefers direct control of each SSD. RAID controllers will add
latency, without much added benefit (Aerospike is already replicated).

Over-provisioning (OP)

Very High

Over-provisioning allocates space on the drive for use by the controller.
The amount the manufacturer has set will amount varies from one
model to the other. Typical amounts are 6% - 28%.

Used before
NCQ

Scheduler

High

If the SSD has been in use for a long time for other purposes, the disk
will be unevenly worn, causing poor performance.

Medium

Native Command Queuing is a SATA extension that allows the disk to
internally optimize how commands are executed. Rarely a problem on
modern equipment.

Low

This is the I/O scheduler for the Linux kernel. Aerospike prefers the
NOOP scheduler and automatically selects it.


Pg. 18

Selecting The Correct SSD Model
Given the most important factors, obviously it is important to choose the
correct model. Aerospike publishes a list that it updates with information on
models that have passed testing.

These SSDs can be found at:
https://support.aerospike.com/customer/portal/articles/1315402-recommended-ssds


Pg. 19

Selecting The Correct Disk Controller
Warning: Be very careful on the disk controller. Aerospike uses them in a
way that goes against traditional conventional wisdom.

Best practices:
➤ Do not use RAID across the SSDs. Aerospike stores small objects and is
much more sensitive to latency than bandwidth.
➤ When possible, use direct attach (SATA or PCIe)
➤ If you can’t use direct attach try one of the following:




➤
➤
➤

➤

Use HBAs without RAID
Configure each SSD as a separate RAID 0 array

Spread the SSDs among as many controllers as possible
All servers will have a limit to the number of drives that will perform
well. 4 is a common number.
If your company has a standard configuration for Hadoop, these often
have similar hardware needs to Aerospike
Some controllers have special software to boost performance. E.g.
The LSI 2208 chip has the fastpath available for specific models.
Check with your vendor.


Pg. 20

Over-provisioning (OP)
OP can make the difference between bad performance
and great performance.
2 types of OP:



Manufacturer’s OP
User OP

Manufacturer’s typically set 6%-8% for consumer rated
drives and 14%-28% for enterprise rated. This varies
depending on the model and capacity.


Pg. 21

Over-Provisioning: What You Can Do
Adding user over-provisioning can be done in one
of 2 ways:





Manufacturer’s software
Host Protected Area (HPA) – Linux has a command
that can use called hdparm that you can use to set
the HPA (Host Protected Area)
Disk partitions – You can also leave some space on
the disk as unpartitioned. The remainder of the
space will be used by the controller.

No matter which method you use, it is good to
reserve 21% for use by the controller.


Pg. 22

Comparing OP Methods
HPA (Host Protected Area) Partitioning
Ease of use

Use hdparm 9.37+

Use built-in fdisk command

Most versions of Linux come with earlier
versions.

Performance

Both methods have the same performance

Device ID

Must specify the basic
device (e.g. /dev/sdb)

Must specify the specific
partition (e.g. /dev/sdb1)

Notes

hdparm may not work
through your RAID
controller

All commands must specify
the full partition. Not doing
so may result in using disks
not OPed.


Pg. 23

OP Using Host Protected Area (HPA)
In order to use the HPA, it is easiest to use the
command hdparm (must have version 9.37+). You
can get a copy of this at:
http://sourceforge.net/projects/hdparm/


Pg. 24

OP Using Host Protected Area (HPA) - Example
First find the number of sectors (must be root or use sudo)
> sudo /opt/hdparm-9.43/hdparm -N /dev/sdb
/dev/sdb:
max sectors
= 500118192/500118192, HPA is disabled

Then multiply by the OP amount (79%):
500,118,192 x 0.79 = 395,093,372 sectors
> sudo /opt/hdparm-9.43/hdparm -Np395093372 --yes-i-know-what-iam-doing /dev/sdb
/dev/sdb:
setting max visible sectors to 395093372 (permanent)
max sectors
= 395093372/500118192, HPA is enabled

Finally reboot. This is actually necessary to make sure the new
settings take hold.


Pg. 25

OP Using Partitions - Example
In this example we will over-provision the disk /dev/sdb by creating a single
partition that is 79% of the overall capacity (15121 = 19140 x 0.79):
> sudo /sbin/fdisk /dev/sdb
Command (m for help): n
Command action
e
extended
p
primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-19140, default 1): 1
Last cylinder, +cylinders or +size{K,M,G} (1-19140, default 19140): 15121
Command (m for help): p
Disk /dev/sdb: 157.4 GB, 157437394944 bytes
255 heads, 63 sectors/track, 19140 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xeff8f3ae
Device Boot
Start
End
Blocks
Id
/dev/sdb1
1
15121
121459401
83
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.

System
Linux

We recommend rebooting the server once this has been done. Note that for this
disk you will need to use /dev/sdb1 as the device.

Pg. 26

Did You Choose Well?
The only way to be sure how these all work in
your environment is to test.
The best way is to use the Aerospike Certification
Test (ACT). This is a tool that has been Open
Sourced by Aerospike for testing SSD
configurations.


Pg. 28

Aerospike ACT
The ACT accesses SSDs similarly to the way the
Aerospike database does: reads with concurrent large
block writes. By default the tests run for a period of
24 hours.
The tests are based on factors of “x”.
1x represents 2,000 reads/s and 1,000 writes/s per SSD
2x represents 4,000 reads/s and 2,000 writes/s per SSD
etc.

1x represents decent performance of an SSD in 2010. Today,
several models of SSDs perform well at 3x. These tests must be
run for 24 hours to ensure stability.
Test with greater and greater “x” levels until the SSD performs
poorly.

Pg. 29

Methodology For Single Disk
The basic methodology is:
➤ Test a single drive at 3x
➤ Retest with different configurations (OP, disk
controller, settings, etc)
➤ If the best of these pass standards, retest at a
higher x. If not, lower test standards to 2x.
➤ Repeat these tests until you have discovered the
limits of performance.
➤ Finally, test at twice the highest level passed to
make sure the disk can handle large bursts of
traffic. If a disk passes the test criteria at Nx and
completes the test at twice that speed, it is said to
pass at Nx.

Pg. 30

What Is Passing?
Aerospike defines passing with the following
criteria:
No more than 5% of all transactions exceed 1 ms
No more than 1% of all transactions exceed 8 ms
No more than 0.1% of all transactions exceed 64 ms
You may determine your own.


Pg. 31

Analyzing The Results
When you run the ACT analysis tool, you will see output like this (time slices are
hourly):
trans
device
%>(ms)
%>(ms)
slice
1
2
4
8 16 32 64
1
2
4
8 16 32 64
----- ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ -----1 21.01 1.59 0.04 0.00 0.00 0.00 0.00 20.88 1.57 0.04 0.00 0.00 0.00 0.00
2 23.34 1.58 0.03 0.00 0.00 0.00 0.00 23.19 1.56 0.03 0.00 0.00 0.00 0.00
3 23.89 1.66 0.04 0.00 0.00 0.00 0.00 23.75 1.64 0.04 0.00 0.00 0.00 0.00
4 25.39 2.06 0.05 0.00 0.00 0.00 0.00 25.24 2.03 0.05 0.00 0.00 0.00 0.00
5 26.72 2.41 0.07 0.00 0.00 0.00 0.00 26.57 2.38 0.07 0.00 0.00 0.00 0.00
6 26.68 2.37 0.07 0.00 0.00 0.00 0.00 26.53 2.34 0.06 0.00 0.00 0.00 0.00
7 24.93 1.82 0.04 0.00 0.00 0.00 0.00 24.78 1.79 0.04 0.00 0.00 0.00 0.00
8 25.61 1.99 0.05 0.00 0.00 0.00 0.00 25.46 1.97 0.05 0.00 0.00 0.00 0.00
9 25.68 1.96 0.05 0.00 0.00 0.00 0.00 25.53 1.94 0.05 0.00 0.00 0.00 0.00
10 26.79 2.28 0.06 0.00 0.00 0.00 0.00 26.64 2.25 0.06 0.00 0.00 0.00 0.00
11 24.69 1.63 0.03 0.00 0.00 0.00 0.00 24.54 1.61 0.03 0.00 0.00 0.00 0.00
12 25.73 1.92 0.04 0.00 0.00 0.00 0.00 25.58 1.90 0.04 0.00 0.00 0.00 0.00
13 26.86 2.26 0.06 0.00 0.00 0.00 0.00 26.70 2.23 0.06 0.00 0.00 0.00 0.00
14 26.17 2.03 0.05 0.00 0.00 0.00 0.00 26.02 2.01 0.05 0.00 0.00 0.00 0.00
15 26.40 2.10 0.05 0.00 0.00 0.00 0.00 26.24 2.07 0.05 0.00 0.00 0.00 0.00
16 26.70 2.18 0.06 0.00 0.00 0.00 0.00 26.54 2.15 0.05 0.00 0.00 0.00 0.00
17 26.57 2.13 0.05 0.00 0.00 0.00 0.00 26.41 2.11 0.05 0.00 0.00 0.00 0.00
18 26.53 2.11 0.05 0.00 0.00 0.00 0.00 26.37 2.09 0.05 0.00 0.00 0.00 0.00
19 26.53 2.11 0.05 0.00 0.00 0.00 0.00 26.37 2.08 0.05 0.00 0.00 0.00 0.00
20 25.43 1.79 0.04 0.00 0.00 0.00 0.00 25.27 1.77 0.04 0.00 0.00 0.00 0.00
21 27.56 2.40 0.06 0.00 0.00 0.00 0.00 27.40 2.37 0.06 0.00 0.00 0.00 0.00
22 27.61 2.43 0.07 0.00 0.00 0.00 0.00 27.45 2.40 0.07 0.00 0.00 0.00 0.00
23 25.21 1.71 0.04 0.00 0.00 0.00 0.00 25.05 1.68 0.04 0.00 0.00 0.00 0.00
24 26.61 2.10 0.05 0.00 0.00 0.00 0.00 26.45 2.08 0.05 0.00 0.00 0.00 0.00
----- ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ -----avg 25.78 2.03 0.05 0.00 0.00 0.00 0.00 25.62 2.00 0.05 0.00 0.00 0.00 0.00
max 27.61 2.43 0.07 0.00 0.00 0.00 0.00 27.45 2.40 0.07 0.00 0.00 0.00 0.00


Pg. 32

Methodology For Multiple Disks
In this case, you already know the performance of
a single drive. What you are actually testing for is
if this will scale linearly with the controller(s) you
have.
➤ Test 2 drives in parallel and increase the
number of drives until the performance is
obviously unacceptable or you have reached the
number of drives you wish to test.
As with the single disk, if a disk setup passes the
test criteria at Nx and completes the test at
twice that speed, it is said to pass at Nx.

Pg. 33

Running ACT Tests
In order to run ACT tests (e.g. for drive /dev/sdb). This will
require root or sudo.
1.

2.
3.
4.

5.

Download and compile the ACT. Follow the included directions to
compile.
http://aerospike.github.io/act/
Prepare the drive(s) for use:
<ACT_DIR>/actprep /dev/sdb
Create a config file for the ACT run
python <ACT_DIR>/act_config_helper.py
Execute the ACT on the config file (since these will run for a long
time, it is useful to put it into the background.
<ACT_DIR>/act [config_file] > [log_file] &
Test to make sure it is running and outputting data. The “-t 10”
means to put the data into 10 second slices (default is 3600).
<ACT_DIR>/latency_calc/act_latency.py –l [log_file] –t 10

6.

Wait for test to complete (24 hours)

Pg. 34

Example: Creating Config Files
> python act_config_helper.py
Enter the number of devices you want to create config for: 1
Enter either raw device if over-provisioned using hdparm or
partition if over-provisioned using fdisk
Enter device name # 1(e.g. /dev/sdb or /dev/sdb1): /dev/sdb
Duration for the test (default :24 hours) [ENTER]
Configure test duration ? (N for using default) (y/N) :n
Use advanced mode for configuration ? (y/N) n
"1x" load is 2000 reads per sec and 1000 writes per sec
Enter the load you want to test the devices ( e.g. enter 1 for 1x
test):3
Do you want to Create the config (Save to a file) ? : (y/N) y
Config File actconfig_3x_1d.txt successfully created

The result will be the output file “actconfig_3x_1d.txt”. If you have multiple SSDs, the
the load will be taken for each device. Defaults for the ACT are for small objects (1.5
KB) and can be changed in the advanced options.


Pg. 35

Analyzing The Results
Analyze the final output log.
<ACT_DIR>/latency_calc/act_latency.py –l [log_file]
trans
device
%>(ms)
%>(ms)
slice
1
2
4
8 16 32 64
1
2
4
8 16 32 64
----- ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ -----1 21.01 1.59 0.04 0.00 0.00 0.00 0.00 20.88 1.57 0.04 0.00 0.00 0.00 0.00
2 23.34 1.58 0.03 0.00 0.00 0.00 0.00 23.19 1.56 0.03 0.00 0.00 0.00 0.00
3 23.89 1.66 0.04 0.00 0.00 0.00 0.00 23.75 1.64 0.04 0.00 0.00 0.00 0.00
4 25.39 2.06 0.05 0.00 0.00 0.00 0.00 25.24 2.03 0.05 0.00 0.00 0.00 0.00
5 26.72 2.41 0.07 0.00 0.00 0.00 0.00 26.57 2.38 0.07 0.00 0.00 0.00 0.00
6 26.68 2.37 0.07 0.00 0.00 0.00 0.00 26.53 2.34 0.06 0.00 0.00 0.00 0.00
7 24.93 1.82 0.04 0.00 0.00 0.00 0.00 24.78 1.79 0.04 0.00 0.00 0.00 0.00
8 25.61 1.99 0.05 0.00 0.00 0.00 0.00 25.46 1.97 0.05 0.00 0.00 0.00 0.00
9 25.68 1.96 0.05 0.00 0.00 0.00 0.00 25.53 1.94 0.05 0.00 0.00 0.00 0.00
10 26.79 2.28 0.06 0.00 0.00 0.00 0.00 26.64 2.25 0.06 0.00 0.00 0.00 0.00
11 24.69 1.63 0.03 0.00 0.00 0.00 0.00 24.54 1.61 0.03 0.00 0.00 0.00 0.00
12 25.73 1.92 0.04 0.00 0.00 0.00 0.00 25.58 1.90 0.04 0.00 0.00 0.00 0.00
13 26.86 2.26 0.06 0.00 0.00 0.00 0.00 26.70 2.23 0.06 0.00 0.00 0.00 0.00
14 26.17 2.03 0.05 0.00 0.00 0.00 0.00 26.02 2.01 0.05 0.00 0.00 0.00 0.00
15 26.40 2.10 0.05 0.00 0.00 0.00 0.00 26.24 2.07 0.05 0.00 0.00 0.00 0.00
16 26.70 2.18 0.06 0.00 0.00 0.00 0.00 26.54 2.15 0.05 0.00 0.00 0.00 0.00
17 26.57 2.13 0.05 0.00 0.00 0.00 0.00 26.41 2.11 0.05 0.00 0.00 0.00 0.00
18 26.53 2.11 0.05 0.00 0.00 0.00 0.00 26.37 2.09 0.05 0.00 0.00 0.00 0.00
19 26.53 2.11 0.05 0.00 0.00 0.00 0.00 26.37 2.08 0.05 0.00 0.00 0.00 0.00
20 25.43 1.79 0.04 0.00 0.00 0.00 0.00 25.27 1.77 0.04 0.00 0.00 0.00 0.00
21 27.56 2.40 0.06 0.00 0.00 0.00 0.00 27.40 2.37 0.06 0.00 0.00 0.00 0.00
22 27.61 2.43 0.07 0.00 0.00 0.00 0.00 27.45 2.40 0.07 0.00 0.00 0.00 0.00
23 25.21 1.71 0.04 0.00 0.00 0.00 0.00 25.05 1.68 0.04 0.00 0.00 0.00 0.00
24 26.61 2.10 0.05 0.00 0.00 0.00 0.00 26.45 2.08 0.05 0.00 0.00 0.00 0.00
----- ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ -----avg 25.78 2.03 0.05 0.00 0.00 0.00 0.00 25.62 2.00 0.05 0.00 0.00 0.00 0.00
max 27.61 2.43 0.07 0.00 0.00 0.00 0.00 27.45 2.40 0.07 0.00 0.00 0.00 0.00


Pg. 36

Final Preparations
Once you have your hardware properly configured,
there are some final steps before you use the SSDs.
You must blank out the drives (similar to a format
with a filesystem) bye running the dd command on
each of the drives. These can be run in parallel, but
must be done by root or with sudo:
> sudo dd if=/dev/zero of=/dev/<DEVICE_ID> bs=128k &
If you used partitioning to OP the drives, make sure to use the partition
id (e.g. /dev/sdb1).
WARNING: Do not run this on the disk with your operating system
(usually /dev/sda)!


Pg. 38

Troubleshooting Common Issues


Tests show much greater than expected latency





Test won’t complete





Make sure you have properly configured over-provisioning. This is a common issue.
If you are doing a multi-disk test, the problem may lie in a single disk. Variances in
manufacturing may lead to a single drive masking poor latencies for all drives. Also make
sure your drives are fresh. Old drives may have hotspots.
Your load may be overwhelming your controller or the drive. A log message will let you
know if it is stopping because it cannot keep up.
If there is no error message in the log, sometimes logging out of the server will stop the
ACT process. You must use nohup or a similar mechanism to ensure the process will run
for the full 24 hours.

Operating system gives odd errors


You may have inadvertently run actprep or dd on the OS drive. Even the best of us have
done this.


Pg. 39

Thank You
Send all questions/comments/complaints to
YOUNG PAIK
YOUNG@AEROSPIKE.COM

Getting The Most Out Of Your Flash/SSDs

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