Wiping disks¶

DANGER:

As mentioned above, the stressant commandline tool can be used to directly wipe a disk with the fio(1) command which is actually a disk-testing command that is abused for that purpose. You may not have fio(1) installed on your machine, however, so you may also use the venerable badblocks(8) command to test disks, without wiping them:

badblocks -nsv /dev/sdc

You can also wipe disks with the -w flag:

badblocks -wsv /dev/sdc

Be aware, however, that the effect of this will vary according to the physical medium. For example, data may be recovered old spinning hard drives (HDD) if only the above technique is used. For that purpose, you should use a tool like nwipe(1) that erases disks using multiple passes and patterns:

nwipe --autonuke --nogui --method=random --verify=off --logfile=nwipe.log /dev/sdc

Those tools are also ineffective on solid state drives (SSD) as they have a more complex logic layer and different layout semantics. For this, you need to use a “ATA secure erase” procedure using the hdparm(8) command:

hdparm --user-master u --security-set-pass Eins /dev/sdc
time hdparm --user-master u --security-erase Eins /dev/sdc

More information about this procedure is available in the ATA wiki.

NOTE:

Testing disks¶

A good way to test disks is to wipe them, as above, but that’s obviously destructive. Sometimes you might want to just test the disk’s performance by hand, without wiping anything. Stressant ships with fio(1) and bonnie++(1) for that purpose. The latter is probably the simplest to use:

bonnie++ -s 4G -d /mnt/disk/ -n 1024

Make sure the file size (-s) is at least twice the main memory (see free -h). The /mnt/disk directory should be writable by the current user as well.

Stressant itself, when disk tests are enabled, will run the following commands:

dd bs=1M count=512 conv=fdatasync if=/dev/zero of=/mnt/disk/testfile
dd bs=1M count=512 conv=fdatasync if=/mnt/disk/testfile of=/dev/null
hdparm -Tt /dev/disk
smartctl -t long /dev/disk

Those provide a quick overview of basic disk statistics as well.

More elaborate workloads can be done with fio. A simple benchmark could be:

fio --name=stressant --group_reporting --directory=/mnt/disk --size=100M

That is a basic read test. The result here, on a Western Digital Blue M.2 500GB Internal SSD (WDS500G1B0B) with LUKS encryption, LVM and ext4, looks like:

Run status group 0 (all jobs):


   READ: bw=267MiB/s (280MB/s), 267MiB/s-267MiB/s (280MB/s-280MB/s), io=100MiB (105MB), run=374-374msec
Disk stats (read/write):


    dm-3: ios=323/0, merge=0/0, ticks=484/0, in_queue=484, util=70.99%, aggrios=511/0, aggrmerge=0/0, aggrticks=764/0, aggrin_queue=764, aggrutil=76.86%


    dm-0: ios=511/0, merge=0/0, ticks=764/0, in_queue=764, util=76.86%, aggrios=511/0, aggrmerge=0/0, aggrticks=547/0, aggrin_queue=576, aggrutil=73.55%


  sdb: ios=511/0, merge=0/0, ticks=547/0, in_queue=576, util=73.55%

A more realistic workload will ignore the cache (--direct=1), include random (--readwrite=randrw) or sequential writes (--readwrite=readwrite), and parallelize the test to put more pressure on the disk (--numjobs=4):

$ fio --name=stressant --group_reporting --directory=test --size=100M --readwrite=randrw --direct=1 --numjobs=4
Run status group 0 (all jobs):


   READ: bw=45.8MiB/s (48.0MB/s), 45.8MiB/s-45.8MiB/s (48.0MB/s-48.0MB/s), io=199MiB (209MB), run=4346-4346msec


  WRITE: bw=46.2MiB/s (48.5MB/s), 46.2MiB/s-46.2MiB/s (48.5MB/s-48.5MB/s), io=201MiB (211MB), run=4346-4346msec
Disk stats (read/write):


    dm-3: ios=49674/50087, merge=0/0, ticks=10028/3912, in_queue=13972, util=97.22%, aggrios=50982/51423, aggrmerge=0/0, aggrticks=10204/3852, aggrin_queue=14092, aggrutil=96.62%


    dm-0: ios=50982/51423, merge=0/0, ticks=10204/3852, in_queue=14092, util=96.62%, aggrios=50982/51423, aggrmerge=0/0, aggrticks=9042/2598, aggrin_queue=11224, aggrutil=92.54%


  sdb: ios=50982/51423, merge=0/0, ticks=9042/2598, in_queue=11224, util=92.54%

There is, of course, way more information shown by the default fio output, including latency distribution, but those are the numbers people first look for.

Parameters can be stored in a job file, passed as an argument to fio. Examples are available in /usr/share/doc/fio/examples.

NOTE:

Testing flash memory¶

Flash memory cards are known to sometimes be “fake”, that is, they misreport the actual capacity of the card or the bandwith available. The stressant distribution therefore recommends a tool called f3 which allows you to perform tests on the memory card. For example, this is a probe on a honest memory card:

$ sudo f3probe --destructive --time-ops /dev/sdb
F3 probe 6.0
Copyright (C) 2010 Digirati Internet LTDA.
This is free software; see the source for copying conditions.
WARNING: Probing normally takes from a few seconds to 15 minutes, but


         it can take longer. Please be patient.
Good news: The device `/dev/sdb' is the real thing
Device geometry:


             *Usable* size: 30.00 GB (62916608 blocks)


            Announced size: 30.00 GB (62916608 blocks)


                    Module: 32.00 GB (2^35 Bytes)


    Approximate cache size: 0.00 Byte (0 blocks), need-reset=no


       Physical block size: 512.00 Byte (2^9 Bytes)
Probe time: 4'57"


 Operation: total time / count = avg time


      Read: 3.07s / 4815 = 637us


     Write: 4'51" / 4192321 = 69us


     Reset: 324.5ms / 1 = 324.5ms

WARNING:

Note that older versions of f3probe(1) (6.0 or earlier) will have trouble doing its job unless the card is connected through a USB reader. Newer versions can deal with normal block devices, provided that you pass the magic --reset-type=2 argument. Here’s such an example, on a fake MicroSD card that is labeled and announced as 32GB but is actually closer to 16GB:

root@curie:/home/anarcat/backup# ~anarcat/dist/f3/f3probe --destructive --time-ops --reset-type=2  /dev/mmcblk0
F3 probe 6.0
Copyright (C) 2010 Digirati Internet LTDA.
This is free software; see the source for copying conditions.
WARNING: Probing normally takes from a few seconds to 15 minutes, but


         it can take longer. Please be patient.
Bad news: The device `/dev/mmcblk0' is a counterfeit of type limbo
You can "fix" this device using the following command:
f3fix --last-sec=30983327 /dev/mmcblk0
Device geometry:


             *Usable* size: 14.77 GB (30983328 blocks)


            Announced size: 31.25 GB (65536000 blocks)


                    Module: 32.00 GB (2^35 Bytes)


    Approximate cache size: 7.00 MB (14336 blocks), need-reset=no


       Physical block size: 512.00 Byte (2^9 Bytes)
Probe time: 2'29"


 Operation: total time / count = avg time


      Read: 1.57s / 32937 = 47us


     Write: 2'27" / 200814 = 736us


     Reset: 2us / 2 = 1us

To repair the device, you can repartition it quickly with the f3fix(1) command, as recommended in the output:

f3fix --last-sec=30983327 /dev/mmcblk0

You will also need to reformat the partition so the new size is taken into account, for example if this is a FAT32 filesystem:

mkfs.fat /dev/mmcblk0p1

You can also perform bandwidth tests with f3read(1) and f3write(1):

pmount /dev/sdb1
f3write /media/sdb1
f3read /media/sdb1

This allows you to detect hidden caches and fake sizes directly as well.

Network performance testing¶

The --iperfServer option of stressant runs a bandwidth test against a predefined (or specified) server. You can, of course, call iPerf directly to run your own client/server tests to find issues in specific routes on the network. The iperf3 package was chosen over the older iperf because public servers are available for the test to work automatically. iperf3 also has interesting performance features like --zerocopy and --file, see iperf3(1) for details.

To run a test, start a server:

iperf3 --server

On another machine, connect to the server:

iperf3 --client 192.0.2.1

This runs a TCP test. You can specify UDP test on the client and disable bandwidth limitations (otherwise UDP tests are limited to 1 Mbit/s):

iperf3 -c 192.0.2.1 --udp --bandwidth 0

To simulate a DDOS condition, you can try multiple clients and run the test for a longer period:

iperf3 -c 192.0.2.1 -u -b 0 --parallel 50 --time 30