Monitoring disks

cryptofuture edited this page Jan 2, 2017 · 11 revisions


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Monitoring Disks' Performance with netdata

Live demo of disk monitoring at:

Performance monitoring for Linux disks is quite complicated. The main reason is the plethora of disk technologies available. There are many different hardware disk technologies, but there are even more virtual disk technologies that can provide additional storage features.

Hopefully, the Linux kernel provides many metrics that can provide deep insights of what our disks our doing. The kernel measures all these metrics on all layers of storage: virtual disks, physical disks and partitions of disks.

Let's see the list of metrics provided by netdata for each of the above:

I/O bandwidth/s (kb/s)

The amount of data transferred from and to the disk.

I/O operations/s

The number of I/O operations completed.

Queued I/O operations

The number of currently queued I/O operations. For traditional disks that execute commands one after another, one of them is being run by the disk and the rest are just waiting in a queue.

Backlog size (time in ms)

The expected duration of the currently queued I/O operations.

Utilization (time percentage)

The percentage of time the disk was busy with something. This is a very interesting metric, since for most disks, that execute commands sequentially, this is the key indication of congestion. A sequential disk that is 100% of the available time busy, has no time to do anything more, so even if the bandwidth or the number of operations executed by the disk is low, its capacity has been reached.

Of course, for newer disk technologies (like fusion cards) that are capable to execute multiple commands in parallel, this metric is just meaningless.

Average I/O operation time (ms)

The average time for I/O requests issued to the device to be served. This includes the time spent by the requests in queue and the time spent servicing them.

Average I/O operation size (kb)

The average amount of data of the completed I/O operations.

Average Service Time (ms)

The average service time for completed I/O operations. This metric is calculated using the total busy time of the disk and the number of completed operations. If the disk is able to execute multiple parallel operations the reporting average service time will be misleading.

Merged I/O operations/s

The Linux kernel is capable of merging I/O operations. So, if two requests to read data from the disk are adjacent, the Linux kernel may merge them to one before giving them to disk. This metric measures the number of operations that have been merged by the Linux kernel.

Total I/O time

The sum of the duration of all completed I/O operations. This number can exceed the interval if the disk is able to execute multiple I/O operations in parallel.

Space usage

For mounted disks, netdata will provide a chart for their space, with 3 dimensions:

  1. free
  2. used
  3. reserved for root

inode usage

For mounted disks, netdata will provide a chart for their inodes (number of file and directories), with 3 dimensions:

  1. free
  2. used
  3. reserved for root

disk names

netdata will automatically set the name of disks on the dashboard, from the mount point they are mounted, of course only when they are mounted. Changes in mount points are not currently detected (you will have to restart netdata to change the name of the disk).

performance metrics

By default netdata will enable monitoring metrics only when they are not zero. If they are constantly zero they are ignored. Metrics that will start having values, after netdata is started, will be detected and charts will be automatically added to the dashboard (a refresh of the dashboard is needed for them to appear though).

netdata categorizes all block devices in 3 categories:

  1. physical disks (i.e. block devices that does not have slaves and are not partitions)
  2. virtual disks (i.e. block devices that have slaves - like RAID devices)
  3. disk partitions (i.e. block devices that are part of a physical disk)

Performance metrics are enabled by default for all disk devices, except partitions and not-mounted virtual disks. Of course, you can enable/disable monitoring any block device by editing the netdata configuration file.

netdata configuration

You can get the running netdata configuration using this:

cd /etc/netdata
curl "http://localhost:19999/netdata.conf" >
mv netdata.conf

Then edit netdata.conf and find the following section. This is the basic plugin configuration.

    # enable new disks detected at runtime = yes
    # performance metrics for physical disks = auto
    # performance metrics for virtual disks = no
    # performance metrics for partitions = no
    # performance metrics for mounted filesystems = no
    # performance metrics for mounted virtual disks = auto
    # space metrics for mounted filesystems = auto
    # bandwidth for all disks = auto
    # operations for all disks = auto
    # merged operations for all disks = auto
    # i/o time for all disks = auto
    # queued operations for all disks = auto
    # utilization percentage for all disks = auto
    # backlog for all disks = auto
    # space usage for all disks = auto
    # inodes usage for all disks = auto
    # filename to monitor = /proc/diskstats
    # path to get block device infos = /sys/dev/block/%lu:%lu/%s
    # path to get h/w sector size = /sys/block/%s/queue/hw_sector_size
    # path to get h/w sector size for partitions = /sys/dev/block/%lu:%lu/subsystem/%s/../queue

For each virtual disk, physical disk and partition you will have a section like this:

    # enable = yes
    # enable performance metrics = auto
    # bandwidth = auto
    # operations = auto
    # merged operations = auto
    # i/o time = auto
    # queued operations = auto
    # utilization percentage = auto
    # backlog = auto

For all configuration options:

  • auto = enable monitoring if the collected values are not zero
  • yes = enable monitoring
  • no = disable monitoring

Of course, to set options, you will have to uncomment them. The comments show the internal defaults.

After saving /etc/netdata/netdata.conf, restart your netdata to apply them.

Disabling performance metrics for individual device and to multiple devices by device type

You can pretty easy disable performance metrics for individual device, for ex.:

    enable performance metrics = no

But sometimes you need disable performance metrics for all devices with the same type, to do it you need to figure out device type from /proc/diskstats for ex.:

   7       0 loop0 1651 0 3452 168 0 0 0 0 0 8 168
   7       1 loop1 4955 0 11924 880 0 0 0 0 0 64 880
   7       2 loop2 36 0 216 4 0 0 0 0 0 4 4
   7       6 loop6 0 0 0 0 0 0 0 0 0 0 0
   7       7 loop7 0 0 0 0 0 0 0 0 0 0 0
 251       2 zram2 27487 0 219896 188 79953 0 639624 1640 0 1828 1828
 251       3 zram3 27348 0 218784 152 79952 0 639616 1960 0 2060 2104

All zram devices starts with 251 number and all loop devices starts with 7.
So, to disable performance metrics for all loop devices you could add performance metrics for disks with major 7 = no to [plugin:proc:/proc/diskstats] section.

       performance metrics for disks with major 7 = no