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Read and write operations

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Updated at October 30, 2024

There are some technical restrictions on reads and writes that apply to disks and file storages. The restrictions apply both to the entire disk or storage and to each individual disk space allocation unit. The allocation unit size depends on the disk or storage type.

The maximum read and write operation parameters are as follows:

  • Maximum IOPS: Maximum number of read and write operations per second.
  • Maximum bandwidth: Total number of bytes that can be read or written per second.

The actual IOPS value depends on the disk or storage configuration, total bandwidth, and the size of the request in bytes. The provided IOPS value is calculated using the following formula:

Where:

Read and write operations utilize the same disk resource. The more read operations you do, the fewer write operations you can do, and vice versa. The total number of read and write operations per second is determined by this formula:

Where:

  • is the share of write operations out of the total number of read and write operations per second. Possible values: α∈[0,1].
  • WriteIOPS: IOPS write value obtained using the formula for the actual IOPS value.
  • ReadIOPS: IOPS read value obtained using the formula for the actual IOPS value.

For more information about maximum possible IOPS and bandwidth values, see Quotas and limits.

Disk and file storage performance

The maximum IOPS values are achieved when performing reads and writes that are 4 KB in size. Network SSDs and file storage have much higher IOPS for read operations and process requests faster than HDDs.

To achieve the maximum possible bandwidth, we recommend performing 4 MB reads and writes.

Disk or storage performance depends on its size: with more allocation units, you get higher IOPS and bandwidth values.

For smaller HDDs, there is a performance boosting mechanism in place for them to operate on a par with 1 TB disks during peak load periods. By operating at the basic performance level for 12 hours, a smaller HDD accumulates credits for operations. These will be automatically spent when the load increases, e.g., when the VM starts. Small HDDs can be boosted for about 30 minutes a day. Credits for operations can be spent either all at once or by small intervals. This feature is not available for HDD storages.

Testing disk performance

Note

We do not recommend testing writes to your /dev/vda disk. If you need to test writes to the boot disk, run the fio utility with the --filename=./testfile and --filesize=1G parameters.

You can test the performance of your network disks with fio (Flexible I/O Tester):

  1. Attach a disk to a VM instance.

  2. Install fio on your VM instance.

    Sample command for Ubuntu:

    sudo apt-get update && sudo apt-get install fio -y

  3. Start fio and run the following:

    sudo fio \
--filename=/dev/vdb \
--direct=1 \
--rw=write \
--bs=4k \
--ioengine=libaio \
--iodepth=64 \
--runtime=120 \
--numjobs=8 \
--time_based \
--group_reporting \
--eta-newline=1

    Where:

    • --filename=/dev/vdb: Name of the disk you are testing. To view the attached disks, run the lsblk command.
    • --direct: Flag that toggles buffering; 0 means buffering is used, 1 means buffering is not used.
    • --rw: Load template. The possible values are as follows:
      • read: Sequential reads.
      • write: Sequential writes.
      • rw: Sequential reads and writes.
      • randrw: Random reads and writes.
      • randwrite: Random writes.
      • randread: Random reads.
    • --bs: Read and write block size. To get better results, specify a value that is equal to the disk block size or less.
    • --iodepth: I/O block depth per job.
    • --runtime: Test duration in seconds.
    • --numjobs: Number of read and write jobs.

Test examples

Test IOPS for sequential writes

sudo fio \
--name=readio \
--filename=/dev/vdd \
--direct=1 \
--rw=write \
--bs=4k \
--ioengine=libaio \
--iodepth=96 \
--runtime=120 \
--numjobs=4 \
--time_based \
--group_reporting \
--eta-newline=1

Result:

---
  write: IOPS=39.7k, BW=155MiB/s (162MB/s)(5112MiB/33001msec); 0 zone resets
    slat (usec): min=2, max=19776, avg= 5.25, stdev=47.15
    clat (usec): min=874, max=5035.1k, avg=9677.38, stdev=40976.63
     lat (usec): min=889, max=5035.1k, avg=9682.81, stdev=40976.66
---

Test IOPS for random writes

sudo fio \
--name=randwrite \
--filename=/dev/vdd \
--direct=1 \
--rw=randwrite \
--bs=4k \
--ioengine=libaio \
--iodepth=96 \
--runtime=120 \
--numjobs=1 \
--time_based \
--group_reporting \
--eta-newline=1

Result:

---
write: IOPS=9596, BW=37.5MiB/s (39.3MB/s)(4499MiB/120011msec); 0 zone resets
    slat (usec): min=2, max=338, avg= 5.21, stdev= 4.52
    clat (usec): min=680, max=161320, avg=9996.54, stdev=10695.67
     lat (usec): min=698, max=161323, avg=10001.94, stdev=10695.77
---

Test throughput for sequential writes

sudo fio \
--name=writebw \
--filename=/dev/vdd \
--direct=1 \
--rw=write \
--bs=4M \
--ioengine=libaio \
--iodepth=32 \
--runtime=120 \
--numjobs=1 \
--time_based \
--group_reporting \
--eta-newline=1

Result:

---
   write: IOPS=112, BW=449MiB/s (471MB/s)(52.8GiB/120237msec); 0 zone resets
    slat (usec): min=166, max=270963, avg=8814.82, stdev=10995.16
    clat (msec): min=58, max=661, avg=276.06, stdev=28.21
     lat (msec): min=60, max=679, avg=284.88, stdev=27.91
---

Test IOPS for sequential reads

sudo fio \
--name=readio \
--filename=/dev/vdd \
--direct=1 \
--rw=read \
--bs=4k \
--ioengine=libaio \
--iodepth=128 \
--runtime=120 \
--numjobs=8 \
--time_based \
--group_reporting \
--eta-newline=1

Result:

---
  read: IOPS=62.2k, BW=243MiB/s (255MB/s)(28.5GiB/120008msec)
    slat (usec): min=2, max=123901, avg= 6.88, stdev=151.96
    clat (usec): min=859, max=168609, avg=16450.99, stdev=8226.23
     lat (usec): min=877, max=168611, avg=16458.07, stdev=8229.16
---

Test read throughput

sudo fio \
--name=readbw \
--filename=/dev/vdd \
--direct=1 \
--rw=read \
--bs=4M \
--ioengine=libaio \
--iodepth=32 \
--runtime=120 \
--numjobs=1 \
--time_based \
--group_reporting \
--eta-newline=1

Result:

---
  read: IOPS=112, BW=449MiB/s (470MB/s)(52.7GiB/120227msec)
    slat (usec): min=85, max=177850, avg=8878.47, stdev=9824.19
    clat (msec): min=50, max=4898, avg=276.36, stdev=45.16
     lat (msec): min=52, max=4898, avg=285.24, stdev=44.94
---

Test IOPS for random reads

sudo fio \
--name=randread \
--filename=/dev/vdd \
--direct=1 \
--rw=randread \
--bs=4k \
--ioengine=libaio \
--iodepth=16 \
--runtime=120 \
--numjobs=8 \
--time_based \
--group_reporting \
--eta-newline=1

Result:

---
 read: IOPS=17.0k, BW=66.4MiB/s (69.6MB/s)(7966MiB/120006msec)
    slat (usec): min=2, max=114, avg= 9.05, stdev= 5.36
    clat (usec): min=172, max=251507, avg=7519.93, stdev=6463.84
     lat (usec): min=179, max=251511, avg=7529.25, stdev=6464.41
---

Throttling

If a VM exceeds disk limits at any time, this will trigger throttling.

Throttling is a feature that forcibly limits the performance. When throttled, disk operations are suspended, and the disk operation wait time (iowait) is increased. Since all write and read operations are processed in a single thread (vCPU), overloading system disks may cause network problems. This is true for both VMs and physical servers.

For example, let's assume there is a write limit of 300 IPOS. The limit is split into 10 parts and applies once every 100 ms. 300 / 10 = 30 IOPS per write request will be allowed every 100 ms. If you send 30 requests once and then 30 more requests within 100 ms (evenly distributed across the 100 ms interval), this will trigger throttling and only the first 30 requests will be sent. The rest of them will be enqueued and processed within the next 100 ms. If write requests are executed sporadically, throttling may cause significant delays. At times, there will be up to N IOPS of requests within 100 ms.

Disk performance depends on its size. To improve the overall performance of the disk subsystem, use VMs with SSD network storage (network-ssd). Every increment of 32 GB increases the number of allocation units and, consequently, the performance.

You can select the storage type only when creating a VM. However, you can take a disk snapshot and create a new VM from such a snapshot with a network-ssd.

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