How to measure the sequential write/read speed of a Hard Disk or SSD?

Question: How to measure the sequential write speed of a hard disk or SSD?

Answer

We may use two ways. The most popular one is to use dd tools. For hard disks, we may not avoid using cache to measure the disk performance. As sequential writes can write the data to the cache, which is basically a DRAM and can perform excessively fast masking the original speed of the HDD. Let’s check how can we measure the HDD speed using dd bypassing HDD write cache:

dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct

/hddvz is my HDD mount, oflag=direct instruct the dd to confirm writes to the disk, not just the cache before saying the write is completed.

If it’s a good quality hard disk alone or soft raid 1, you can get speed up to 157-161MBps:

[[email protected] ~]# dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 6.82675 s, 157 MB/s
[[email protected] ~]#

With a good hardware raid controller and raid 10, you may be able to reach rates like 320MBps.

There is something to remember, software raid mdadm is able to read simultaneously from two disks. You may have the evidence, with two dd one by one, both in background as following:

[[email protected] ~]# dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct &
[1] 12039
[[email protected] ~]# dd if=/dev/zero of=/hddvz/testfile1.img bs=1G count=1 oflag=direct &
[2] 12040
[[email protected] ~]# 1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 11.1064 s, 96.7 MB/s
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 10.7155 s, 100 MB/s

[1]-  Done                    dd if=/dev/zero of=/hddvz/testfile.img bs=1G count=1 oflag=direct
[2]+  Done                    dd if=/dev/zero of=/hddvz/testfile1.img bs=1G count=1 oflag=direct

As you can see, both the request was able to reach 100MBps roughly while running in parallel.

Testing Read Speed Using hdparm

To check the read speed, you may use hdparm like the following:

[[email protected] ~]# hdparm -Tt /dev/sda

/dev/sda:
 Timing cached reads:   40444 MB in  1.99 seconds = 20365.12 MB/sec
 Timing buffered disk reads: 396 MB in  3.06 seconds = 129.43 MB/sec
[[email protected] ~]# hdparm -Tt /dev/sdb

/dev/sdb:
 Timing cached reads:   41006 MB in  1.99 seconds = 20649.43 MB/sec
 Timing buffered disk reads: 562 MB in  3.01 seconds = 186.81 MB/sec
[[email protected] ~]#

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