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Drew Noakes
Asked: 2014-06-12 11:17:40 +0800 CST

SD card write speed not as advertised, on 14.04 at least

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I bought a new SD card and the claimed performance is a long way from my benchmark. Is there something I can do improve the transfer speed when working with this SD card?

Information on the disk:

$ df --output=source,fstype,size,target -h
Filesystem     Type      Size Mounted on
/dev/mmcblk0p1 fuseblk    60G /media/drew/6564-6433

Partition type: HPFS/NTFS
Contents:       exFAT (version 1.0)

The card's packaging claims:

Speed up to 30 MB/s* (200X)

Following the asterisk leads to:

Up to 30MB/s transfer speed; write speed lower. Based on internal testing; performance may be lower depending on host device. 1MP = 1,000,000 bytes. X = 150KB/s.

My machine didn't recognise the card's formatting at first, so I installed exfat-fuse the package and it worked at the next attempt.

I then ran a benchmark using gnome-disks:

enter image description here

As you can see, the average read speed of 7.1 MB/s which is a long way from 30 MB/s. That's 4.2 times slower (23%).

Is there anything I can do to improve the transfer speed when working with this SD card?

EDIT output of hdparam as requested in comments:

$ sudo hdparm -tT /dev/mmcblk0

/dev/mmcblk0:
 Timing cached reads:   12186 MB in  1.99 seconds = 6111.71 MB/sec
 Timing buffered disk reads:  34 MB in  3.17 seconds =  10.73 MB/sec

I executed this four times on an unoccupied machine and the results were consistent across runs.

The first number (6GB/s) is described as:

the speed of reading directly from the Linux buffer cache without disk access. This measurement is essentially an indication of the throughput of the processor, cache, and memory of the system under test.

The second number (10 MB/s) is described as:

This displays the speed of reading through the buffer cache to the disk without any prior caching of data. This measurement is an indication of how fast the drive can sustain sequential data reads under Linux, without any filesystem overhead.

So 10 MB/s may be the throughput for a different type of load, or possibly just for a shorter time (the incomplete chart above ran for around 5 minutes before I took that screenshot.) Either way, it's still not close to 30 MB/s!

14.04

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