system response issue

[jd1008]

On 07/13/2015 05:15 PM, Patrick O'Callaghan wrote:
> On Mon, 2015-07-13 at 15:42 -0600, jd1008 wrote:
>> On 07/13/2015 03:13 PM, Patrick O'Callaghan wrote:
>>> On Mon, 2015-07-13 at 13:50 -0600, jd1008 wrote:
>>>> I started a tar command from
>>>> one external eSATA drive (ext4), connected to eSATA port,
>>>> out to a USB flash drive (with vfat). The USB stick is touted
>>>> to support 50MB/s write, 160MB/s read.
>>> Have you actually measured its real write speed for large files?
>>> Try
>>> something like:
>>>
>>> time dd if=/dev/zero of=/the/usb/drive count=1000 bs=1M
>>>
>>> You might be surprised.
>>>
>>> poc
>> As  I just replied with the actual read/write performance data
>> on this USB 3.0 SUperTalentExpress Drive (256GB), I am
>> much more surprised by the fact that while tarring a large
>> dir to it from a fast eSATA drive, the system becomes nearly
>> unusable!!!
>> So, if this "shtick" is so slow, why is writing to it killing the
>> response
>> time for everything else?
> IIRC USB drives are not DMA driven, i.e. each filesystem block written
> to the drive has to be pushed over the serial bus and handled by the
> CPU. Also, check if the drive is mounted with the 'flush' option, which
> sacrifices speed for greater integrity (i.e. less damage of the user
> pulls the drive without ejecting it).
>
> poc
Good tip. I will check next time when I rerun the test.