wedgec
Posts: 4
Joined: Wed Dec 26, 2012 12:41 pm

Raspbmc - is USB install worthwhile?

Sun Dec 08, 2013 7:06 pm

Hi

Sorry for a newbie question but I'm currently running Raspbmc on a class 10, 16GB Sandisk SD card, which according to the label has 30MB/s. I have a Lexar 8 Gb USB Flash Drive USB Device, which according to http://usbspeed.nirsoft.net/?g=8gb has 16.92 MB/Sec read and 8.75 MB/Sec write speeds.

So I'm thinking it's not worth having a USB install? I'm just confused as my SD card seems to eclipse pretty much all the USB drives on that page, which goes against a lot of what I've read about USB installs.

Can anyone shed any light? Thanks

Dilligaf
Posts: 283
Joined: Wed May 23, 2012 6:48 pm

Re: Raspbmc - is USB install worthwhile?

Sun Dec 08, 2013 7:34 pm

You have a slow usb drive, check the speeds of usb 3 drives in usb 2 ports and they about double what your sd gives (sd cards on PI max out about 20 regardless of what they are rated)

masterluke
Posts: 200
Joined: Tue Apr 17, 2012 4:10 pm

Re: Raspbmc - is USB install worthwhile?

Mon Dec 09, 2013 5:35 pm

Even the fastest USB3 drives will max out at about 22mb/sec on the pi. Try benchmarking your sdcard, it will probably only be marginally slower than that so would likely not be worth it.

dom
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 5318
Joined: Wed Aug 17, 2011 7:41 pm
Location: Cambridge

Re: Raspbmc - is USB install worthwhile?

Mon Dec 09, 2013 5:42 pm

Pay no attention to the class or speed on the label of sdcard.
These report the speed of continuous writing to sdcard. Useful for recording video in a camcorder, but not relevant to Raspberry Pi.

The speed of reading random 4K blocks is more important but harder to get information on.

Typically USB sticks tend to beat sdcards for this, and I use a USB install for xbmc, and it is faster than the fastest sdcard I own.

But obviously not all USB devices are equal, and no doubt some are slower than sdcard,
but I am sure the fastest USB device will outperform the fastest sdcard when running xbmc.

kbarb
Posts: 4
Joined: Mon Dec 09, 2013 5:53 pm

Re: Raspbmc - is USB install worthwhile?

Mon Dec 09, 2013 5:58 pm

This is a bit playing dumb, but, does any of this actually make a real world difference with the RPi . . . . playing videos, writing files, whatever ?
What's the main advantage ?

= = = = = = =
Btw, a couple of years ago I was benchmarking some flash drives, and found that it can be a little diff. than benchmarking regular disk drives.
Atto Disk Benchmark is pretty good and fairly reliable :
http://www.attotech.com/disk-benchmark/

If you really want to get into it, read this pdf :
"Don’t Let Your Favorite Benchmarks Lie to You"
http://www.lsi.com/downloads/Public/Fla ... MS2013.pdf

gregor3000
Posts: 114
Joined: Tue Nov 20, 2012 12:57 pm

Re: Raspbmc - is USB install worthwhile?

Tue Dec 10, 2013 1:37 pm

for me the SD card works fast enough. most writing, reading (media) is done from external USB disk anyway.

RoyLongbottom
Posts: 287
Joined: Fri Apr 12, 2013 9:27 am
Location: Essex, UK
Contact: Website

Re: Raspbmc - is USB install worthwhile?

Fri Dec 13, 2013 4:07 pm

Following are some results from my DriveSpeed benchmark where details and DL link are in:

http://www.roylongbottom.org.uk/Raspber ... m#anchor20

There are three sets of tests, writing/reading large files, random reading and writing, writing and reading small files.Results are for:

SD Main - original Kingston HC Class 4 card, Ext4 format

Sd 4 - SanDisk Extreme Pro microSDHC UHS-1 Class 10 card, rated at up to 633X or 95 MB/second, Ext4 format.

St3 - Patriot Rage XT USB 2 Flash drive, rated at up to 25 MB/second writing and 27 MB/second reading speeds, FAT format.

St4 - SanDisk Extreme USB 3.0 Flash drive, with speed ratings of up to 190 MB/s read and 110 MB/s write, FAT format, then as St5 Ext4 format.

Code: Select all

      Large Files MB/second 16 MB files
                        Write1   Write2   Write3    Read1    Read2    Read3
    
      SD Main  16 GB       9.9     10.3     10.3     22.8     22.8     22.8
      SD 4     16 GB      19.8     19.9     20.0     22.7     22.8     22.8
    
      USB2 St3  8 GB       9.3      9.3      9.3     25.8     25.8     25.1
      USB3 St4 32 GB      17.0     16.8     16.6     26.6     26.7     26.5
      USB3 St5 32 GB Ext4 26.7     26.3     26.5     26.5     26.8     26.7
    
    
      Random milliseconds
                          Read                      Write
      From MB                4        8       16        4        8       16
    
      SD Main Kingston   0.019    0.019    0.043      9.1     15.3      9.5
      SD 4   SanDisk EP  0.016    0.017    0.075      5.1     11.0      6.7
    
      USB2 St3 Pat Rage  0.018    0.019    0.364      5.9      3.7      8.5    
      USB3 St4 SanEx     0.018    0.017    0.061      1.2      0.8      0.8
      USB3 St5 Ext4      0.019    0.019    0.075      1.2      1.2      1.2
    
    
      200 Small Files Milliseconds per file
                         Write                       Read                     Delete
      File KB                4        8       16        4        8       16  Seconds
    
      SD Main            46.73    72.37   105.14     0.95     1.09     1.44    0.039
      SD 4               11.84    13.28    10.55     0.77     0.96     1.34    0.016
    
      USB2 St3          113.83   111.02   114.11     1.08     1.28     1.54    0.030
      USB3 St4           86.56    87.60    87.94     0.94     1.15     1.27    0.022
      USB3 St5 Ext4       5.93     5.25     5.51     1.01     1.32     1.42    0.016
 
Large Files - Reading performance is limited by USB bus speed, with similar results on all these devices. For writing, one of the more expensive and faster SD cards or USB drives is needed to provide performance approaching the bus limit. Ext4 formatting is advantageous.

Random Access - Using this benchmark, lots of Flash memory cards or sticks produce wide variations in random access speeds, with response times upp to 100 msecs on reading and 300 msecs on writing. This bunch are all quite good, the USB3/Ext4 drive having the edge.

Small Files - The FAT formatted USB sticks are particularly slow on writing. This is due to a combination of drive capacity, sector size and allocation table updating, A fast writing speed specification clearly helps, but the clear winner is the USB 3 drive with Ext4 formatting. Average reading times are good in all cases.

I suppose that the answer to the original question is that, if you don’t care about the exorbitant cost, a top notch USB 3 stick can provide the best possible performance, if you can effectively use it with Ext4 (or other non-FAT) formatting.

masterluke
Posts: 200
Joined: Tue Apr 17, 2012 4:10 pm

Re: Raspbmc - is USB install worthwhile?

Sat Dec 14, 2013 3:44 pm

Very interesting results Roy thanks. Did you run any USB2 results with ext4? There is a massive difference between the USB3 fat drive and the USB3 ext4. I cant help thinking your Patriot Rage XT got a raw deal as it was stuck on fat.

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