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Author Topic: Benefits/Downsides to moving to SDHC based booting.  (Read 4089 times)
mgillespie
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« on: October 25, 2009, 07:41:26 AM »

I currently running Ubuntu from the internal flash, and whilst everything is working well, the impeding ditching of ARMv5 support in Ubuntu is making me consider doing the switch to Debian (the testing one).   As the installer does not work on internal flash and the only current method is to SDHC (or USB flashdisk), I and thinking of taking the plunge and getting a 2GB SD card and moving my setup to that.

before I do so, I want to understand if there are any downsides to doing this.  Is there a read or write speed penalty, is the wear leveling the same?  I assume the card itself deals with that, as opposed to the filesystem having to do it curently? (do I format it with my filesystem of choice?)

I already have the latest u-boot (the one that comes with Sheeva Installer 1.0), is there ANYTHING that I need to know before I jump in and do this?

Thanks.
« Last Edit: October 25, 2009, 08:03:12 AM by mgillespie » Logged

knireis
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« Reply #1 on: October 25, 2009, 12:48:41 PM »

I followed same route, you might run into this issue: http://plugcomputer.org/plugforum/index.php?topic=657.0

But it is easily solved by the steps described in the thread.

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mgillespie
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« Reply #2 on: October 25, 2009, 01:11:06 PM »

Looking into the read/write performance I am likely to suffer moving from internal flash to SHDC, I'm thinking it's better to stay on the internal flash. 

I have a plan on how to get the Debian Squeeze testing onto the internal flash.  I'm going to setup a rootfilesystem using debootstrap: http://www.openplug.org/plugwiki/index.php/Debian

then i'm going to use the Sheeva Installer 1.0 to get the rootfilesystem installed (replacing the ubuntu part of the installer with the newly created Debian one).

Will this work?
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fragfutter
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« Reply #3 on: October 26, 2009, 12:43:11 AM »

you need to have mtd device drivers in the kernel (or initrd) to access the nand. And you need to have the drivers for the filesystem. Then it should be possible to install to the nand.

But what is your performance problem with the SD-Card? A decent class 6 card is fast enough, has lots of space, can be replaced when/if cells are worn out, can be taken out for backups...
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mgillespie
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« Reply #4 on: October 26, 2009, 12:48:49 AM »

I'm trying to determine how much slower SDHC actually is.  I'm getting 37MB/sec write 33MB/sec read from the internal flash, which is very quick.  I'm thinking I will be lucky to get half that from a SD card.  I have also seem people saying about 60 second reboot times (mine is about 20 second), which is also indicative of the performance hit I am likely to see.

As for "wearing out", I asking this a while back, and never got a satisfactory answer, with nobody knowing for sure.   I went back to the assumption that the wear levelling will sort things out (SSD harddrives are fine, so why would the internal sheeva memory be any less?)
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DamonHD
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« Reply #5 on: October 26, 2009, 01:37:39 AM »

SSD drives are intended for fairly heavy/continuous use whereas SD cards and the like are for occasional use in, for example, retail digital cameras used for a duty cycle of << 1%, I suggest...

So thus I think it is good to assume that an SD card may have far fewer write cycles and far less wear-levelling than an SSD drive.

Rgds

Damon
« Last Edit: October 26, 2009, 12:04:17 PM by DamonHD » Logged

fragfutter
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« Reply #6 on: October 26, 2009, 02:30:12 PM »

speed of an class 6 sd card

dd if=/dev/zero of=binary.dat bs=4096 count=20000
20000+0 records in
20000+0 records out
81920000 bytes (82 MB) copied, 5.93425 s, 13.8 MB/s


as far as i know ubifs is caching and compressing, the same dd might be invalid when comparing.
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darkscout
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« Reply #7 on: October 30, 2009, 07:36:08 AM »

Code:
plug:~# hdparm -tT /dev/mmcblk0

/dev/mmcblk0:
 Timing cached reads:   432 MB in  2.00 seconds = 215.84 MB/sec
 Timing buffered disk reads:   56 MB in  3.00 seconds =  18.64 MB/sec
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mhtsaras
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« Reply #8 on: December 28, 2009, 05:58:12 AM »

Here are some tips to reduce wear
Tips for running Linux on a flash device
http://www.cyrius.com/debian/nslu2/linux-on-flash.html
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mgillespie
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« Reply #9 on: December 28, 2009, 06:29:43 AM »

Code:
plug:~# hdparm -tT /dev/mmcblk0

/dev/mmcblk0:
 Timing cached reads:   432 MB in  2.00 seconds = 215.84 MB/sec
 Timing buffered disk reads:   56 MB in  3.00 seconds =  18.64 MB/sec

Mine:  (Transcend 4GB SDHC)

sheeva:~# hdparm -tT /dev/mmcblk0

/dev/mmcblk0:
 Timing cached reads:   634 MB in  2.00 seconds = 316.65 MB/sec
 Timing buffered disk reads:   58 MB in  3.03 seconds =  19.12 MB/sec
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notjeff
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« Reply #10 on: December 29, 2009, 08:45:08 PM »

I am using the latest Debian (squeeze) off internal flash with no problems.  I tried switching to Debian on an 8GB Class 6 SDHC, but the resulting system was way too slow.  I quickly jumped back to using the internal flash.
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mgillespie
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« Reply #11 on: December 30, 2009, 01:57:10 AM »

I boot from NAND, but have mounted my /var on SDHC card to reduce wear.
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UnaClocker
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« Reply #12 on: December 30, 2009, 06:29:09 PM »

I am using the latest Debian (squeeze) off internal flash with no problems.  I tried switching to Debian on an 8GB Class 6 SDHC, but the resulting system was way too slow.  I quickly jumped back to using the internal flash.
Something seriously doesn't add up there. It was either a chinese fake from eBay or the like, or you formatted it in a completely different file system, one that doesn't perform well on solid state memory.
Me, I just picked up a 8gb class 4, I fully intend to make it my main drive, and leave the onboard drive alone. I wanted a class 6, but I'm cheap. Wink No point getting such a cheap computer and spending almost that much again on the storage medium. Smiley
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SheevaPlug - 8gb class 4 SDHC primary drive, 4tb 3.5" media drive, Debian Wheezy, nginx, Samba, Shorewall

knireis
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« Reply #13 on: December 31, 2009, 09:48:19 AM »

on my plug with a 4GB SDHC class 6 (brand ATP)

/dev/mmcblk0:
 Timing cached reads:   634 MB in  2.00 seconds = 316.58 MB/sec
 Timing buffered disk reads:   60 MB in  3.07 seconds =  19.54 MB/sec
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ruivilela
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« Reply #14 on: December 31, 2009, 12:13:24 PM »

I use a Class 4 SDHC with 8 GB for booting a Linux Gentoo system and I find it enough fast. Kernel boots from NAND. I can say that it takes around 1 minute to have a login shell. Not sure for what you are using the plug. I use it as a server (for many services). And I find quite good the response time when I read from file system on card. The extenal disk handles most IO (e.g.: rtorrent). I will try in other a Class 2 SDHC with 4GB (last installation needed ~3GB without X.

So the card for me is not as slow as some people says.
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