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Re: [Xen-users] [XCP] Install to Flash Media

What file system are you going to use?

With flash media you will eventually wear out your device, wearing out specific 
blocks and then using up spare blocks which will then cause the device to go 
offline or the filesystem to go readonly. While each block can be written 
millions of times, all it takes is a the same block to be repeatedly written 
to. There are parts of the filesystem that are like that. Such as a journal.

You can go with extending the commit interval but that is dangerous. Maybe not 
so much if you consider that you're not writing important data. Just boot files 
etc. But still your not going to like that if the writes were needless.  So 
some extra work on your part can help identify and weed out those processes.

I'd recommend a ext2, and also mount with noatime to reduce the unnecessary 

Beware of lock or stat files that get touched regularly. Those too create lots 
of writes unnecessarily. You could use a really small ramfs for those dirs. 
This is great for a file or directory that has state information which won't 
matter after a reboot.

You can do some tests watching for io to the disks to see what processes are 
writing to what parts.
echo 1 > /proc/sys/vm/block_dump
Use dmesg or redirect with klog to a file to see the process name, id, inode, 
filename, and device.
On a journaled file system like ext3, you'll see your write, and then a few 
seconds later you'll see the journal commit the changes.
Notice that the same inodes get written to over and over. Sort and word count 
those to see how many times in a minute or hour and you'll see the heavily used 

Doing this upfront can give you many years of reliable usage. Without it, you 
could easily wear out your device in a year.

Also, flash blocks or pages are not the same as your file system blocks. It's 
not a 1 to 1 relationship. So 1 512byte FS block, could be sharing a page with 
other blocks. Writes to 2 separate FS blocks could affect the same flash page.

There are also flash file systems (i.e. jffs2 and a number of others) that are 
supported by the linux kernel. Most distributions don't support those by 
default, so you may have to recompile your kernel to include it. They are more 
common on embedded systems. But they are out there and they do the extra work 
for you of leveling the where on on the flash device.


On Feb 10, 2012, at 8:30 AM, Nick Couchman wrote:

> On Thu, 2012-02-09 at 10:01 +0100, Roger Pau Monné wrote:
>> 2012/2/8 Nick Couchman <Nick.Couchman@xxxxxxxxx>:
>>> Well, now that I've figured out that network booting XCP is not really
>>> going to be feasible, I'm moving on to my next option: flash media
>>> installation.  The systems that I purchased have SDHC slots on them, so
>>> I figured I'd attempt to install XCP to one of these cards.  I had
>>> already read that there's a minimum disk size in the installer
>>> (apparently 12 GB in XCP) and so have factored in that.  However,
>>> whenever I try to install I get an error message that the "Partition
>>> size in sectors is negative."  Any ideas on this one?
>> I've successfully installed XCP on a SD card some time ago, but I
>> don't recommend that. Running an OS from a SD is not recommended
>> because the access time is large, and the system can feel
>> irresponsible some times.
> I'm willing to take the risk - I'm using Class 10 SDHC cards (10MB/s),
> and there isn't going to be a lot going on at the dom0 level that is
> going to require responsiveness.  Additionally, I'll be doing syslog to
> a remote machine, so this should take care of some of the normal
> read/write traffic that would be on the local disk.
>>> Any better way to
>>> install to flash media?  I suppose I could go ahead and do the install
>>> to a regular HD, then use a rescue system and dd to grab an image of the
>>> system partition, then transfer it to the flash media.  Any other
>>> possible paths?
>> Installing the image and doing a dd will not work straight away, you
>> will have to use syslinux to make the usb bootable, but again you will
>> face access problems.
>>> Anyone running XCP or XenServer off SDHC or other
>>> flash-based media?
>> I'm using a SD card to boot a regular Xen install (4.1.2) on some of
>> my servers, using Alpine Linux which is easy to install to a SD card,
>> has a very small footprint and works great with Xen. Don't have
>> experience with XenServer.
> I did throw a 16GB SDHC card into one of the systems and ran the XCP
> install, and it worked fine.  It installed to the SDHC card.  The only
> issue I ran into is that in the case the SDHC card is seen as a USB
> storage device by the O/S, and the default XCP initrd did not have
> usb-storage loaded.  So, I had to boot into a rescue system, mount the
> SDHC card, and regenerate the initrd, and reboot.  It is working well,
> now, and I've ordered SDHC cards for the rest of the systems so that I
> can install XCP this way.  I will have to see longer-term if there are
> any adverse side-effects of install XCP this way - performance,
> degradation of the flash devices, etc.
> -Nick
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