USB Live Ubuntu Persistence

[Eric Shubert]

Joseph Sinclair wrote:
> Eric Shubert wrote:
>> Eric Shubert wrote:
>>> Dazed_75 wrote:
>>>> Saturday I was helping Matthew create a Live Ubuntu USB stick.  We 
>>>> succeeded but for some reason persistence was not working and I could 
>>>> not figure out why.  I did the same thing here at home and persistence 
>>>> works fine.  In fact, I think I know the answer now.  Mathew was using a 
>>>> 16 GB flash drive and wanted the rest of the drive used for persistent 
>>>> storage. 
>>>>
>>>> The problem was, I believe, that the flash drive was formatted for FAT32 
>>>> which has a file size limit of 4GB.  It appears the utility to create 
>>>> the LiveUSB ubuntu stick creates a special file to use for the "overlay" 
>>>> (my term) file system that is merged onto the read only filesystem from 
>>>> the Live image.  Since we were asking it to make a 14.4GB overlay file 
>>>> on a FAT32 partition, that part of the install failed silently and 
>>>> persistence was was working.
>>>>
>>>> Mathew, I believe there are several ways to resolve this with the 
>>>> simplest being to only ask for 4GB of persistent storage.  The rest of 
>>>> the stick should still be usable though you may find it handy to make 
>>>> the rest a separate partition and mount it within the LiveUSB Ubuntu.  
>>>> You should even be able to make the mount persistent.
>>>>
>>>> -- 
>>>> Dazed_75 a.k.a. Larry
>>>>
>>> That sounds like a possibility all right. TTBOMK though, FAT32 has a 2G 
>>> file size limit. :(
>>>
>> Now that I think of it though, are we confusing partition vs file sizes? 
>> The 'overlay' partition would be what's over 2G, not a file. From what 
>> I've seen of overlay filesystems, there are still going to be 
>> independent files within the partition. It's not just one big file. As 
>> such, I doubt this is the problem. As always, I could be wrong. I think 
>> I was on that last post (except for the 2G part!).
>>
> Most mixed live/persistent distros use a single file loopback mounted as the overlay filesystem (via overlayfs or unionfs), including IIRC Ubuntu live/USB.  As such, the entire persistent partition is, indeed, limited to the *file* size limit of the host partition.
> Most virtual machine systems have similar problems (which is one more reason why it's good to *not* put virtual disk files on systems using FAT or NTFS, poor windows users...).
> 

Thanks for chiming in on this, Joseph. I'm not terribly familiar with 
unionfs/aufs, just enough to be dangerous. ;)

Can you say then, whether this single-file aspect is true for all 
union/aufs implementations, or is it only because of the way it's used 
by persistent distros? TIA.

-- 
-Eric 'shubes'