Re: why am I getting "No space left on device" here?

[Martin Steigerwald <Martin@lichtvoll.de>]

Am Mittwoch, 15. Januar 2014, 19:05:41 schrieb Duncan:
> But just as my already allocated mixed-mode chunks were just about full 
> and I needed another one allocated to complete the job, so your data 
> chunks are full or very close, according to btrfs fi df, and you need a 
> new one allocated (and if the file is greater than a gig in size, likely 
> more than one) to finish the job.
> 
> And in both our cases, there's plenty of unallocated space in the pool, 
> but for whatever reason, btrfs isn't allocating that new chunk when it 
> should!  Why, I can't say, but as I mentioned, I was able to work around 
> the problem here by trying the remaining files in a different order, and 
> at some point, btrfs figured out it needed that new chunk allocated, and 
> everything went fine after that.
> 
> So... why btrfs is failing to allocate a new chunk when it needs to I 
> can't say, but I *CAN* say you're not the only one to have run into the 
> problem recently; I did too.
> 
> And just as I did, here, with a bit of monkeying around, you can 
> /probably/ get btrfs to allocate that new data chunk and get on with 
> things.  But the trouble is, since I don't know what the exact problem is 
> or what exactly I did to persuade btrfs to do that new chunk allocation, 
> I can't tell you exactly what to do to get it to happen, all I can do is 
> suggest you try copying smaller files or files of different sizes around 
> a bit, hoping to trigger that allocation.
> 
> Once that chunk allocation happens, you should be good for at least a 
> gig, since that's the data-chunk size, but if your file is over a gig in 
> size, you may run into the problem again.  In that case...  Well, you 
> could try copying several gigs of smaller files, then once it allocates 
> what you need, delete them, leaving the data chunks allocated but with 
> enough unused space to copy the original multi-gig file over.

I think you can cause BTRFS to allocate a chunk or more by creating a file with 
the fallocate command. Unless BTRFS doesn´t do it due to a bug and errors out 
with no space left.

fallocate gives also an easy way to find out how much it might still allocate 
and at what point it fails – and that without writing tons of data first. 
fallocate just triggers allocation and does not write any actual data.

-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
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