Re: [Help] Can't login to my systemd-homed user account due to fallocate failure

[Goffredo Baroncelli <kreijack@libero.it>]

On 8/24/20 5:23 PM, Andrei Borzenkov wrote:
> 24.08.2020 10:13, Chris Murphy пишет:
>>
>> I think we've got a pretty good idea what's going on. The current work
>> around in your case will be to use the --luks-discard=true option when
>> activating. This will avoid the fallocate step. But the question
>> remains why the fallocate fails. I suspect it has something to do with
>> shared extents.
> 
> 
> Actually I see the same behavior without any shared extent at all.
> fallocate attempts to allocate full file size, not just additional
> unallocated space:

I don't remember if at the time it was reached a conclusion, but this topic was already discussed in the past:

https://lore.kernel.org/linux-btrfs/798a9077-bcbd-076c-a458-3403010ce8ac@libero.it/

I think that this information should be forwarded back to the systemd mailing list...

Anyway IMHO a failing fallocate for a small file is a reasonable failure.
However I am not convinced that this is still true for a filesystem image (like the systemd-homed
case is). In this case it is reasonable to thinking the image as sparse image as a way of
over-provisioning.
However it is true that not all the stack (btrfs <-> loopback <-> luks <-> btrfs) is able to manage
a case where there is no space, so fallocat'ing may be the least worst scenario...

> 
> tw:/mnt # truncate -s $[112*5]M src/file
> tw:/mnt # dd if=/dev/urandom of=src/file conv=notrunc,block,sync bs=112M
> count=20+2 records in
> 2+0 records out
> 234881024 bytes (235 MB, 224 MiB) copied, 1.53186 s, 153 MB/s
> tw:/mnt # sync
> tw:/mnt # btrfs fi us /mnt
> Overall:
>      Device size:		   1.00GiB
>      Device allocated:		 356.00MiB
>      Device unallocated:		 668.00MiB
>      Device missing:		     0.00B
>      Used:			 224.59MiB
>      Free (estimated):		 787.81MiB	(min: 787.81MiB)
>      Data ratio:			      1.00
>      Metadata ratio:		      1.00
>      Global reserve:		   3.25MiB	(used: 0.00B)
>      Multiple profiles:		        no
> 
> Data,single: Size:344.00MiB, Used:224.19MiB (65.17%)
>     /dev/vdc	 344.00MiB
> 
> Metadata,single: Size:8.00MiB, Used:400.00KiB (4.88%)
>     /dev/vdc	   8.00MiB
> 
> System,single: Size:4.00MiB, Used:16.00KiB (0.39%)
>     /dev/vdc	   4.00MiB
> 
> Unallocated:
>     /dev/vdc	 668.00MiB
> tw:/mnt # fallocate -l $[112*5]M -n src/file
> tw:/mnt # sync
> tw:/mnt # btrfs fi us /mnt
> Overall:
>      Device size:		   1.00GiB
>      Device allocated:		 916.00MiB
>      Device unallocated:		 108.00MiB
>      Device missing:		     0.00B
>      Used:			 560.78MiB
>      Free (estimated):		 451.62MiB	(min: 451.62MiB)
>      Data ratio:			      1.00
>      Metadata ratio:		      1.00
>      Global reserve:		   3.25MiB	(used: 0.00B)
>      Multiple profiles:		        no
> 
> Data,single: Size:904.00MiB, Used:560.38MiB (61.99%)
>     /dev/vdc	 904.00MiB
> 
> Metadata,single: Size:8.00MiB, Used:400.00KiB (4.88%)
>     /dev/vdc	   8.00MiB
> 
> System,single: Size:4.00MiB, Used:16.00KiB (0.39%)
>     /dev/vdc	   4.00MiB
> 
> Unallocated:
>     /dev/vdc	 108.00MiB
> tw:/mnt # btrfs fi du src/file
>       Total   Exclusive  Set shared  Filename
>   560.00MiB   560.00MiB       0.00B  src/file
> tw:/mnt # uname -a
> Linux tw.0.2.15 5.8.0-1-default #1 SMP Tue Aug 4 07:30:59 UTC 2020
> (9bc0044) x86_64 x86_64 x86_64 GNU/Linux
> tw:/mnt #
> 


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