Formalizing the use of Boot Area B

Formalizing the use of Boot Area B

[H. Peter Anvin]

It turns out that the primary 64K "Boot Area A" is too small for some
applications and/or some architectures.

When I discussed this with Chris Mason, he pointed out that the area
beyond the superblock is also unused, up until at least the megabyte
point (from my reading of the mkfs code, it is actually slightly more
than a megabyte.)

This is present in all versions of mkfs.btrfs that has the superblock at
64K (some very early ones had the superblock at 16K, but that format is
no longer supported), so all that is needed is formalizing the specs as
to the use of this area.

My suggestion is that 64-128K is reserved for extension of the
superblock and/or any other filesystem uses, and 128-1024K is defined as
Boot Area B.  However, if there may be reason to reserve more, then we
should do that.  Hence requesting a formal decision as to the extent and
ownership of this area.

	-hpa

Re: Formalizing the use of Boot Area B

[H. Peter Anvin]

On 05/14/2014 05:01 PM, H. Peter Anvin wrote:
> It turns out that the primary 64K "Boot Area A" is too small for some
> applications and/or some architectures.
> 
> When I discussed this with Chris Mason, he pointed out that the area
> beyond the superblock is also unused, up until at least the megabyte
> point (from my reading of the mkfs code, it is actually slightly more
> than a megabyte.)
> 
> This is present in all versions of mkfs.btrfs that has the superblock at
> 64K (some very early ones had the superblock at 16K, but that format is
> no longer supported), so all that is needed is formalizing the specs as
> to the use of this area.
> 
> My suggestion is that 64-128K is reserved for extension of the
> superblock and/or any other filesystem uses, and 128-1024K is defined as
> Boot Area B.  However, if there may be reason to reserve more, then we
> should do that.  Hence requesting a formal decision as to the extent and
> ownership of this area.
> 
> 	-hpa
> 

Ping on this?  If I don't hear back on this I will probably just go
ahead and use 128K-1024K.
	
	-hpa

Re: Formalizing the use of Boot Area B

[Chris Mason]

On 05/20/2014 07:29 PM, H. Peter Anvin wrote:
> On 05/14/2014 05:01 PM, H. Peter Anvin wrote:
>> It turns out that the primary 64K "Boot Area A" is too small for some
>> applications and/or some architectures.
>>
>> When I discussed this with Chris Mason, he pointed out that the area
>> beyond the superblock is also unused, up until at least the megabyte
>> point (from my reading of the mkfs code, it is actually slightly more
>> than a megabyte.)
>>
>> This is present in all versions of mkfs.btrfs that has the superblock at
>> 64K (some very early ones had the superblock at 16K, but that format is
>> no longer supported), so all that is needed is formalizing the specs as
>> to the use of this area.
>>
>> My suggestion is that 64-128K is reserved for extension of the
>> superblock and/or any other filesystem uses, and 128-1024K is defined as
>> Boot Area B.  However, if there may be reason to reserve more, then we
>> should do that.  Hence requesting a formal decision as to the extent and
>> ownership of this area.
>>
>> 	-hpa
>>
> 
> Ping on this?  If I don't hear back on this I will probably just go
> ahead and use 128K-1024K.

Hi Peter,

We do leave the first 1MB of each device alone.  Can we do 256K-1024K
for the boot loader?  We don't have an immediate need for the extra
space, but I'd like to reserve a little more than the extra 64KB.

-chris

Re: Formalizing the use of Boot Area B

[H. Peter Anvin]

On 05/20/2014 04:37 PM, Chris Mason wrote:
> On 05/20/2014 07:29 PM, H. Peter Anvin wrote:
>> On 05/14/2014 05:01 PM, H. Peter Anvin wrote:
>>> It turns out that the primary 64K "Boot Area A" is too small for some
>>> applications and/or some architectures.
>>>
>>> When I discussed this with Chris Mason, he pointed out that the area
>>> beyond the superblock is also unused, up until at least the megabyte
>>> point (from my reading of the mkfs code, it is actually slightly more
>>> than a megabyte.)
>>>
>>> This is present in all versions of mkfs.btrfs that has the superblock at
>>> 64K (some very early ones had the superblock at 16K, but that format is
>>> no longer supported), so all that is needed is formalizing the specs as
>>> to the use of this area.
>>>
>>> My suggestion is that 64-128K is reserved for extension of the
>>> superblock and/or any other filesystem uses, and 128-1024K is defined as
>>> Boot Area B.  However, if there may be reason to reserve more, then we
>>> should do that.  Hence requesting a formal decision as to the extent and
>>> ownership of this area.
>>>
>>> 	-hpa
>>>
>>
>> Ping on this?  If I don't hear back on this I will probably just go
>> ahead and use 128K-1024K.
> 
> Hi Peter,
> 
> We do leave the first 1MB of each device alone.  Can we do 256K-1024K
> for the boot loader?  We don't have an immediate need for the extra
> space, but I'd like to reserve a little more than the extra 64KB.
> 

Works for me.  So 64-256K (192K) is reserved for the file system, and
Boot Area B is 256-1024K (768K).

	-hpa

Re: Formalizing the use of Boot Area B

[H. Peter Anvin]

On 05/20/2014 04:37 PM, Chris Mason wrote:
> 
> Hi Peter,
> 
> We do leave the first 1MB of each device alone.  Can we do 256K-1024K
> for the boot loader?  We don't have an immediate need for the extra
> space, but I'd like to reserve a little more than the extra 64KB.
> 

Incidentally, the current version of mkfs.btrfs actually leaves not
1 MB (1024K) but rather 1104K (64K+16K+1024K).  Not sure if that is
intentional.

	-hpa