Formalizing the use of Boot Area B
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
Quoted 13 lines · 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.
Quoted 8 lines
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
Quoted 20 lines · H. Peter Anvin
Quoted 19 lines · 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.
Quoted 5 lines
Quoted 2 lines
-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
Quoted 24 lines · Chris Mason
Quoted 23 lines · H. Peter Anvin
Quoted 21 lines · 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
Quoted 9 lines
Quoted 2 lines
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
Quoted 7 lines · Chris Mason
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