"- 8" in EXT2_MAX_BLOCKS_PER_GROUP
When looking at the maximal filesystem size issue, I found myself wondering what the "- 8" is in here, it's not commented or documented anywhere:
#define EXT2_MAX_BLOCKS_PER_GROUP(s) (((1 << 16) - 8) * \
(EXT2_CLUSTER_SIZE(s) / \
EXT2_BLOCK_SIZE(s)))(pre-bigalloc, it was just ((1 << 16) - 8) )
Anyone know?
-Eric
Quoted 4 lines · Eric Sandeen
When looking at the maximal filesystem size issue, I found myself wondering what the "- 8" is in here, it's not commented or documented anywhere:
Quoted 8 lines
#define EXT2_MAX_BLOCKS_PER_GROUP(s) (((1 << 16) - 8) * \ (EXT2_CLUSTER_SIZE(s) / \ EXT2_BLOCK_SIZE(s)))(pre-bigalloc, it was just ((1 << 16) - 8) )
Anyone know?
Ah, Darrick pointed out
http://osdir.com/ml/file-systems.ext2.devel/2006-03/msg00032.html
So it would have been - 1, to not overflow __u16, but since we have multiples of 8, we get - 8.
But now we have bg_free_blocks_count_hi, giving us 32 bits of counter. With EXT4_FEATURE_INCOMPAT_64BIT, MAX_BLOCKS_PER_GROUP should grow, no?
-Eric
Quoted 16 lines · Eric Sandeen
Quoted 4 lines · Eric Sandeen
When looking at the maximal filesystem size issue, I found myself wondering what the "- 8" is in here, it's not commented or documented anywhere:
Quoted 8 lines
#define EXT2_MAX_BLOCKS_PER_GROUP(s) (((1 << 16) - 8) * \ (EXT2_CLUSTER_SIZE(s) / \ EXT2_BLOCK_SIZE(s)))(pre-bigalloc, it was just ((1 << 16) - 8) )
Anyone know?
Ah, Darrick pointed out
Quoted 7 lines
http://osdir.com/ml/file-systems.ext2.devel/2006-03/msg00032.html
So it would have been - 1, to not overflow
__u16, but since we have multiples of 8, we get - 8.But now we have
bg_free_blocks_count_hi, giving us 32 bits of counter. WithEXT4_FEATURE_INCOMPAT_64BIT,MAX_BLOCKS_PER_GROUPshould grow, no?
As far as I know, each group contains a block bitmap that is exactly 1 block long, and block can be no longer than 4096 bytes in length. Therefore, a group can have no more than 4096 * 8 = 32768 blocks, correct? To get more we'd have to allow blocks larger than 4K (mkfs won't allow that) or change the disk format to allow multi-block block bitmaps.
Unless of course a block bitmap can be the size of a cluster.... which afaict is not the case.
--D
Quoted 6 lines
-Eric
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Quoted 25 lines · Darrick J. Wong
Quoted 16 lines · Eric Sandeen
Quoted 4 lines · Eric Sandeen
When looking at the maximal filesystem size issue, I found myself wondering what the "- 8" is in here, it's not commented or documented anywhere:
Quoted 8 lines
#define EXT2_MAX_BLOCKS_PER_GROUP(s) (((1 << 16) - 8) * \ (EXT2_CLUSTER_SIZE(s) / \ EXT2_BLOCK_SIZE(s)))(pre-bigalloc, it was just ((1 << 16) - 8) )
Anyone know?
Ah, Darrick pointed out
Quoted 7 lines
http://osdir.com/ml/file-systems.ext2.devel/2006-03/msg00032.html
So it would have been - 1, to not overflow
__u16, but since we have multiples of 8, we get - 8.But now we have
bg_free_blocks_count_hi, giving us 32 bits of counter. WithEXT4_FEATURE_INCOMPAT_64BIT,MAX_BLOCKS_PER_GROUPshould grow, no?
Quoted 8 lines
As far as I know, each group contains a block bitmap that is exactly 1 block long, and block can be no longer than 4096 bytes in length. Therefore, a group can have no more than 4096 * 8 = 32768 blocks, correct? To get more we'd have to allow blocks larger than 4K (mkfs won't allow that) or change the disk format to allow multi-block block bitmaps.
Unless of course a block bitmap can be the size of a cluster.... which afaict is not the case.
You are right:
if (fs_param.s_blocks_per_group) {
if (fs_param.s_blocks_per_group < 256 ||
fs_param.s_blocks_per_group > 8 * (unsigned) blocksize) {
com_err(program_name, 0,
_("blocks per group count out of range"));So I guess 64k blocks on ia64 or ppc64 would get us up to 512k blocks per group.
but then MAX_BLOCKS_PER_GROUP limits us to less than that, at least for now.
So still not sure what bg_free_blocks_count_hi does for us.
-Eric
Quoted 1 line
--D