On 2013-01-14, at 2:03 PM, Theodore Ts'o wrote: > On Sun, Jan 13, 2013 at 05:08:14PM +0800, Zheng Liu wrote: >> From: Zheng Liu <wenqing.lz@xxxxxxxxxx> >> >> There are two bugs need to be fixed, which are about cluster-size. >> Now the range of cluster-size is from 1024 to 512M bytes. Although >> with '-C 1024', the cluster-size will be 4096 after making a >> filesystem because in ext2fs_initialize() set_field() needs to check >> 'param->s_log_cluster_size' and s_log_cluster_size is 0 as >> cluster-size is 1024. Then s_log_cluster_size will be assigned to >> s_log_block_size+4. So we never set cluster-size to 1024. >> >> Another bug is that when cluster-size is 512M EXT2FS_C2B will return >> 0. So s_blocks_per_group will be assigned to zero and we will meet >> a 'division by zero' error. > > There are a couple of things going on here. The first is that it > makes no senes when the cluster size is less than or equal to the > block size. (Actually, nothing bad should happen in the case when > cluster size == block size, but if the user specified the bigalloc > feature, that's something which they almost certainly don't want.) > > So the more general check is we should be complaining if the cluster > size is <= the block size. That is, the combination of -b 4096 and > -C 2048 makes no sense, either. > > Also, there's technically nothing wrong with a cluster size of 512MB. > The problem is in how we calculate the default number of clusters per > group --- if it translates to a number of blocks per group which > overals 2**32, that's when we run into problems. > > Which leads to another bug in the current mke2fs command. The range > checking for the -g (which allows you to specify the number of blocks > per group is bogus in the case when the bigalloc feature is enabled). > I think the best way of fixing this is to document that the -g option > specifies the number of clusters per block if the bigalloc feature is > enabled. Presumably you mean "the -g option specifies the number of clusters _per_group_ if the bigalloc feature is enabled"? Cheers, Andreas -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
