On 2011-10-03, at 10:00 PM, Ted Ts'o <tytso@xxxxxxx> wrote:
> On Mon, Oct 03, 2011 at 04:55:11PM -0500, Eric Sandeen wrote:
>> Has anyone tried mke2fs at its limits? The latest git tree seems to fail in several ways.
>> (Richard Jones reported the initial failure)
>>
>> # truncate --size 1152921504606846976 reallybigfile
>> # mke2fs -t ext4 reallybigfile
>>
>> first,
>>
>> Warning: the fs_type huge is not defined in mke2fs.conf
>>
>> (when types "big" and "huge" got added, they never got a mke2fs.conf update?)
>
> It used to be that an undefined file system type didn't flag an error.
> It now does, so we should have definitions for them in mke2fs.conf.
>
>> reallybigfile: Not enough space to build proposed filesystem while setting up superblock
Isn't there also a problem with the number of block group descriptor blocks in the first group, if meta_bg is not used? With 64-byte group descriptors per 128MB group this is 1024 bytes of descriptors for 2GB of blocks, or 128MB of descriptors for 256TB of blocks. At this point group 0 is full of primary block descriptors and group 1 is full of backup descriptors, and we are out of luck to make a larger filesystem.
That is only 2^48 bytes, not 2^48 blocks (2^60 bytes), so it means meta_bg needs to get into more testing, and online resize with flex_bg needs to move forward.
>> because:
>>
>> fs->group_desc_count = (blk_t) ext2fs_div64_ceil(
>> ext2fs_blocks_count(super) - super->s_first_data_block,
>> EXT2_BLOCKS_PER_GROUP(super));
>> if (fs->group_desc_count == 0) {
>> retval = EXT2_ET_TOOSMALL;
>>
>> The div64_ceil returns > 2^32 (2^33, actually), and the cast to blk_t
>> (which should be dgrp_t?) turns that into a 0.
>
> Yep, that should be dgrp_t. Oops.
>
>> Trying it with "-O bigalloc" (which should be automatic at this size,
>> I think?) just goes away for a very long time, I'm not sure what it's
>> thinking about, or if it's in a loop somewhere (looking now).
>
> Well, we probably do want to engage bigalloc automatically, at some
> point (I want to wait until bigalloc is in commonly used kernels, at
> least for community distro's). I'm not sure what the best cluster
> size to pick by default should be, though. 16k? 64k?
>
> - Ted
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