Re: [RFC][PATCH 0/4] BIG_BG: support of large block groups

[Valerie Clement <valerie.clement@bull.net>]

Theodore Tso wrote:
> So what's the current limitation on the maximum size of the filesystem
> without big block groups?  Well, the block group number is an unsigned
> 32 bit number, so we can have 2**32 block group.  Using a 4k (2**12)
> block group, have a limit of 32768 blocks per block group, or 2**15
> blocks.  So the limit is 2**(32+15) or 2**47 blocks, or 2**59 bytes
> (512 petabytes).
Hi Ted,
thanks for your comments.

In fact, there is another limitation related to the block group size:
all the group descriptors are stored in the first group of the filesystem.
Currently, with a 4-KB block size, the maximum size of a group is
2**15 blocks = 2**27 bytes.
With a group descriptor size of 32 bytes, we can store a maximum of
2**32 / 32 = 2**22 group descriptors in the first group.
So the maximum number of groups is limited to 2**22 which limits the 
size of the filesystem to
2**22(groups) * 2**15(blocks) * 2**12(blocksize) = 2**49 bytes = 512TB

With big block groups, we can grow beyond this limit of 512TB.

> 
> Based on this analysis, it's clear that the big block groups patch has
> some benefits, but I'm wondering if they are sufficiently large to be
> worth it, especially since we also have to consider the changes
> necessary to the e2fsprogs (which haven't been written yet as far as I
> know).

I already made changes in the e2fsprogs to support larger block groups,
but there is still some work to do.
The first thing is when creating a large filesystem (over 512TB or 
perhaps before) which is the optimal value for the block group size ?
How to set its default value used by mkfs ?
It is why I do tests now to see the behavior of a system when increasing 
  the size of the block groups.

   Valérie