Re: Size of journal?

[Eric Sandeen <sandeen@redhat.com>]

Jeff Layton wrote:
> Good afternoon!
> 
> I was wondering what the default journal size is for ext4
> when it's built on a single partition? Or does it vary in size
> as needed?
> 
> TIA!
> 
> Jeff

See figure_journal_size() in mke2fs.c.  It's called w/ size == -1
if no other size is specified on the commandline so that goes
to ext2fs_default_journal_size(), which is pretty straightforward
in its scaling with nr of filesystem blocks:

/*
 * Find a reasonable journal file size (in blocks) given the number of blocks
 * in the filesystem.  For very small filesystems, it is not reasonable to
 * have a journal that fills more than half of the filesystem.
 */
int ext2fs_default_journal_size(__u64 blocks)
{
        if (blocks < 2048)
                return -1;
        if (blocks < 32768)
                return (1024);
        if (blocks < 256*1024)
                return (4096);
        if (blocks < 512*1024)
                return (8192);
        if (blocks < 1024*1024)
                return (16384);
        return 32768;
}