Reiser3/4 & Ext2/3 was: First impressions of reiserfs4

[Mike Fedyk <mfedyk@matchmail.com>]

On Thu, Sep 11, 2003 at 02:29:38PM +0400, Oleg Drokin wrote:
> Hello!
> 
> On Tue, Sep 09, 2003 at 12:10:44PM -0700, Mike Fedyk wrote:
> 
> > > But my experiments quickly shown that if you ask mkfs to create inode tables with
> > > free inodes that exceed blocks count for the device, then mkfs will only create as much free inodes
> > > as there are free blocks on the device (I was needing that when I experimented with 60 millions files
> > > on ext2/reiserfs/xfs and stuff and I only had 20G partition.)
> > Hmm, didn't know this, but it makes sence for ext2/3 since they use 1 block
> > per file/directory.  It wouldn't do much good to waste more space for inode
> > tables than you could even theoretically use.
> 
> Well, in fact empty files do not need this block.
> 

True.  Do you know if ext2/3 allocates the block even for empty files?  So
if you create the file, it should be sparse until you write something to it,
right?  Does the touch command do this?

> > > > tail merging off, 1k files per directory and all files the same size as
> > > > block size with 40M files.  How would the table look as far as space effency
> > > Hm. I will probably try this once.
> > > For reiserfs:
> > > I can tell you that 60M+ empty files (cannot remember exact number, but I still have the script to create those)
> > > took ~5.5G of space. 
> > With how many directories?  Do you run into drive speed limitations with
> 
> Ok. I looked at the script. There should be 182900000 files created. (182.9M)
> 100 files per dir.
> the dir structure was like this (in number of dirs per level):
> 31/59/25/40
> Files were only created at the end of hierarchy.
> See the script at the end if you are interested or want to try it yourself.
> (script was donated to us by somebody, only it was shell script,
> also I changed variable-names to hide identity).
> 

Hmm, any experiments with more files per dir (maybe 500 or 1000)?  I'm not
sure if you're going to use a directory full block with 100 files per dir in
ext2/3.

> > inodes?  In ext2/3 they're currently 128 bytes I believe plus some static
> > bitmaps in the block groups.  The only thing variable in ext2/3 are the
> > directory sizes (and they don't shrink... :( )
> 
> Well in reiserfs we have statdata (each object should have one), this is sort of
> like ext2 inode, only not static. It's size is 44 bytes (plus 24 bytes item
> header overhead). Each metadata block has header of 24 bytes.
> If you write to file (not looking at tail case yet), you create "indirect"
> items in which block pointers are stored.
> (4 bytes per pointer, when you use all space in metadata block, next block is
> allocated (24 bytes of overhead + pointer in higher level block) plus
> new indirect item (24 bytes of overhead again))

Are these indirects stored in the tree, or do you have many partially filled
indirect blocks?

> Also bitmaps are static (1 block per 128M of space in case of 4k blocksize)
> as are superblock, journal and journal header.
> 

How many superblocks are there in reiser3?  Also, the bitmap locations are
static, and allocated at mkfs time?  How is that done so fast for large
filesystems?