Re: [RFC] BIG_BG vs extended META_BG in ext4

["Jose R. Santos" <jrs@us.ibm.com>]

On Sat, 30 Jun 2007 11:06:16 -0400
Laurent Vivier <Laurent.Vivier@bull.net> wrote:

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> Le 29 juin 07 à 18:09, Jose R. Santos a écrit :
> Hi Jose,

Hi Laurent,

Seems like your emails are not making it to the mailing list.  I got
them fine though.

> Thank you for the question ;-)
> 
> BIG_BG allows to limit the number of groups (at least in the group  
> counter).
> IMHO, I think it could be important in some cases.

Yes, I think bigger block groups will benefit extents a great deal
since not only can we have larger extents, but I believe that as the
filesystem ages the chances of getting large number contiguous block can
be reduce with small block groups.
 
> For instance, if we keep the same inode table allocation politic, we  
> divide the total number of inode in the FS by the total number of  
> groups.
> For the moment, number of inode < 2^32 and if we have number of block  
> group > 2^32 the number of inode per group is 0.... is META_BG able  
> to manage this case ?

Good point.  It is a scenario that needs to be looked, although I
sincerely hope that we get 64-bit inodes implemented by the time
storage devices get that big. ;)
 
> With META_BG, a 2^48 blocks FS will have 2^48 / 2^12 = 2^36 groups.  
> Perhaps it could be interesting to have less groups ?

Agree...
 
> With less groups, we load less group descriptors in memory, we have  
> less I/O to read bitmap and inode array (because we manage less group  
> descriptors again, because we load bigger bitmap and array in one time)

Presumably, we would still need to access the same amount data but
latencies should be reduce since we could do larger IO's and less seeks
to read the bitmaps.  I also wonder if there are benefits in terms of
locality to having the bitmaps closer to its blocks vs having them far
away like in xMETA_BG.

> Regards,
> Laurent

-JRS