On Tue, Apr 22, 2008 at 10:32:12AM -0500, Jose R. Santos wrote:
> I see that now, guess I should not read code with out having
> breakfast. I think 8 is a very safe and conservative number, maybe to
> conservative. The 64 group packing was the number I found to be a
> overall improvement with the limited number of drives that I had to
> test with. Haven't done any testing on old drives or laptop drive with
> slow spindle speed but I would think 16 or 32 would be safe here unless
> the drive is really old and small.
Let's stay with 16 then for now. Spindle speed doesn't actually
matter here; what matters is seek speed, and the density of the disk
drive. The other thing which worries me though is that the size of
each flex_bg block group cluster is dependent on the size of the block
group, which in turn is related to the square of the filesystem
blocksize. i.e., assuming a fs blockgroup size of 16, then:
Blocksize Blocks/blockgroup Blockgroup Size Flex_BG cluster size
1k 8192 8 Meg 128 Meg
2k 16384 32 Meg 512 Meg
4k 32768 128 Meg 2 Gig
8k 65536 512 Meg 8 Gig
16k 131072 2 Gig 32 Gig
32k 262144 8 Gig 128 Gig
64k 524288 32 Gig 512 Gig
So using a fixed default of 16, the flexible blockgroup size can range
anything from 128 megs to half a terabyte!
How much a difference in your numbers are you seeing, anyway? Is it
big enough that we really need to worry about it?
- Ted
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