On Tue, 22 Apr 2008 14:57:28 -0400 Theodore Tso <tytso@xxxxxxx> wrote: > 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 Well higher spindle speed affect cylinder seek times which affect overall seek time, which is why I think it should be tested as well. > 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 I do not have any data on multiple block size and I have not done testing with the 64K equivalent of 4096 groups for a 4k filesystem. The testing scenarios in a 4k filesystem should also be different than those for a 64k filesystem, so the testing I did in 4k does not necessarily apply to a bigger block size. The default of 16 is a safe number for 4k block size. I would think that the larger the block size, the smaller the flex_bg packing size should be since larger block size address some of the issues that flex_bg tries to address. -JRS -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
