On 18 December 2012 03:23, Greg 'groggy' Lehey <g...@freebsd.org> wrote:
> On Monday, 17 December 2012 at 16:44:11 -0500, Dieter BSD wrote:
>> The newfs man page says:
>>
>> -a maxcontig
>> Specify the maximum number of contiguous blocks that will be laid
>> out before forcing a rotational delay. The default value is 16.
>> See tunefs(8) for more details on how to set this option.
>>
>> Is this still a good idea with modern drives where the number of
>> sectors per track varies, and no one but the manufacturer knows how
>> many sectors a particular track has?
>
> No.
>
> It looks as if this, and also a number of comments in sys/ufs/ffs/fs.h
> and sys/ufs/ffs/ffs_alloc.c, are leftovers from the Olden Days. The
> value isn't used anywhere that I can see. Unless somebody can show
> that I'm wrong, I'd suggest that this is a documentation issue that I
> can take a look at.
[performance@ list trimmed]
I'm not sure about this.
In UFS fs_maxcontig controls fs_contigsumsize during newfs, both saved
in superblock, and fs_contigsumsize is widely used throughout FFS.
sblock.fs_maxcontig = maxcontig;
if (sblock.fs_maxcontig < sblock.fs_maxbsize / sblock.fs_bsize) {
sblock.fs_maxcontig = sblock.fs_maxbsize / sblock.fs_bsize;
printf("Maxcontig raised to %d\n", sblock.fs_maxbsize);
}
if (sblock.fs_maxcontig > 1)
sblock.fs_contigsumsize = MIN(sblock.fs_maxcontig,FS_MAXCONTIG);
For ext2 this is instead "reconstructed" in the in-memory sblock copy
in mountfs().
/*
* Calculate the maximum contiguous blocks and size of cluster summary
* array. In FFS this is done by newfs; however, the superblock
* in ext2fs doesn't have these variables, so we can calculate
* them here.
*/
ump->um_e2fs->e2fs_maxcontig = MAX(1, MAXPHYS /
ump->um_e2fs->e2fs_bsize);
if (ump->um_e2fs->e2fs_maxcontig > 0)
ump->um_e2fs->e2fs_contigsumsize =
MIN(ump->um_e2fs->e2fs_maxcontig, EXT2_MAXCONTIG);
else
ump->um_e2fs->e2fs_contigsumsize = 0;
--
wbr,
pluknet
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