CD9660/md(4)/UFS22 silly behaviour

[Kostik Belousov]

On Sun, Jan 08, 2012 at 04:09:00PM -0800, Don Lewis wrote:
> On  8 Jan, Poul-Henning Kamp wrote:
> > 
> > I'm doing som data-mining on a pile of ISO images right now.
> > 
> > I stuck the ISOs on a UFS2 on a flash-disk for speed, and mdconfig(8)'d
> > them so I could mount them.
> > 
> > The traffic pattern his "interesting":
> > 
> > dT: 1.003s  w: 1.000s
> >  L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w   %busy Name
> > [...]
> >     1    733    733   1466    1.3      0      0    0.0   98.2| md39
> >     1    733    733  23449    1.3      0      0    0.0   93.2| da0
> > 
> > Notice the 1:16 ratio on kBps but 1:1 ratio on ops/s ?
> > 
> > da0's UFS2 has 32k block-size:
> > 
> > 	magic   19540119 (UFS2) time    Wed Jan  4 16:41:47 2012
> > 	superblock location     65536   id      [ 4f046cf5 c30697ee ]
> > 	ncg     104     size    19537685        blocks  19228156
> > 	bsize   32768   shift   15      mask    0xffff8000
> > 	fsize   4096    shift   12      mask    0xfffff000
> > 	[...]
> > 
> > It looks like every 2k read from CD9660 turns into a 32k block
> > read in the UFS filesystem, without any beneficial caching happening.
> 
> Probably some confusion about which filesystem layer owns the cached
> data.  It would probably be inefficient to cache the data in both
> places.  The best fix would probably be for CD9660 to think that the
> underlying device has 32Kb sectors.
I discussed the issue with phk further. The reason for discarding
the 30K of the read 32K block is that md(4) supplies IO_DIRECT flag
for VOP_READ, and FFS avoids putting the read data into any cache.

Most likely, we can implement a sysctl that would disable direct
reads, at the cost of double-buffering the data. For obvious reasons,
it is impossible to disable caching from the filesystem living on
top of md(4) volume.
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