CD9660/md(4)/UFS22 silly behaviour
kostikbel at gmail.com
Mon Jan 9 00:22:12 UTC 2012
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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