Nikita Danilov wrote:
> Rogier Wolff writes:
> >
> > Hi,
> >
> > the "ls" command is horribly slow on my system....
> >
> > I'm doing some stuff with large files. From the old days when files
> > couldn't be larger than 2G, I'm manipulating 1G files.
> >
> > There is currently a program runnning that will make a file sparse if
> > it finds only zeroes in a block. It's reading between 20 and 30Mb per
> > second off the disks, and currently finding only zeros, so there is no
> > writing going on.
>
> Reiserfs puts on-disk inode (stat-data) near file "body" which is an
> array of pointers to actual blocks of the file. This optimizes the case
> of short files, because inode and file "body" can be read in one io. But
> when there are many large files in the same directory, this results in
> inodes of the files from the same directory being far from each other on
> the disk, making readdir() or sequential stat() slower. Reiser4 uses
> (will use, that is) different allocation policy that tries to address
> this.
OK. so the layout on the disk is non-optimal.
But I'd expect performance on the order of:
times
in ms
0 my first stat finishes
0.1 the other program has already issued a 1Mb read from the
other part of the disk.
my ls issues a second stat.
10 seek finishes
18 rotational latency.
58 read (1Mb) finishes (25Mb per second -> 40ms /Mb).
68 seek to "my ls" finishes.
76 rotational latency.
76.1 my stat has transferred its 1k, in the intervening time the
other program has already issued a new read for 1Mb of data.
86 seek to other program's data finishes...
So I'd expect something like 10-14 stats per second. While in fact my
trace showed 3 to 5 stats per second. Still at 50 files in a directory
waiting around 4-5 seconds to see results from an "ls" is kind of
annoyingly slow, and I would've reported it as well.
Maybe the stat requires 2 to 3 IOs to complete? That would explain the
difference.
So the question becomes: Why does a stat require 2 or 3 IOs? OR why is
too much independent stuff scheduled inbetween that one IO takes
200-300 ms?...
Roger.
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