Re: Abysmal disk performance, how to debug?

[Bill Davidsen]

Willy Tarreau wrote:
> On Sat, Jan 20, 2007 at 02:56:20PM -0500, Stephen Clark wrote:
> > Sunil Naidu wrote:
> >
> > > On 1/20/07, Willy Tarreau <w@xxxxxx> wrote:
> > >
> > >
> > > > It is not expected to increase write performance, but it should help
> > > > you do something else during that time, or also give more responsiveness
> > > > to Ctrl-C. It is possible that you have fast and slow RAM, or that your
> > > > video card uses shared memory which slows down some parts of memory
> > > > which are not used anymore with those parameters.
> > > >   
> > > I did test some SATA drives, am getting these value for 2.6.20-rc5:-
> > >
> > > [sukhoi@Typhoon ~]$ time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024
> > > 1024+0 records in
> > > 1024+0 records out
> > > 1073741824 bytes (1.1 GB) copied, 21.0962 seconds, 50.9 MB/s
> > >
> > > What can you suggest here w.r.t my RAM & disk?
> > >
> > >
> > >
> > > > Willy
> > > >   
> > > Thanks,
> > >
> > > ~Akula2
> > > -
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> > Hi,
> > whitebook vbi s96f core 2 duo t5600 2gb hitachi ATA      HTS721060G9AT00 
> > using libata
> > time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024
> > 1024+0 records in
> > 1024+0 records out
> > 1073741824 bytes (1.1 GB) copied, 10.0092 seconds, 107 MB/s
> >
> > real    0m10.196s
> > user    0m0.004s
> > sys     0m3.440s
>
> You have too much RAM, it's possible that writes did not complete before
> the end of your measurement. Try this instead :
>
> $ time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024 | sync
I'm not sure that does what you think it does, the sync runs first, and 
data is still in the cache. Replace sync with "/bin/echo RAN" if you 
doubt me.

What you want is this:
  sync;
  time bash -c "dd if=/dev/zero of=/tmp/1GB bs=1M count=1024; sync"
which will actually time the write with the time command.


--
Bill Davidsen <davidsen@xxxxxxx>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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