[ntp:questions] Re: ntp sanity limit kills ntp daily

Jan Ceuleers janspam.ceuleers at computer.org
Sat May 21 09:19:08 UTC 2005

Brad Knowles wrote:
> At 9:44 AM +0200 2005-05-21, Michael Heiming wrote:
>>  Rubbish, have been using Linux as ntp servers on a larger network
>>  for ages, not a single problem. The problem might be related to
>>  hdparm settings as other already pointed out.
>     That's one I hadn't heard of before.  I knew about the HZ and ACPI 
> issues, which are not unique to Linux, but hdparm settings seems to me 
> like it would be something that probably would be unique to Linux.
>     Does anyone have any more in-depth information on the issue of 
> hdparm settings causing massive loss of interrupts like this?

The only direct reference in the hdparm manpage is the following:

        -u     Get/set interrupt-unmask flag  for  the  drive.   A
               setting  of  1  permits  the driver to unmask other
               interrupts during processing of a  disk  interrupt,
               which  greatly  improves Linux's responsiveness and
               eliminates "serial port overrun" errors.  Use  this
               feature  with caution: some drive/controller combi­
               nations do not tolerate the increased I/O latencies
               possible when this feature is enabled, resulting in
               massive  filesystem  corruption.   In   particular,
               CMD-640B  and RZ1000 (E)IDE interfaces can be unre­
               liable (due to a hardware flaw) when this option is
               used  with  kernel  versions  earlier  than 2.0.13.
               Disabling the IDE prefetch feature of these  inter­
               faces (usually a BIOS/CMOS setting) provides a safe
               fix for the problem for use with earlier kernels.

I don't know how relevant the above cautionary statement still is in 
kernels that are somewhat more recent than 2.0.13...

There is another reference to interrupts in the manpage where it deals 
with IDE Block Mode, but my interpretation is that this merely reduces 
the frequency of disk-related interrupts, without having any influence 
over the masking of other interrupts.

Here you go:

        -m     Get/set sector count for multiple sector I/O on the
               drive.  A setting of 0 disables this feature.  Mul­
               tiple  sector  mode (aka IDE Block Mode), is a fea­
               ture of most modern IDE hard drives, permitting the
               transfer  of  multiple  sectors  per I/O interrupt,
               rather than the usual  one  sector  per  interrupt.
               When  this feature is enabled, it typically reduces
               operating system overhead for disk I/O  by  30-50%.
               On  many  systems,  it also provides increased data
               throughput  of  anywhere  from  5%  to  50%.   Some
               drives,   however   (most  notably  the  WD  Caviar
               series), seem to  run  slower  with  multiple  mode
               enabled.   Your mileage may vary.  Most drives sup­
               port the minimum settings of 2, 4, 8, or  16  (sec­
               tors).   Larger  settings  may  also  be  possible,
               depending on the drive.  A  setting  of  16  or  32
               seems  optimal  on  many  systems.  Western Digital
               recommends lower settings of 4  to  8  on  many  of
               their  drives,  due  tiny  (32kB) drive buffers and
               non-optimized buffering algorithms.   The  -i  flag
               can  be  used to find the maximum setting supported
               by an installed drive (look for MaxMultSect in  the
               output).   Some  drives  claim  to support multiple
               mode, but lose data at some settings.   Under  rare
               circumstances,  such failures can result in massive
               filesystem corruption.


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