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