Re: [concept & "good taste" review] persistent store
From: david
Date: Tue Dec 21 2010 - 19:56:01 EST
On Wed, 22 Dec 2010, Huang Ying wrote:
On Tue, 2010-12-21 at 18:10 +0800, David Howells wrote:
Huang Ying <ying.huang@xxxxxxxxx> wrote:
- for OOPS messages will not cause system panic, it will go to disk and
will not use up the persistent storage.
You can't guarantee that an oops didn't just kill your ability to actually
write your syslog to disk or out across the network.
I do not need to guarantee that. If the OOPS message can not be written
to disk, just keeping it in persistent storage, and that is the very
value of persistent storage. But for OOPS can go to disk safely, we do
not need to waste persistent storage for it.
My point is how do you know an oops message will actually manage to get to
disk? There's a userspace program (syslogd) between the kernel log and the
disk or network.
The user space program (syslogd) is in my big picture, it will guarantee
an oops meesage actually go to disk via something like fsync. After
doing that, the user space program can erase the corresponding record in
persistent storage to free the space. So all in all, oops messages not
causing system panic or disk error will go to disk eventually and being
freed and will not use up the persistent storage.
almost nobody runs syslog with a fsync after each message anymore. the
problem is that doing so reduced throughput so much that you ended up
loosing more messages (and causing processes to block, resulting in
user-visible problems) because the messages had to queue up for
processing.
so if you want to record critical messages and be guaranteed that they are
on disk, you will be needing a specific application, and not just using
standard syslog.
David Lang
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