[CentOS] reboot - is there a timeout on filesystem flush?

Wed Jan 7 19:37:38 UTC 2015
Gary Greene <ggreene at minervanetworks.com>

> On Jan 6, 2015, at 9:23 PM, Gordon Messmer <gordon.messmer at gmail.com> wrote:
> On 01/06/2015 04:37 PM, Gary Greene wrote:
>> This has been discussed to death on various lists, including the
>> LKML...
>> Almost every controller and drive out there now lies about what is
>> and isn’t flushed to disk, making it nigh on impossible for the
>> Kernel to reliably know 100% of the time that the data HAS been
>> flushed to disk. This is part of the reason why it is always a Good
>> Idea™ to have some sort of pause in the shut down to ensure that it
>> IS flushed.
> That's pretty much entirely irrelevant to the original question.
> (Feel free to correct me if I'm wrong in the following)
> A filesystem has three states: Clean, Dirty, and Dirty with errors.
> When a filesystem is unmounted, the cache is flushed and it is marked clean last.  This is the expected state when a filesystem is mounted.
> Once a filesystem is mounted read/write, then it is marked dirty.  If a filesystem is dirty when it is mounted, then it wasn't unmounted properly.  In the case of a journaled filesystem, typically the journal will be replayed and the filesystem will then be mounted.
> The last case, dirty with errors indicates that the kernel found invalid data while the filesystem was mounted, and recorded that fact in the filesystem metadata.  This will normally be the only condition that will force an fsck on boot.  It will also normally result in logs being generated when the errors are encountered.  If your filesystems are force-checked on boot, then the logs should usually tell you why.  It's not a matter of a timeout or some device not flushing its cache.
> Of course, the other possibility is simply that you've formatted your own filesystems, and they have a maximum mount count or a check interval.  Use 'tune2fs -l' to check those two values.  If either of them are set, then there is no problem with your system.  It is behaving as designed, and forcing a periodic check because that is the default behavior.
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Problem is, Gordon, the layer I’m talking about is _below_ the logical layer that filesystems live at, in the block layer, at the mercy of drivers, and firmware that the kernel has zero control over. While in a perfect world, the controller would do strictly only what the Kernel tells it, that just isn’t true for a while now with the large caches that drives and controllers have now.

In most cases, this should never trigger, however in some buggy drivers, or controllers that have buggy firmware, the writes can be seriously delayed to disk, which can cause data to never make it to the platter.

Gary L. Greene, Jr.
Sr. Systems Administrator
IT Operations
Minerva Networks, Inc.
Cell: +1 (650) 704-6633