Re: bus error when disk is full, with mmap & sparse file

When I was speaking about a SIGBUS handler, I was not really thinking
about something to recover from errors, but simply check if the SIGBUS
signal have a file descriptor associated with, and so check the free
space to print an hint to users. Like:

Bus error (your disk seem full, the error could be a result of an
impossibility to allocate disk space for a file)

I believe that the strongest way seem to writes zeros before mapping
the file, using plain old write()s, and so simply check write return
code / errno and make rrd_open return nicely and error.

On Mon, Apr 18, 2011 at 12:01 AM, Tobias Oetiker <[email protected]> wrote:
> Hi Francois,
>
> Yesterday Francois-Xavier Bourlet wrote:
>
>> Hi Tobi,
>>
>> Yes it happen at create time.
>>
>> Checking available free space before the creation process would lead
>> to some race condition, because between the time you check the free
>> space and the time you allocate it you can still have some others
>> process/thread allocating it.
>
> yes ...
>
>> But it could be used in another way, by setting up an handler for bus
>> error that check the free space and print a little hint message before
>> exiting the application? The advantage would be zero overhead (until
>> you crash... but do you really care at crash time ;) ) and no
>> modification of the current rrd_open function. What do you think?
>
> having a handler for sigbus sounds like a sensible idea ...
>
> http://www.linuxprogrammingblog.com/code-examples/SIGBUS-handling
>
> as for early vfsstat this could save time for people who try to
> create unreasonably large rrd files by alerting them before
> gigabytes of 0s have been allocated ...
>
> cheers
> tob
>
>> On Sun, Apr 17, 2011 at 10:07 PM, Tobias Oetiker <[email protected]> wrote:
>> > Hi Francois,
>> >
>> > Yesterday Francois-Xavier Bourlet wrote:
>> >
>> >> Hello,
>> >>
>> >> On my system rrd_open use mmap and my system support sparse file.
>> >> That's mean when my disk get full rdd_open can bus error. Here's the
>> >> scenario in rrd_open:
>> >>
>> >> Disk really close to full, few kbytes free
>> >> open file -> ok
>> >> seek to end -1 -> ok
>> >> write 1 -> ok
>> >> the system will only write the last chunk of the file, every others
>> >> will be allocated lazily later because of the sparse file feature.
>> >> So we have a file bigger than the free space available on the system.
>> >> Next attempt to write on this file, even without extending the size of
>> >> it will fail with a disk full error.
>> >>
>> >> next rrd_open map the file and then
>> >> memset to zero the whole file... leading to a buserror since the
>> >> kernel can't write into the file because the filesystem is full.
>> >
>> > this happens at create time, right ?
>> >
>> >> In my case I just have to extend the disk space available and it's
>> >> fine. But the problem is you don't have any clue that the bus error
>> >> happen because you're disk is full, and I really wasted a lots of time
>> >> before I thought simply checking the free space...
>> >>
>> >> I don't really now how to fix the code, maybe we can catch SIGBUS
>> >> signals, and when discovering that the error is about a file mapping,
>> >> provide an human readable message on terminal/log?
>> >>
>> >> Trying to recover from a bus error on file mapped memory seem to be
>> >> another challenge...
>> >>
>> >> Or rather than memsetting the file to zero, we could simply write
>> >> zeros in the file before mapping it, and so it would be easy to catch
>> >> write error.
>> >
>> >> Let me know what do you think about it, I am available to patch rrd
>> >> with the best proposed solution.
>> >
>> > how about a cal to statvfs before starting the whole creation
>> > process ? (for win32 this would bprobably be GetDiskFreeSpaceEx)
>> >
>> > cheers
>> > tobi
>> >
>> >
>> >
>> >>
>> >> Regards,
>> >>
>> >
>> > --
>> > Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
>> > http://it.oetiker.ch [email protected] ++41 62 775 9902 / sb: -9900
>> >
>>
>>
>>
>>
>
> --
> Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
> http://it.oetiker.ch [email protected] ++41 62 775 9902 / sb: -9900
>



-- 
François-Xavier Bourlet