Open Stack

A present:

 >>> import contextlib
 >>> import os
 >>>
 >>> @contextlib.contextmanager
... def synced_file(path, mode='wb'):
...   with open(path, mode) as fh:
...      yield fh
...      os.fdatasync(fh.fileno())
...
 >>> with synced_file("/tmp/b.txt") as fh:
...    fh.write("b")
...

Have fun :-P

-Josh

Robert Collins wrote:
> On 23 September 2015 at 09:52, Chris Friesen
> <chris.friesen at windriver.com>  wrote:
>> Hi,
>>
>> I recently had an issue with one file out of a dozen or so in
>> "/opt/cgcs/cinder/data/volumes/" being present but of size zero.  I'm
>> running stable/kilo if it makes a difference.
>>
>> Looking at the code in volume.targets.tgt.TgtAdm.create_iscsi_target(), I'm
>> wondering if we should do a fsync() before the close().  The way it stands
>> now, it seems like it might be possible to write the file, start making use
>> of it, and then take a power outage before it actually gets written to
>> persistent storage.  When we come back up we could have an instance
>> expecting to make use of it, but no target information in the on-disk copy
>> of the file.
>
> If its being kept in sync with DB records, and won't self-heal from
> this situation, then yes. e.g. if the overall workflow is something
> like
>
> receive RPC request
> do some work, including writing the file
> reply to the RPC with 'ok'
> ->  gets written to the DB and the state recorded as ACTIVE or whatever..
>
> then yes, we need to behave as though its active even if the machine
> is power cycled robustly.
>
>> Even then, fsync() explicitly says that it doesn't ensure that the directory
>> changes have reached disk...for that another explicit fsync() on the file
>> descriptor for the directory is needed.
>> So I think for robustness we'd want to add
>>
>> f.flush()
>> os.fsync(f.fileno())
>
> fdatasync would be better - we don't care about the metadata.
>
>> between the existing f.write() and f.close(), and then add something like:
>>
>> f = open(volumes_dir, 'w+')
>> os.fsync(f.fileno())
>> f.close()
>
> Yup, but again - fdatasync here I believe.
>
> -Rob
>
>