Trond Myklebust wrote:
> On Thu, 2007-07-12 at 10:12 +1000, Neil Brown wrote:
>> When a write to a local filesystem hits a space limitation such as
>> filesystem-full or quota-exhausted, the write fails synchronously.
>> You get ENOSPC or EDQUOT immediately.
>>
>> NFS cannot do that efficiently.
>>
>> Currently, you don't get these errors until 'fsync' or 'close'.  That
>> is very different from local filesystem behaviour, and is less than
>> ideal.
>>
>> The following patch causes these two errors to be returned through the
>> next write call once they are known about.
>>
>> A possible extension would be to set a flag when we first get such an
>> error, clear it when a write succeeds, and while the flag is set, do
>> all writes synchronously.  This would be even closer to
>> local-filesystem semantics, but I'm not sure it is worth it.
> 
> Well... I've been thinking along these last lines myself (i.e. forcing
> all subsequent writes to be synchronous whenever an error occurs).

OK, but...

After it is triggered by a write error, how would the synchronous 
behavior be turned off?  Both EDQUOT and ENOSPC, for example, are really 
a temporary error; if applications are smart enough, they can retry an 
EDQUOT and might get a successful write.  In that case, performance 
would be hosed until.... the client decides it's OK to use unstable 
writes again.