Re: [LSF/MM ATTEND] memory allocation scope

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

On 02/14/2018 09:53 PM, NeilBrown wrote:
> On Wed, Feb 14 2018, Goldwyn Rodrigues wrote:
> 
>> Discussion with the memory folks towards scope based allocation
>> I am working on converting some of the GFP_NOFS memory allocation calls
>> to new scope API [1]. While other allocation types (noio, nofs,
>> noreclaim) are covered. Are there plans for identifying scope of
>> GFP_ATOMIC allocations? This should cover most (if not all) of the
>> allocation scope.
>>
>> Transient Errors with direct I/O
>> In a large enough direct I/O, bios are split. If any of these bios get
>> an error, the whole I/O is marked as erroneous. What this means at the
>> application level is that part of your direct I/O data may be written
>> while part may not be. In the end, you can have an inconsistent write
>> with some parts of it written and some not. Currently the applications
>> need to overwrite the whole write() again.
> 
> So?
> If that is a problem for the application, maybe it should use smaller
> writes.  If smaller writes cause higher latency, then use aio to submit
> them.
> 
> I doubt that splitting bios is the only thing that can cause a write
> that reported as EIO to have partially completed.  An application should
> *always* assume that EIO from a write means that the data on the device
> is indistinguishable from garbage - shouldn't it?
> 

Yes, and that is what I got from others as well. The scenario is not
deterministic of the contents of the file in case of overwriting a file.
And no, splitting bios is not the only reason you can have partial
write. This is different from what buffered I/O would result in, where a
partial write may not be an error and returns the bytes written.

Perhaps this needs to be documented in the man pages. I will put in one
shortly.

-- 
Goldwyn

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