Re: Why does one get mismatches?

[Bill Davidsen <davidsen@tmr.com>]

Neil Brown wrote:
> On Wed, 24 Feb 2010 09:54:17 -0500
> Bill Davidsen <davidsen@tmr.com> wrote:
>
>   
>> Neil Brown wrote:
>>     
>>> md is not in a position to lock the page - there is simply no way it can stop
>>> the filesystem from changing it.
>>> The only thing it could do would be to make a copy, then write the copy out.
>>> This would incur a performance cost.
>>>
>>>     
>>>       
>> Two thoughts on that - one is that for critical data, give me the option 
>> at array start time, make the copy, slow the performance and make it 
>> more consistent. My second thought is that a checksum of the page before 
>> initiating write and after all writes are complete might be less of a 
>> performance hit, and still could detect that the buffer had changed.
>>     
>
>
> The idea of calculating a checksum before and after certainly has some merit,
> if we could choose a checksum algorithm which was sufficiently strong and
> sufficiently fast, though in many cases a large part of the cost would just be
> bringing the page contents into cache - twice.
>
> It has the advantage over copying the page of not needing to allocate extra
> memory.
>
> If someone wanted to try an prototype this and see how it goes, I'd be happy
> to advise....
>   

Disagree if you wish, but MD5 should be fine for this. While it is not 
cryptographically strong on files, where the size can be changed and 
evil doers can calculate values to add at the end of the data, it should 
be adequate on data of unchanging size. It's cheap, fast, and readily 
available.

-- 
Bill Davidsen <davidsen@tmr.com>
  "We can't solve today's problems by using the same thinking we
   used in creating them." - Einstein