On 4/30/13 9:01 AM, "Kumar Sundararajan" <kumar@xxxxxx> wrote: > > >On 4/29/13 11:48 PM, "David Brown" <david.brown@xxxxxxxxxxxx> wrote: > >>On 30/04/13 02:05, Dan Williams wrote: >>> On Mon, Apr 29, 2013 at 12:28 PM, David Brown >>><david.brown@xxxxxxxxxxxx> wrote: >>>> For each data block you are changing, you will need to remove the old >>>>g^i * >>>> Di_old then add in the new g^i * Di_new, so you can still use this >>>> simplification to reduce the number of multiplies. If you want to >>>>change >>>> blocks "i" and "j", you thus do: >>>> >>>> Q_new = Q_old + g^i * (Di_old + Di_new) + g^j * (Dj_old + Dj_new) >>>> >>>> But as I say, I only know the maths - not the code. >>> >>> The issue is where to store those intermediate Di_old + Di_new results >>> without doubling the size of the stripe cache. >>> >> >>(As before, I haven't looked at the code. I justify my laziness by >>claiming that I might come up with fresh ideas without how to implement >>things. But only you folks can say what takes more work, and what is >>riskier in the code.) >> >>I don't see that you would need to double the size of the stripe cache. >> You might need an extra few block spaces, but not double the cache. >> >>Also, once you have done this calculation (assuming you did the easy >>P_new first), you no longer need to keep Di_old lying around - it's >>going to be replaced with the new stripe data. So maybe you can do the >>operation as "Di_old += Di_new" - i.e., in place and without using more >>memory. That is going to be faster too, as it is more cache friendly. >>On the other hand, it might involve more locking or other tracking >>mechanisms to avoid problems if something else is trying to access the >>same caches. >> > >Yes, I had seen your earlier mails on this topic and implemented it this >way initially. >However, this required us to either ask for stable pages or allocate an >extra "spare" page >for each disk in the array to hold the intermediate results. Sorry, that should read -- one extra "spare" page per cpu for each disk in the array. > >> > -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
