> > Nikolai Joukov wrote: > > > replication. In case of RAID4 and RAID5-like configurations, RAIF performed > > > about two times *better* than software RAID and even better than an Adaptec > > > 2120S RAID5 controller. This is because RAIF is located above file system > > > caches and can cache parity as normal data when needed. We have more > > > performance details in a technical report, if anyone is interested. > > > > This doesn't make sense to me. You do not want to cache the parity > > data. It only needs to be used to validate the data blocks when the > > stripe is read, and after that, you only want to cache the data, and > > throw out the parity. Caching the parity as well will pollute the cache > > and thus, should lower performance due to more important data being > > thrown out. > > This happens automatically: unused parity pages are treated as unused > pages and get reused to cache something else. Also, the parity > never gets cached if you do not write the data (or recover the data). > However, if you use the same parity page over and over you do not need to > fetch it from the disk again. To avoid confusion here: data recovery is not the only situation when it is necessary to read the parity. Existing parity is also necessary for writes that are smaller than the page size. Nikolai. --------------------- Nikolai Joukov, Ph.D. Filesystems and Storage Laboratory Stony Brook University - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
