On Thu, Nov 10, 2016 at 12:46:20PM -0800, Song Liu wrote: > Recovery of write-back cache has different logic to write-through only > cache. Specifically, for write-back cache, the recovery need to scan > through all active journal entries before flushing data out. Therefore, > large portion of the recovery logic is rewritten here. > > To make the diffs cleaner, we split the rewrite as follows: > > 1. In this patch, we: > - add new data to r5l_recovery_ctx > - add new functions to recovery write-back cache > The new functions are not used in this patch, so this patch does not > change the behavior of recovery. > > 2. In next patch, we: > - modify main recovery procedure r5l_recovery_log() to call new > functions > - remove old functions > > With cache feature, there are 2 different scenarios of recovery: > 1. Data-Parity stripe: a stripe with complete parity in journal. > 2. Data-Only stripe: a stripe with only data in journal (or partial > parity). > > The code differentiate Data-Parity stripe from Data-Only stripe with > flag STRIPE_R5C_WRITE_OUT. > > For Data-Parity stripes, we use the same procedure as raid5 journal, > where all the data and parity are replayed to the RAID devices. > > For Data-Only strips, we need to finish complete calculate parity and > finish the full reconstruct write or RMW write. For simplicity, in > the recovery, we load the stripe to stripe cache. Once the array is > started, the stripe cache state machine will handle these stripes > through normal write path. > > r5c_recovery_flush_log contains the main procedure of recovery. The > recovery code first scans through the journal and loads data to > stripe cache. The code keeps tracks of all these stripes in a list > (use sh->lru and ctx->cached_list), stripes in the list are > organized in the order of its first appearance on the journal. > During the scan, the recovery code assesses each stripe as > Data-Parity or Data-Only. > > During scan, the array may run out of stripe cache. In these cases, > the recovery code will also call raid5_set_cache_size to increase > stripe cache size. If the array still runs out of stripe cache > because there isn't enough memory, the array will not assemble. > > At the end of scan, the recovery code replays all Data-Parity > stripes, and sets proper states for Data-Only stripes. The recovery > code also increases seq number by 10 and rewrites all Data-Only > stripes to journal. This is to avoid confusion after repeated > crashes. More details is explained in raid5-cache.c before > r5c_recovery_rewrite_data_only_stripes(). I'm ok with this patch in current stage. This one has a lot of areas to improve: - the r5c_recovery_lookup_stripe algorithm looks silly - r5c_recovery_analyze_meta_block will read the data twice. One in verify checksum and the other in loading to stripe. We could estimate the maximum pages a meata could use, then read data to the pages. we could do the read in very checksum and then copy the data to stripe. - r5c_recovery_rewrite_data_only_stripes doesn't need to use sync io, we can dispatch several io and wait in batch. Please remember to fix these later. Thanks, Shaohua -- 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
