Hello, these are only my thoughts; no code here, but I would like to share it hoping that it could be useful.
As reported several times by Zygo (and others), one of the problem of raid5/6 is the write hole. Today BTRFS is not capable to address it. The problem is that the stripe size is bigger than the "sector size" (ok sector is not the correct word, but I am referring to the basic unit of writing on the disk, which is 4k or 16K in btrfs). So when btrfs writes less data than the stripe, the stripe is not filled; when it is filled by a subsequent write, a RMW of the parity is required. On the best of my understanding (which could be very wrong) ZFS try to solve this issue using a variable length stripe. On BTRFS this could be achieved using several BGs (== block group or chunk), one for each stripe size. For example, if a filesystem - RAID5 is composed by 4 DISK, the filesystem should have three BGs: BG #1,composed by two disks (1 data+ 1 parity) BG #2 composed by three disks (2 data + 1 parity) BG #3 composed by four disks (3 data + 1 parity). If the data to be written has a size of 4k, it will be allocated to the BG #1. If the data to be written has a size of 8k, it will be allocated to the BG #2 If the data to be written has a size of 12k, it will be allocated to the BG #3 If the data to be written has a size greater than 12k, it will be allocated to the BG3, until the data fills a full stripes; then the remainder will be stored in BG #1 or BG #2. To avoid unbalancing of the disk usage, each BG could use all the disks, even if a stripe uses less disks: i.e DISK1 DISK2 DISK3 DISK4 S1 S1 S1 S2 S2 S2 S3 S3 S3 S4 S4 S4 [....] Above is show a BG which uses all the four disks, but has a stripe which spans only 3 disks. Pro: - btrfs already is capable to handle different BG in the filesystem, only the allocator has to change - no more RMW are required (== higher performance) Cons: - the data will be more fragmented - the filesystem, will have more BGs; this will require time-to time a re-balance. But is is an issue which we already know (even if may be not 100% addressed). Thoughts ? BR G.Baroncelli -- gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it> Key fingerprint BBF5 1610 0B64 DAC6 5F7D 17B2 0EDA 9B37 8B82 E0B5 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
