2016-11-18 21:15 GMT+03:00 Goffredo Baroncelli <kreijack@xxxxxxxxx>: > 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 AFAIK, it's difficult to do such things with btrfs, because btrfs use chuck allocation for metadata & data, i.e. AFAIK ZFS work with storage more directly, so zfs directly span file to the different disks. May be it's can be implemented by some chunk allocator rework, i don't know. Fix me if i'm wrong, thanks. -- Have a nice day, Timofey. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
