2016-11-18 23:32 GMT+03:00 Janos Toth F. <toth.f.ja...@gmail.com>: > Based on the comments of this patch, stripe size could theoretically > go as low as 512 byte: > https://mail-archive.com/linux-btrfs@vger.kernel.org/msg56011.html > If these very small (0.5k-2k) stripe sizes could really work (it's > possible to implement such changes and it does not degrade performance > too much - or at all - to keep it so low), we could use RAID-5(/6) on > <=9(/10) disks with 512 byte physical sectors (assuming 4k filesystem > sector size + 4k node size, although I am not sure if node size is > really important here) without having to worry about RMW, extra space > waste or additional fragmentation. > > On Fri, Nov 18, 2016 at 7:15 PM, Goffredo Baroncelli <kreij...@libero.it> > wrote: >> 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 > -- > 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
AFAIK all drives at now use 4k physical sector size, and use 512b only logically So it's create another RWM Read 4k -> Modify 512b -> Write 4k, instead of just write 512b. -- Have a nice day, Timofey. -- 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
