Announcement

Collapse
No announcement yet.

Btrfs Gets More RAID 5/6 Fixes In Linux 4.16

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #41
    Originally posted by caligula View Post
    Having 10 disks in RAID-1 is just dumb.
    This entirely depends on your use case. For BTRFS a 10 disk "RAID"-1 can actually yield better performance than a RAID10. Keep in mind that BTRFS "RAID"-1 is actually very close to RAID1-E. Also BTRFS's implementation of "RAID"5/6 or single or dual parity is far from ok (but there has been lots of patches on the mailing list to improve this recently) and therefore RAID-1 is not dumb at all.


    Originally posted by caligula View Post
    Well, MD-RAID is a dumb RAID. AFAIK it has no access to filesystem level metadata and checksums. With Btrfs you can do "smart" RAID. You can even double sequential reads assuming the application can communicate the need for "readahead" without wasting perf on small intermediate buffers. The big difference is, with Btrfs you can check the integrity of data after a single read from a single disk. If the data differs from stuff on the other disk, this can be done in the background as a separate delayed job or even as late as in scrub / only when a read fails. So it performs basically like RAID-0, perf-wise.
    MD RAID absolutely does not know anything about the filesystem metadata and/or checksums. MD does BLOCK LEVEL and don't care about what filesystem people put on top of it.
    I am not sure I follow you, but assuming we still discuss RAID1 like behavior here - usually a program will either execute a read() with a large buffer or a small buffer. BTRFS will NOT double sequential read speed unless you have multiple processes asking for the same data, at which point you rely on the process PID and the number of mirrors you have. You may be lucky or not depending on what PID's you get. There where patches trying to improve this based on congestion that I helped to test , but I don't think they where merged.

    BTRFS also does not check integrity by comparing the same read from two disks. It first read off one disk and verifies that to a checksum. If that fails then BTRFS will try another copy which may be on a different disk (or even the same disk depending on what dataprofile you have chosen). This is not done "in the background", but transparently to the program trying to read some data. The read() will not return until the filesystem have either been able to return the correct data, restore it from a working copy (in which case BTRFS will also write out a extra good copy to restore the redundancy), or failed the operation completely.

    BTRFS "RAID"-1 does NOT perform like RAID0 as this is a very different. It can achieve impressive speed if you have lots of processes reading from a larger array at the same time (one of the reason a 10 disk RAID-1 is not dumb). RAID0 is better for large sequential reads (video editing for example) and *may* not perform as well for many smaller reads.

    http://www.dirtcellar.net

    Comment


    • #42
      Originally posted by pal666 View Post
      your math is wrong. raid1 can read all N disks in parallel. though btrfs didn't do that in the past, it was supposed to be fixed. but write on raid5 is read-modify-write
      Nitpick , but BTRFS have always since it was merged in kernel v2.6.29 been able to read all disks in parallel (if you have enough processes accessing) PID%mirrors https://git.kernel.org/pub/scm/linux...=v2.6.29#n2504

      http://www.dirtcellar.net

      Comment


      • #43
        Originally posted by gbcox View Post
        Do a quick Google search - or simply go back and review previous Phoronix articles concerning bcachefs and btrfs.
        "do your own research" is not an answer.

        I already did google searches before posting the question and did not find anything particulary interesting, it's still more or less working on single-drive features, and now he is getting 1500$ per month on patreon.

        Comment


        • #44
          Originally posted by starshipeleven View Post
          "do your own research" is not an answer.

          I already did google searches before posting the question and did not find anything particulary interesting, it's still more or less working on single-drive features, and now he is getting 1500$ per month on patreon.
          Hummmm.... you need to improve your search skills...
          Last edited by gbcox; 01 February 2018, 10:41 AM.

          Comment


          • #45
            Originally posted by gbcox View Post
            Hummmm.... you need to improve your search skills...
            i can help you with search. if it's not mainline, it does not exist

            Comment


            • #46
              Originally posted by pal666 View Post
              i can help you with search. if it's not mainline, it does not exist
              Correct, because it's your strawman.

              Comment


              • #47
                it would only be a strawman if you were not insisting on its existence

                Comment


                • #48
                  Originally posted by pal666 View Post
                  it would only be a strawman if you were not insisting on its existence
                  ¯\_(ツ)_/¯ You need to read the definition of strawman argument, then go back and re-read the thread.

                  Comment

                  Working...
                  X