On 09/13/2017 04:15 PM, Marat Khalili wrote: > On 13/09/17 16:23, Chris Murphy wrote: >> Right, known problem. To use o_direct implies also using nodatacow (or >> at least nodatasum), e.g. xattr +C is set, done by qemu-img -o >> nocow=on >> https://www.spinics.net/lists/linux-btrfs/msg68244.html > Can you please elaborate? I don't have exactly the same problem as described > by the link, but I'm still worried that particularly qemu can be less > resilient to partial raid1 failures even on newer kernels, due to missing > checksums for instance. (BTW I didn't find any xattrs on my VM images, nor do > I plan to set any.)
>From what Josef Bacik wrote, I understood that it is not only related to >RAID1. I tried to ask further clarifications without success :( It seems that simply using O_DIRECT could allow checksums mismatch. My understand is that to avoid to copy the data between buffer, the checksum computation is subject to data race: i.e. it is possible that the kernel computes the checksum *and* the user space program change the data. This lead to an io error during a subsequent read. To avoid that BTRFS should copy in a temporary buffer the data, and then compute the checksum. But this is what the common sense suggest that O_DIRECT should avoid. If I understood correctly (which is a BIG if), i think that O_DIRECT should be unsupported (i.e. return -EINVAL) if the file is "not marked" as "nodatacsum" I looked to what ZFSOL does: it seems that it doesn't support O_DIRECT [1] for the same reason (see the comments ' ryao commented on Jul 23, 2015' for further details). Anyway I suggest to read what the open(2) man page says about O_DIRECT: it seems that O_DIRECT has to be used carefully when doing fork; the man page concludes: [...] In summary, O_DIRECT is a potentially powerful tool that should be used with caution. It is recommended that applications treat use of O_DIRECT as a performance option which is disabled by default. [...] [1] https://github.com/zfsonlinux/zfs/issues/224 > > -- > > With Best Regards, > Marat Khalili > -- > 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 > 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
