On 02/01/2018 01:12 AM, Anand Jain wrote: > > > On 02/01/2018 01:26 PM, Edmund Nadolski wrote: >> On 1/31/18 7:36 AM, Anand Jain wrote: >>> >>> >>> On 01/31/2018 09:42 PM, Nikolay Borisov wrote: >>> >>> >>>>>> So usually this should be functionality handled by the raid/san >>>>>> controller I guess, > but given that btrfs is playing the role of a >>>>>> controller here at what point are we drawing the line of not >>>>>> implementing block-level functionality into the filesystem ? >>>>> >>>>> Don't worry this is not invading into the block layer. How >>>>> can you even build this functionality in the block layer ? >>>>> Block layer even won't know that disks are mirrored. RAID >>>>> does or BTRFS in our case. >>>>> >>>> >>>> By block layer I guess I meant the storage driver of a particular raid >>>> card. Because what is currently happening is re-implementing >>>> functionality that will generally sit in the driver. So my question was >>>> more generic and high-level - at what point do we draw the line of >>>> implementing feature that are generally implemented in hardware devices >>>> (be it their drivers or firmware). >>> >>> Not all HW configs use RAID capable HBAs. A server connected to a SATA >>> JBOD using a SATA HBA without MD will relay on BTRFS to provide all >>> the >>> features and capabilities that otherwise would have provided by such a >>> presumable HW config. >> >> That does sort of sound like means implementing some portion of the >> HBA features/capabilities in the filesystem. >> >> To me it seems this this could be workable at the fs level, provided it >> deals just with policies and remains hardware-neutral. > > Thanks. Ok. > >> However most >> of the use cases appear to involve some hardware-dependent knowledge >> or assumptions. > >> What happens when someone sets this on a virtual disk, >> or say a (persistent) memory-backed block device? > > Do you have any policy in particular ?
No, this is your proposal ;^) You've said cases #3 thru #6 are illustrative only. However they make assumptions about the underlying storage, and/or introduce potential for unexpected behaviors. Plus they could end up replicating functionality from other layers as Nikolay pointed out. Seems unlikely these would be practical to implement. Case #2 seems concerning if it exposes internal, implementation-dependent filesystem data into a de facto user-level interface. (Do we ensure the devid is unique, and cannot get changed or re-assigned internally to a different device, etc?) Thanks, Ed >> Case #6 seems to >> open up some potential for unexpected interactions (which may be hard >> to reproduce, esp. in error/recovery scenarios). > > Yep. Even the #1 pid based (current default) which motivated > me to provide the devid based policy. > >> Case #2 takes a devid, but I notice btrfs_device::devid says, "the >> internal btrfs device id". How does a user obtain that internal value >> so it can be set as a mount option? > > btrfs fi show -m > > Thanks, Anand > >> Thanks, >> Ed >> >> >>>>>>> :: >>>>>>>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c >>>>>>>>> index 39ba59832f38..478623e6e074 100644 >>>>>>>>> --- a/fs/btrfs/volumes.c >>>>>>>>> +++ b/fs/btrfs/volumes.c >>>>>>>>> @@ -5270,6 +5270,16 @@ static int find_live_mirror(struct >>>>>>>>> btrfs_fs_info *fs_info, >>>>>>>>> num = map->num_stripes; >>>>>>>>> switch(fs_info->read_mirror_policy) { >>>>>>>>> + case BTRFS_READ_MIRROR_BY_DEV: >>>>>>>>> + optimal = first; >>>>>>>>> + if (test_bit(BTRFS_DEV_STATE_READ_MIRROR, >>>>>>>>> + &map->stripes[optimal].dev->dev_state)) >>>>>>>>> + break; >>>>>>>>> + if (test_bit(BTRFS_DEV_STATE_READ_MIRROR, >>>>>>>>> + &map->stripes[++optimal].dev->dev_state)) >>>>>>>>> + break; >>>>>>>>> + optimal = first; >>>>>>>> >>>>>>>> you set optimal 2 times, the second one seems redundant. >>>>>>> >>>>>>> No actually. When both the disks containing the stripe does not >>>>>>> have the BTRFS_DEV_STATE_READ_MIRROR, then I would just want to >>>>>>> use first found stripe. >>>>>> >>>>>> Yes, and the fact that you've already set optimal = first right after >>>>>> BTRFS_READ_MIRROR_BY_DEV ensures that, no ? Why do you need to again >>>>>> set >>>>>> optimal right before the final break? What am I missing here? >>>>> >>>>> Ah. I think you are missing ++optimal in the 2nd if. >>>> >>>> You are right, but I'd prefer you index the stripes array with >>>> 'optimal' >>>> and 'optimal + 1' and leave just a single assignment >>> >>> Ok. Will improve that. >>> >>> Thanks, Anand >>> >>> >>>>> >>>>> Thanks, Anand >>>>> >>>> -- >>>> 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 >>> >> >> -- >> 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 > -- 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