Re: [PATCH v2 0/3] btrfs: add read mirror policy

[Anand Jain <anand.jain@oracle.com>]

Thanks Austin and Jeff for the suggestion.

I am not particularly a fan of mount option either mainly because
those options aren't persistent and host independent luns will
have tough time to have them synchronize manually.

Properties are better as it is persistent. And we can apply this
read_mirror_policy property on the fsid object.

But if we are talking about the properties then it can be stored
as extended attributes or ondisk key value pair, and I am doubt
if ondisk key value pair will get a nod.
I can explore the extended attribute approach but appreciate more
comments.

-Anand


On 05/17/2018 10:46 PM, Jeff Mahoney wrote:
> On 5/17/18 8:25 AM, Austin S. Hemmelgarn wrote:
>> On 2018-05-16 22:32, Anand Jain wrote:
>>>
>>>
>>> On 05/17/2018 06:35 AM, David Sterba wrote:
>>>> On Wed, May 16, 2018 at 06:03:56PM +0800, Anand Jain wrote:
>>>>> Not yet ready for the integration. As I need to introduce
>>>>> -o no_read_mirror_policy instead of -o read_mirror_policy=-<devid>
>>>>
>>>> Mount option is mostly likely not the right interface for setting such
>>>> options, as usual.
>>>
>>>    I am ok to make it ioctl for the final. What do you think?
>>>
>>>
>>>    But to reproduce the bug posted in
>>>      Btrfs: fix the corruption by reading stale btree blocks
>>>    It needs to be a mount option, as randomly the pid can
>>>    still pick the disk specified in the mount option.
>>>
>> Personally, I'd vote for filesystem property (thus handled through the
>> standard `btrfs property` command) that can be overridden by a mount
>> option.  With that approach, no new tool (or change to an existing tool)
>> would be needed, existing volumes could be converted to use it in a
>> backwards compatible manner (old kernels would just ignore the
>> property), and you could still have the behavior you want in tests (and
>> in theory it could easily be adapted to be a per-subvolume setting if we
>> ever get per-subvolume chunk profile support).
> 
> Properties are a combination of interfaces presented through a single
> command.  Although the kernel API would allow a direct-to-property
> interface via the btrfs.* extended attributes, those are currently
> limited to a single inode.  The label property is set via ioctl and
> stored in the superblock.  The read-only subvolume property is also set
> by ioctl but stored in the root flags.
> 
> As it stands, every property is explicitly defined in the tools, so any
> addition would require tools changes.  This is a bigger discussion,
> though.  We *could* use the xattr interface to access per-root or
> fs-global properties, but we'd need to define that interface.
> btrfs_listxattr could get interesting, though I suppose we could
> simplify it by only allowing the per-subvolume and fs-global operations
> on root inodes.
> 
> -Jeff
>