Re: [PATCH v7 rebased 0/5] readmirror feature (sysfs and in-memory only approach; with new read_policy device)

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

David,

  I am not sure if this will be integrated in 5.8 and worth the time to
  rebase. Kindly suggest.

-Anand

On 6/4/20 7:51 pm, Anand Jain wrote:
> v7:
> Fix switch's fall through warning. Changle logs updates where necessary.
> 
> v6:
> Patch 4/5 - If there is no change in device's read prefer then don't log
> Patch 4/5 - Add pid to the logs
> Patch 5/5 - If there isn't read preferred device in the chunk don't reset
> read policy to default, instead just use stripe 0. As this is in
> the read path it avoids going through the device list to find
> read preferred device. So inline to this drop to check if there
> is read preferred device before setting read policy to device.
> 
> __ Original email: __
> 
> v5:
> Worked on review comments as received in its previous version.
> Please refer to individual patches for the specific changes.
> Introduces the new read_policy 'device'.
> 
> v4:
> Rename readmirror attribute to read_policy. Drop separate kobj for
> readmirror instead create read_policy attribute in fsid kobj.
> merge v2:2/3 and v2:3/3 into v4:2/2. Patch titles have changed.
>   
> v3:
> v2:
> Mainly fixes the fs_devices::readmirror declaration type from atomic_t
> to u8. (Thanks Josef).
> 
> v1:
> As of now we use only %pid method to read stripped mirrored data. So
> application's process id determines the stripe id to be read. This type
> of routing typically helps in a system with many small independent
> applications tying to read random data. On the other hand the %pid
> based read IO distribution policy is inefficient if there is a single
> application trying to read large data as because the overall disk
> bandwidth would remains under utilized.
> 
> One type of readmirror policy isn't good enough and other choices are
> routing the IO based on device's waitqueue or manual when we have a
> read-preferred device or a readmirror policy based on the target storage
> caching. So this patch-set introduces a framework where we could add more
> readmirror policies.
> 
> This policy is a filesystem wide policy as of now, and though the
> readmirror policy at the subvolume level is a novel approach as it
> provides maximum flexibility in the data center, but as of now its not
> practical to implement such a granularity as you can't really ensure
> reflinked extents will be read from the stripe of its desire and so
> there will be more limitations and it can be assessed separately.
> 
> The approach in this patch-set is sys interface with in-memory policy.
> And does not add any new readmirror type in this set, which can be add
> once we are ok with the framework. Also the default policy remains %pid.
> 
> Previous works:
> ----------------------------------------------------------------------
> There were few RFCs [1] before, mainly to figure out storage
> (or in memory only) for the readmirror policy and the interface needed.
> 
> [1]
> https://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg86368.html
> 
> https://lore.kernel.org/linux-btrfs/20190826090438.7044-1-anand.jain@oracle.com/
> 
> https://lore.kernel.org/linux-btrfs/5fcf9c23-89b5-b167-1f80-a0f4ac107d0b@oracle.com/
> 
> https://patchwork.kernel.org/cover/10859213/
> 
> Mount -o:
> In the first trial it was attempted to use the mount -o option to carry
> the readmirror policy, this is good for debugging which can make sure
> even the mount thread metadata tree blocks are read from the disk desired.
> It was very effective in testing radi1/raid10 write-holes.
> 
> Extended attribute:
> As extended attribute is associated with the inode, to implement this
> there is bit of extended attribute abuse or else makes it mandatory to
> mount the rootid 5. Its messy unless readmirror policy is applied at the
> subvol level which is not possible as of now.
> 
> An item type:
> The proposed patch was to create an item to hold the readmirror policy,
> it makes sense when compared to the abusive extended attribute approach
> but introduces a new item and so no backward compatibility.
> -----------------------------------------------------------------------
> 
> Anand Jain (5):
>    btrfs: add btrfs_strmatch helper
>    btrfs: create read policy framework
>    btrfs: create read policy sysfs attribute, pid
>    btrfs: introduce new device-state read_preferred
>    btrfs: introduce new read_policy device
> 
>   fs/btrfs/sysfs.c   | 128 +++++++++++++++++++++++++++++++++++++++++++++
>   fs/btrfs/volumes.c |  39 +++++++++++++-
>   fs/btrfs/volumes.h |  16 ++++++
>   3 files changed, 182 insertions(+), 1 deletion(-)
>