Re: [PATCH RFC v2 0/2] readmirror feature

[Josef Bacik <josef@toxicpanda.com>]

On Thu, Sep 12, 2019 at 06:10:08PM +0800, Anand Jain wrote:
> 
> 
> > On 12 Sep 2019, at 6:03 PM, Josef Bacik <josef@toxicpanda.com> wrote:
> > 
> > On Thu, Sep 12, 2019 at 06:00:21PM +0800, Anand Jain wrote:
> >> 
> >> 
> >>> On 12 Sep 2019, at 5:50 PM, Josef Bacik <josef@toxicpanda.com> wrote:
> >>> 
> >>> On Thu, Sep 12, 2019 at 03:41:42PM +0800, Anand Jain wrote:
> >>>> 
> >>>> 
> >>>> Thanks for the comments. More below.
> >>>> 
> >>>> On 12/9/19 3:16 AM, Josef Bacik wrote:
> >>>>> On Wed, Sep 11, 2019 at 03:13:21PM -0400, Eli V wrote:
> >>>>>> On Wed, Sep 11, 2019 at 2:46 PM Josef Bacik <josef@toxicpanda.com> wrote:
> >>>>>>> 
> >>>>>>> On Mon, Aug 26, 2019 at 05:04:36PM +0800, Anand Jain wrote:
> >>>>>>>> Function call chain  __btrfs_map_block()->find_live_mirror() uses
> >>>>>>>> thread pid to determine the %mirror_num when the mirror_num=0.
> >>>>>>>> 
> >>>>>>>> This patch introduces a framework so that we can add policies to determine
> >>>>>>>> the %mirror_num. And also adds the devid as the readmirror policy.
> >>>>>>>> 
> >>>>>>>> The new property is stored as an item in the device tree as show below.
> >>>>>>>>    (BTRFS_READMIRROR_OBJECTID, BTRFS_PERSISTENT_ITEM_KEY, devid)
> >>>>>>>> 
> >>>>>>>> To be able to set and get this new property also introduces new ioctls
> >>>>>>>> BTRFS_IOC_GET_READMIRROR and BTRFS_IOC_SET_READMIRROR. The ioctl argument
> >>>>>>>> is defined as
> >>>>>>>>        struct btrfs_ioctl_readmirror_args {
> >>>>>>>>                __u64 type; /* RW */
> >>>>>>>>                __u64 device_bitmap; /* RW */
> >>>>>>>>        }
> >>>>>>>> 
> >>>>>>>> An usage example as follows:
> >>>>>>>>        btrfs property set /btrfs readmirror devid:1,3
> >>>>>>>>        btrfs property get /btrfs readmirror
> >>>>>>>>          readmirror devid:1 3
> >>>>>>>>        btrfs property set /btrfs readmirror ""
> >>>>>>>>        btrfs property get /btrfs readmirror
> >>>>>>>>          readmirror default
> >>>>>>>> 
> >>>>>>>> This patchset has been tested completely, however marked as RFC for the
> >>>>>>>> following reasons and comments on them (or any other) are appreciated as
> >>>>>>>> usual.
> >>>>>>>> . The new objectid is defined as
> >>>>>>>>      #define BTRFS_READMIRROR_OBJECTID -1ULL
> >>>>>>>>   Need consent we are fine to use this value, and with this value it
> >>>>>>>>   shall be placed just before the DEV_STATS_OBJECTID item which is more
> >>>>>>>>   frequently used only during the device errors.
> >>>>>>>> 
> >>>>>>>> .  I am using a u64 bitmap to represent the devices id, so the max device
> >>>>>>>>   id that we could represent is 63, its a kind of limitation which should
> >>>>>>>>   be addressed before integration, I wonder if there is any suggestion?
> >>>>>>>>   Kindly note that, multiple ioctls with each time representing a set of
> >>>>>>>>   device(s) is not a choice because we need to make sure the readmirror
> >>>>>>>>   changes happens in a commit transaction.
> >>>>>>>> 
> >>>>>>>> v1->RFC v2:
> >>>>>>>>  . Property is stored as a dev-tree item instead of root inode extended
> >>>>>>>>    attribute.
> >>>>>>>>  . Rename BTRFS_DEV_STATE_READ_OPRIMIZED to BTRFS_DEV_STATE_READ_PREFERRED.
> >>>>>>>>  . Changed format specifier from devid1,2,3.. to devid:1,2,3..
> >>>>>>>> 
> >>>>>>>> RFC->v1:
> >>>>>>>>  Drops pid as one of the readmirror policy choices and as usual remains
> >>>>>>>>  as default. And when the devid is reset the readmirror policy falls back
> >>>>>>>>  to pid.
> >>>>>>>>  Drops the mount -o readmirror idea, it can be added at a later point of
> >>>>>>>>  time.
> >>>>>>>>  Property now accepts more than 1 devid as readmirror device. As shown
> >>>>>>>>  in the example above.
> >>>>>>>> 
> >>>>>>> 
> >>>>>>> This is a lot of infrastructure
> >>>> 
> >>>> Ok. Any idea on a better implementation?
> >>>> How about extended attribute approach? v1 patches proposed
> >>>> it, but it abused the extended attribute as commented here [1]
> >>>> and v2 got changed to an item-key.
> >>>> 
> >>>> [1]
> >>>> https://lore.kernel.org/linux-btrfs/be68e6ea-00bc-b750-25e1-9c584b99308f@gmx.com/
> >>>> 
> >>> 
> >>> That's a NAK on the prop interface.  This is a fs wide policy, not a
> >>> directory/inode policy.
> >>> 
> >>>> 
> >>>>>>> to just change which mirror we read to based on
> >>>>>>> some arbitrary user policy.  I assume this is to solve the case where you have
> >>>>>>> slow and fast disks, so you can always read from the fast disk?  And then it's
> >>>>>>> only used in RAID1, so the very narrow usecase of having a RAID1 setup with a
> >>>>>>> SSD and a normal disk?  I'm not seeing a point to this much code for one
> >>>>>>> particular obscure setup.  Thanks,
> >>>>>>> 
> >>>>>>> Josef
> >>>>>> 
> >>>>>> Not commenting on the code itself, but as a user I see this SSD RAID1
> >>>>>> acceleration as a future much have feature. It's only obscure at the
> >>>>>> moment because we don't have code to take advantage of it. But on
> >>>>>> large btrfs filesystems with hundreds of GB of metadata, like I have
> >>>>>> for backups, the usability of the filesystem is dramatically improved
> >>>>>> having the metadata on an SSD( though currently only half of the time
> >>>>>> due to the even/odd pid distribution.)
> >>>>> 
> >>>>> But that's different from a mirror.  100% it would be nice to say "put my
> >>>>> metadata on the ssd, data elsewhere".  That's not what this patch is about, this
> >>>>> patch is specifically about changing which drive we choose in a mirrored setup,
> >>>>> which is super unlikely to mirror a SSD with a slow drive, cause it's just going
> >>>>> to be slow no matter what.  Sure we could make it so reads always go to the SSD,
> >>>>> but we can accomplish that by just adding a check for nonrotational in the code,
> >>>>> and then we don't have to encode all this nonsense in the file system.  Thanks,
> >>>> 
> >>>> I wrote about the readmirror policy framework here[2],
> >>>> I forgot to link it here, sorry about that, my mistake.
> >>>> 
> >>>> [2]
> >>>> 
> >>>> https://lore.kernel.org/linux-btrfs/1552989624-29577-1-git-send-email-anand.jain@oracle.com/
> >>>> 
> >>>> Readmirror policy is for raid1, raid10 and future N way mirror.
> >>>> Yes for now its only for raid1.
> >>>> 
> >>>> Here the idea is to create a framework so that readmirror policy
> >>>> can be configured as needed. And nonrotational can be one such policy.
> >>>> 
> >>>> The example of hard-coded nonrotational policy does not work in case
> >>>> of ssd and a remote iscsi ssd, OR in case of local ssd and a NVME block
> >>>> device, as all these are still nonrotational devices. So hard-coded
> >>>> policy is not a good idea. If we have to hardcode then there is Q-depth
> >>>> based readmirror routing is better (patch in the ML), but that is
> >>>> not good enough, because some configs wants it based on the disk-LBA
> >>>> so that SAN storage target cache is balanced and not duplicated.
> >>>> So in short it must be a configurable policy.
> >>>> 
> >>> 
> >>> Again, if you are mixing disk types you likely always want non-rotational, but
> >>> still mixing different speed devices in a mirror setup is just asking for weird
> >>> latency problems.  I don't think solving this use case is necessary.  If you mix
> >>> ssd + network device in a serious production setup then you probably should be
> >>> fired cause you don't know what you are doing.  Having the generic
> >>> "nonrotational gets priority" is going to cover 99% of the actual use cases that
> >>> make sense.
> >>> 
> >>> The SAN usecase I can sort of see, but again I don't feel like it's a problem we
> >>> need to solve with on-disk format.  Add a priority to sysfs so you can change it
> >>> with udev or something on the fly.  Thanks,
> >>> 
> >> 
> >> Ok.
> >> Sysfs is fine however we need configuration to be persistent across reboots.
> >> Any idea?
> >> 
> > 
> > Udev rules.  Thanks,
> > 
> 
>  Josef, configs moving along with the luns in san seems to be more
>  easy to manage, otherwise when the host fails, each potential new
>  server has to be pre-configured with the udev rules. 
> 

It's 2019, if people haven't figured out how to do persistent configuration by
now then all hope is lost.  Facebook persistently configures millions of
machines, I'm sure people can figure out how to make sure a udev rule ends up on
the right host connected to a SAN that doesn't move.  Thanks,

Josef