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    SubjectRe: dm-ioband + bio-cgroup benchmarks

    > > Hi All,
    > >
    > > I have got excellent results of dm-ioband, that controls the disk I/O
    > > bandwidth even when it accepts delayed write requests.
    > >
    > > In this time, I ran some benchmarks with a high-end storage. The
    > > reason was to avoid a performance bottleneck due to mechanical factors
    > > such as seek time.
    > >
    > > You can see the details of the benchmarks at:
    > >


    > Secondly, why do we have to create an additional dm-ioband device for
    > every device we want to control using rules. This looks little odd
    > atleast to me. Can't we keep it in line with rest of the controllers
    > where task grouping takes place using cgroup and rules are specified in
    > cgroup itself (The way Andrea Righi does for io-throttling patches)?

    It isn't essential dm-band is implemented as one of the device-mappers.
    I've been also considering that this algorithm itself can be implemented
    in the block layer directly.

    Although, the current implementation has merits. It is flexible.
    - Dm-ioband can be place anywhere you like, which may be right before
    the I/O schedulers or may be placed on top of LVM devices.
    - It supports partition based bandwidth control which can work without
    cgroups, which is quite easy to use of.
    - It is independent to any I/O schedulers including ones which will
    be introduced in the future.

    I also understand it's will be hard to set up without some tools
    such as lvm commands.

    > To avoid creation of stacking another device (dm-ioband) on top of every
    > device we want to subject to rules, I was thinking of maintaining an
    > rb-tree per request queue. Requests will first go into this rb-tree upon
    > __make_request() and then will filter down to elevator associated with the
    > queue (if there is one). This will provide us the control of releasing
    > bio's to elevaor based on policies (proportional weight, max bandwidth
    > etc) and no need of stacking additional block device.

    I think it's a bit late to control I/O requests there, since process
    may be blocked in get_request_wait when the I/O load is high.
    Please imagine the situation that cgroups with low bandwidths are
    consuming most of "struct request"s while another cgroup with a high
    bandwidth is blocked and can't get enough "struct request"s.

    It means cgroups that issues lot of I/O request can win the game.

    > I am working on some experimental proof of concept patches. It will take
    > some time though.
    > I was thinking of following.
    > - Adopt the Andrea Righi's style of specifying rules for devices and
    > group the tasks using cgroups.
    > - To begin with, adopt dm-ioband's approach of proportional bandwidth
    > controller. It makes sense to me limit the bandwidth usage only in
    > case of contention. If there is really a need to limit max bandwidth,
    > then probably we can do something to implement additional rules or
    > implement some policy switcher where user can decide what kind of
    > policies need to be implemented.
    > - Get rid of dm-ioband and instead buffer requests on an rb-tree on every
    > request queue which is controlled by some kind of cgroup rules.
    > It would be good to discuss above approach now whether it makes sense or
    > not. I think it is kind of fusion of io-throttling and dm-ioband patches
    > with additional idea of doing io-control just above elevator on the request
    > queue using an rb-tree.
    > Thanks
    > Vivek
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