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Re: [PATCH v3 0/9] block-backend: Introduce I/O hang

From: cenjiahui
Subject: Re: [PATCH v3 0/9] block-backend: Introduce I/O hang
Date: Tue, 3 Nov 2020 20:19:32 +0800
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2

On 2020/10/30 21:21, Stefan Hajnoczi wrote:
> On Thu, Oct 29, 2020 at 05:42:42PM +0800, cenjiahui wrote:
>> On 2020/10/27 0:53, Stefan Hajnoczi wrote:
>>> On Thu, Oct 22, 2020 at 09:02:54PM +0800, Jiahui Cen wrote:
>>>> A VM in the cloud environment may use a virutal disk as the backend 
>>>> storage,
>>>> and there are usually filesystems on the virtual block device. When backend
>>>> storage is temporarily down, any I/O issued to the virtual block device 
>>>> will
>>>> cause an error. For example, an error occurred in ext4 filesystem would 
>>>> make
>>>> the filesystem readonly. However a cloud backend storage can be soon 
>>>> recovered.
>>>> For example, an IP-SAN may be down due to network failure and will be 
>>>> online
>>>> soon after network is recovered. The error in the filesystem may not be
>>>> recovered unless a device reattach or system restart. So an I/O rehandle is
>>>> in need to implement a self-healing mechanism.
>>>> This patch series propose a feature called I/O hang. It can rehandle AIOs
>>>> with EIO error without sending error back to guest. From guest's 
>>>> perspective
>>>> of view it is just like an IO is hanging and not returned. Guest can get
>>>> back running smoothly when I/O is recovred with this feature enabled.
>>> Hi,
>>> This feature seems like an extension of the existing -drive
>>> rerror=/werror= parameters:
>>>   werror=action,rerror=action
>>>       Specify which action to take on write and read errors. Valid
>>>       actions are: "ignore" (ignore the error and try to continue),
>>>       "stop" (pause QEMU), "report" (report the error to the guest),
>>>       "enospc" (pause QEMU only if the host disk is full; report the
>>>       error to the guest otherwise).  The default setting is
>>>       werror=enospc and rerror=report.
>>> That mechanism already has a list of requests to retry and live
>>> migration integration. Using the werror=/rerror= mechanism would avoid
>>> code duplication between these features. You could add a
>>> werror/rerror=retry error action for this feature.
>>> Does that sound good?
>>> Stefan
>> Hi Stefan,
>> Thanks for your reply. Extending the rerror=/werror= mechanism is a feasible
>> way for the retry feature.
>> However, AFAIK, the rerror=/werror= mechanism in block-backend layer only
>> provides ACTION, and the real handler of errors need be implemented several
>> times in device layer for different devices. While our I/O Hang mechanism
>> directly handles AIO errors no matter which type of devices it is. Is it a
>> more common way to implement the feature in block-backend layer? Especially 
>> we
>> can set retry timeout in a common structure BlockBackend.
>> Besides, is there any reason that QEMU implements the rerror=/werror 
>> mechansim
>> in device layer rather than in block-backend layer?
> Yes, it's because failed requests can be live-migrated and retried on
> the destination host. In other words, live migration still works even
> when there are failed requests.
> There may be things that can be refactored so there is less duplication
> in devices, but the basic design goal is that the block layer doesn't
> keep track of failed requests because they are live migrated together
> with the device state.
> Maybe Kevin Wolf has more thoughts to share about rerror=/werror=.
> Stefan

Hi Kevin,

What do you think about extending rerror=/werror= for the retry feature?

And which place is better to set retry timeout, BlockBackend in
block layer or per device structure in device layer?


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