[Devel] [PATCH RH9] dm-ploop: port the standby mode feature.

[Konstantin Khorenko]

Hi Liu, Alexey,

to summarize a bit:

In case we have ploops without iSCSI targets on top (but may be with vStorage underneath), in case we 
get these errors EBUSY/ENOTCONN/EIO we get stuck io on ploop (which reason by the way is quite 
difficult to detect) - and this is not we want to get.

There can be setups with a Node + vStorage + ploops with iSCSI ontop AND ploops without iSCSI on top 
on the same node,
thus i have to suggest a per-ploop flag, otherwise we will not distingiush those ploop setups.

And we also do not want to have a performance degradation if our setup is not
"vStorage + ploops with iSCSI ontop", so i suggest to put that per-ploop checks under a static key and 
enable this key, for example, on the first iSCSI over ploop creation.

Please suggest the implementation and what would be the trigger for static key switch and how to mark 
ploop devices with that flag,
from my point of view it might be either some kernel code trigger like scst module load (for static 
key switch) and iSCSI target creation in scst module (for marking ploop devices), or may be we can 
introduce a new ploop device construction userspace command line option which will say "hey, this 
ploop is gonna be used for iSCSI, please put the appropriate flag for the device".

Thank you.

--
Best regards,

Konstantin Khorenko,
Virtuozzo Linux Kernel Team

On 14.10.2022 11:42, Alexander Atanasov wrote:
> Hello,
> 
> On 14.10.22 8:57, Kui Liu wrote:
>>
>>
>> -----Original Message-----
>> From: Alexander Atanasov <alexander.atanasov at virtuozzo.com>
>> Date: Thursday, 13 October 2022 at 11:26 PM
>> To: Kui Liu <Kui.Liu at acronis.com>, Konstantin Khorenko <khorenko at virtuozzo.com>, Alexey Kuznetsov <kuznet at acronis.com>
>> Cc: Devel <devel at openvz.org>, Alexander Mikhalitsyn <alexander.mikhalitsyn at virtuozzo.com>
>> Subject: Re: [Devel] [PATCH RH9] dm-ploop: port the standby mode feature.
>>
>>       On 13.10.22 16:33, Kui Liu wrote:
>>       > Hello,
>>       >
>>       > First of all, please bear in mind that this patch is a port of a patchset currently present in VZ 7 kernel.
>>       > Original patches were added just to address a problem that would only happen in our particular use case,
>>       > where we need to use ploop devices as the back store for  iSCSI target, while ploop devices are backed
>>       > by files stored in vstorage filesystem.
>>
>>
>>       Ok, i don't quite get this yet. Since it is specific to vstorage can we
>>       check the queue on init and see if it is backed by that iSCSI target and
>>       set a static key or flag to enable the checks only if that is the case?
>>       check can be done via queue->disk->major/minor/diskname/fops/pick-one
>>       from a quick look.
>>
>> [LIU]: Do you mean that you want a 'check standby mode enable' flag so that the check
>> is done only when the enable flag is true, and the flag should be set by iSCSI target when the
>> ploop device is attached iSCSI target?
>> Well, it can be done, but I'm not sure why that's necessary?  Current code is kind copied
>> directly from original implementation, where there wasn't such 'enable' flag, the question would
>> be why it wasn't implemented back then?
> 
> Yes, I think it is better to be explicit and not blindly assume that it
> wouldn't affect anything. Old ploop code might not needed it. But
> dm-ploop is new code and it might need that flag - point is that it is
> better be safe and we know that we run on exactly that target.
> 
>>
>>       [snip]
>>
>>       >      >> +    /* move to standby if delta lease was stolen or mount is gone */
>>       >      >> +    if (res != -EBUSY && res != -ENOTCONN && res != -EIO) {
>>       >      >> +        return;
>>       >      >> +    }
>>       >      >> +
>>       >      >> +    spin_lock_irq(&q->queue_lock);
>>       >      >> +    prev = blk_queue_flag_test_and_set(QUEUE_FLAG_STANDBY, q);
>>       >      >> +    spin_unlock_irq(&q->queue_lock);
>>       >      >> +
>>       >      >> +    if (!prev)
>>       >      >> +        pr_info("ploop: switch into standby mode\n");
>>       >      >> +}
>>       >
>>       >
>>       >      What guarantees that we got EBUSY, EIO or ENOTCONN for the reasons
>>       >      listed in the description - "This mode shows that a delta lease was
>>       >      stolen and it is impossible to handle any requests." - what if we got
>>       >      such an error for other reason? If the problem comes from nfs why not
>>       >      suspend device mapper from there ?
>>       >
>>       > [LIU]: I would assume vstorage filesystem guarantees that only EBUSY, EIO, or ENOTCONN
>>       > will be returned when the described event happens, and it doesn't matter whether we could
>>       > get these errors for other reasons.  And what matters is, in our use case, once the 3 errors are
>>       > returned, the ploop device need to be flagged as 'standby mode' which needs to be passed to
>>       >   iSCSI target driver.  As for what happens when used with other filesystems, we actually don't care,
>>       >   however you can see that the changes don't affect ploop's normal behaviour either. >
>>       >      Also do this errors actually reach dm-ploop ? From my recent tests i
>>       >      observed that if you have and EIO the filesystem gets it and remounts RO
>>       >      - it might not reach ploop code at all. Even if you suspend/resume the
>>       >      queue it would not help with the RO fs.
>>       >
>>       > [LIU]:  When used with vstorage filesystem, these errors do reach dm-ploop.  Again, we really
>>       > don’t care about other filesystems as long as it doesn't break anything.
>>
>>       If there is no check for the specific target and some of the errors
>>       comes from other device - ploop will say it is into standby but no
>>       device will check the flags - so it will not be true. My point is that
>>       it is hard to say it won't break anything.
>>
>> [LIU]: Currently only iSCSI target driver is aware of this flag, and of course we have code in iSCSI target driver
>> to deal with this flag.  However for any other ploop users who are not aware of the standby flag,  the flag is just
>>    transparent, and it doesn't matter whether the flag is set or clear anyway, then how does it break anything?
>>
>> Or is your concern that there may be users that would test the entire flag without masking out uninterested bits?
>> I don’t believe such use exists in the kernel code.
> 
> My concern is more about the debug message in case some other driver
> return one of the errors.
> 
>>
>>
>>
>>       [snip]
>>       There is ploop->ti->table->md->queue, why not use it but cache queue ptr
>>       here ? Is it guaranteed that queue won't change and leave ploop with
>>       dangling ptr?
>>
>> [LIU]: Because of convenience,  and yes, it is guaranteed that the queue won't change
>> during ploop's lifespan. Looking at dm-mapper's code, apparently  the 'md', hence 'md->queue',
>> outlives 'ploop', and md->queue can't be changed while ploop is still alive.
>> In case of table reload,  a new ploop instance will allocated and initialized,  the old one will be destroyed.
> 
> 
> Ok, i double checked and you are right about this.
> 
>