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Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no management

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  Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no  management Lee Duncan <lduncan@suse.com> - 2015-11-12 17:40 +0100
    Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no management "Martin K. Petersen" <martin.petersen@oracle.com> - 2015-11-13 23:00 +0100
      Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no  management Hannes Reinecke <hare@suse.de> - 2015-11-16 13:20 +0100
        Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no  management Lee Duncan <lduncan@suse.com> - 2015-11-16 22:50 +0100
          Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no management "Martin K. Petersen" <martin.petersen@oracle.com> - 2015-11-18 00:30 +0100

#1268099 — Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no management

On 10/14/2015 08:53 PM, James Bottomley wrote:
> On Wed, 2015-10-14 at 11:34 -0700, Lee Duncan wrote:
>> On 10/14/2015 06:55 AM, James Bottomley wrote:
>>> On Wed, 2015-10-07 at 16:51 -0700, Lee Duncan wrote:
>>>> Update the SCSI hosts module to use the ida_simple*() routines
>>>> to manage its host_no index instead of an ATOMIC integer. This
>>>> means that the SCSI host number will now be reclaimable.
>>>
>>> OK, but why would we want to do this?  We do it for sd because our minor
>>> space for the device nodes is very constrained, so packing is essential.
>>> For HBAs, there's no device space density to worry about, they're
>>> largely statically allocated at boot time and not reusing the numbers
>>> allows easy extraction of hotplug items for the logs (quite useful for
>>> USB) because each separate hotplug has a separate and monotonically
>>> increasing host number.
>>>
>>> James
>>>
>>
>> Good question, James. Apologies for not making the need clear.
>>
>> The iSCSI subsystem uses a host structure for discovery, then throws it
>> away. So each time it does discovery it gets a new host structure. With
>> the current approach, that number is ever increasing. It's only a matter
>> of time until some user with a hundreds of disks and perhaps thousands
>> of LUNs, that likes to do periodic discovery (think super-computers)
>> will run out of host numbers. Or, worse yet, get a negative number
>> number (because the value is signed right now).
>>
>> And this use case is a real one right now, by the way.
> 
> Um, so even if you do discovery continuously, say one every second, it
> still will take 68 years before we wrap the sign.
> 
>> As you can see from the patch, it's a small amount of code to ensure
>> that the host number management is handled more cleanly.
> 
> Well, I'm a bit worried about the loss of a monotonically increasing
> host number from the debugging perspective.  Right now, if you look at
> any log, hostX always refers to one and only one incarnation throughout
> the system lifetime for any given value of X.  With your patch, the
> lowest host number gets continually reused ... probably for every hot
> plug event.  If the USB and other hotplug system people don't mind this,
> I suppose I can live with it, but I'd like to hear their view before
> making this change.
> 
> James

James?

It looks like both Hannes and GregKH agreed this was the proper approach.

-- 
Lee Duncan
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#1269262 — Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no management

>>>>> "Lee" == Lee Duncan <lduncan@suse.com> writes:

>> Well, I'm a bit worried about the loss of a monotonically increasing
>> host number from the debugging perspective.  Right now, if you look
>> at any log, hostX always refers to one and only one incarnation
>> throughout the system lifetime for any given value of X.

That's a feature that I would absolutely hate to lose. I spend a huge
amount of time looking at system logs.

-- 
Martin K. Petersen	Oracle Linux Engineering
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#1270063

On 11/13/2015 10:54 PM, Martin K. Petersen wrote:
>>>>>> "Lee" == Lee Duncan <lduncan@suse.com> writes:
> 
>>> Well, I'm a bit worried about the loss of a monotonically increasing
>>> host number from the debugging perspective.  Right now, if you look
>>> at any log, hostX always refers to one and only one incarnation
>>> throughout the system lifetime for any given value of X.
> 
> That's a feature that I would absolutely hate to lose. I spend a huge
> amount of time looking at system logs.
> 
Right. Then have it enabled via a modprobe parameters.

We actually had customers running into a host_no overflow due to
excessive host allocations and freeing done by iSCSI.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@suse.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
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#1270623

On 11/16/2015 04:10 AM, Hannes Reinecke wrote:
> On 11/13/2015 10:54 PM, Martin K. Petersen wrote:
>>>>>>> "Lee" == Lee Duncan <lduncan@suse.com> writes:
>>
>>>> Well, I'm a bit worried about the loss of a monotonically increasing
>>>> host number from the debugging perspective.  Right now, if you look
>>>> at any log, hostX always refers to one and only one incarnation
>>>> throughout the system lifetime for any given value of X.
>>
>> That's a feature that I would absolutely hate to lose. I spend a huge
>> amount of time looking at system logs.
>>
> Right. Then have it enabled via a modprobe parameters.
> 
> We actually had customers running into a host_no overflow due to
> excessive host allocations and freeing done by iSCSI.
> 
> Cheers,
> 
> Hannes
> 

Martin: I will be glad to update the patch, creating a modprobe
parameter as suggested, if you find this acceptable.
-- 
Lee Duncan
SUSE Labs
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#1271739 — Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no management

>>>>> "Lee" == Lee Duncan <lduncan@suse.com> writes:

Lee> Martin: I will be glad to update the patch, creating a modprobe
Lee> parameter as suggested, if you find this acceptable.

For development use a module parameter would be fine. But I am concerned
about our support folks that rely on the incrementing host number when
analyzing customer log files.

Ewan: How do you folks feel about this change?

-- 
Martin K. Petersen	Oracle Linux Engineering
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