Re: [PATCH] [RFC] Advanced TCA SCSI Disk Hotswap

[Steven Dake]

James
Some responses below:

James Bottomley wrote:

>[e-mail] said:
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>>I plan to produce a now patch that dumps the filesystem interface and
>>replaces it with driverfs files in /sys/bus/scsi.  These things take
>>time, but I hope to be  finished by October 25th. 
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>OK, that's good, thanks.
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>>The current remove interface is unmaintained, doesn't contain locking,
>> and requires laborious string processing resulting in slow results.
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>It is maintained (well, I was planning on looking after it).  The locking can 
>be added (the 1st part of your patch).  It does two in kernel strncmps.  
>That's not really slow by most definitions.
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The locking most definately needs to be added to the kernel.  I'm 
surprised the original
patch didn't contain any locking, but then again, my first patch didn't 
either :)

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>>Further there is  no usage information (which means the usage must
>>come by looking at  drivers/scsi/scsi.c which is beyond most typical
>>users).
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>I don't really think it's the job of the kernel to conatin usage information.  
>That's the job of the user level documentation.
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I've gotten mixed feedback on this.  I'll add you to the list that 
doesn't like this.

perhaps it should be removed (even though it takes up minimal memory).

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>>Imagine scanning each disk in driverfs looking at its WWN attribute
>>(if  it has one) until a match is found.  Assume there are 16 FC
>>devices.  That is  several hundred syscalls just to complete one
>>hotswap operation. 
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>Why is speed so important?
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Telecoms and Datacoms have told me in numerous conversations that a hotswap
operation should occur in 20msec.  I've arbitrarily set 10msec as my 
target to
ensure that I meet the worse-case bus-is-loaded responses during scans, etc.

I can't mention the names of the telecoms, but several with 10000+ employees
have mentioned it.

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>>This requires the adaptor to maintain a mapping of WWNs to SCSI IDs,
>>however, this is already required by most FibreChannel firmware I've
>>seen (and hence is available  in the driver database already). 
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>There will be a point where for a large number of drivers, a linear scan even 
>in the kernel will be slower than a good DB lookup in userspace.
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This may be true, but most systems will only have at most 4-5 devices. 
 Theres only
so much room on PCI for FC devices :)

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>>Hotplugs on FibreChannel don't trigger "events".  What they can do is
>>LIP (loop initialization procedure) if the device has been configured
>>in it's SCSI code pages to  do such a thing.  Since this is device
>>specific I'd hate to rely on it for hotswap. 
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>They don't now, but they should.  The LIP protocol makes the FC driver aware 
>of the gain or loss of devices.  This should be communicated to the mid-layer 
>and then trigger a hotplug event.  Someone needs to write this, I was just 
>wondering if you might.
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I like the idea and it was something I was considering for early next 
year.  Its driver
dependent and until a FC driver is in the kernel, theres not much point 
yet :)

Keep in mind also that a LIP is not always generated on an insertion and 
isn't generated
on a removal at all.  This makes insertion easy but removal still 
requires user intervention.

In Advanced TCA (what spawned this work) a button is pressed to indicate 
hotswap removal
which makes for easy detection of hotswap events.  This is why there are 
kernel interfaces
for removal and insertion (so a kernel driver can be written to detect 
the button press
and remove the devices from the os data structures and then light a blue 
led indicating
safe for removal).

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>>I think this would be too slow.  10 msec for my entire hotswap is
>>available.  If you calculate 2msec for the actual hotswap disk
>>operation, that leaves 8 msec for the rest of the mess.  Scanning
>>through tables or scanning tens or hundreds of files through hundreds
>>of  syscalls may betoo slow. 
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>Where does the 10ms figure come from?
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See above

Thanks James for reading the code and giving comments!

>James
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