[lustre-devel] [PATCH 1/2] staging: lustre: replace uses of class_devno_max by MAX_OBD_DEVICES
andreas.dilger at intel.com
Mon Nov 7 13:50:32 PST 2016
On Nov 7, 2016, at 02:07, Aya Mahfouz <mahfouz.saif.elyazal at gmail.com> wrote:
> On Mon, Nov 7, 2016 at 6:22 AM, Oleg Drokin <oleg.drokin at intel.com> wrote:
>> On Nov 4, 2016, at 4:37 AM, Aya Mahfouz wrote:
>> > On Thu, Nov 3, 2016 at 1:05 AM, Dilger, Andreas <andreas.dilger at intel.com> wrote:
>> > On Oct 25, 2016, at 10:47, Aya Mahfouz <mahfouz.saif.elyazal at gmail.com> wrote:
>> > >
>> > > On Mon, Oct 17, 2016 at 10:38:31PM +0000, Dilger, Andreas wrote:
>> > >> On Oct 17, 2016, at 15:46, Aya Mahfouz <mahfouz.saif.elyazal at gmail.com> wrote:
>> > >>>
>> > >>> class_devno_max is an inline function that returns
>> > >>> MAX_OBD_DEVICES. Replace all calls to the function
>> > >>> by MAX_OBD_DEVICES.
>> > >>
>> > >> Thanks for your patch, but unfortunately it can't be accepted.
>> > >>
>> > >> This function was added in preparation of being able to tune the maximum
>> > >> number of storage devices dynamically, rather than having to hard code it
>> > >> to the maximum possible number of servers that a client can possibly
>> > >> connect to.
>> > >>
>> > >> While the current maximum of 8192 servers has been enough for current
>> > >> filesystems, I'd rather move in the direction of dynamically handling this
>> > >> limit rather than re-introducing a hard-coded constant throughout the code.
>> > >>
>> > > Hello,
>> > >
>> > > I would like to proceed with implementing the function if possible.
>> > > Kindly direct me to some starting pointers.
>> > Hi Aya,
>> > thanks for offering to look into this.
>> > There are several ways to approach this problem to make the allocation
>> > of the obd_devs array dynamic. In most cases, there isn't any value
>> > to dynamically shrink this array, since the filesystem(s) will typically
>> > be mounted until the node is rebooted, and it is only in the tens of KB
>> > size range, so this will not affect ongoing operations, and that simplifies
>> > the implementation.
>> > The easiest way would be to have a dynamically-sized obd_devs array that
>> > is reallocated in class_newdev() in PAGE_SIZE chunks whenever the current
>> > array has no more free slots and copied to the new array, using obd_dev_lock
>> > to protect the array while it is being reallocated and copied. In most
>> > cases, this would save memory over the static array (not many filesystems
>> > have so many servers), but for the few sites that have 10000+ servers they
>> > don't need to change the source to handle this. Using libcfs_kvzalloc()
>> > would avoid issues with allocating large chunks of memory.
>> > There are a few places where obd_devs is accessed outside obd_dev_lock
>> > that would need to be fixed now that this array may be changed at runtime.
>> > A second approach that may scale better is to change obd_devs from an array
>> > to a doubly linked list (using standard list_head helpers). In many cases
>> > the whole list is seached linearly, and most of the uses of class_num2obd()
>> > are just used to walk that list in order, which could be replaced with
>> > list_for_each_entry() list traversal. The class_name2dev() function should
>> > be changed to return the pointer to the obd_device structure, and a new
>> > helper class_dev2num() would just return the obd_minor number from the
>> > obd_device struct for the one use in class_resolve_dev_name(). Using a
>> > linked list has the advantage that there is no need to search for free slots
>> > in the array, since devices would be removed from the list when it is freed.
>> > Cheers, Andreas
>> > Thanks Andreas! Will start looking into it.
>> I also would like to point out that Alexey Lyashkov had an implementation
>> of this in http://review.whamcloud.com/347 but it needed to be reverted
>> as it was way too race-prone in the end.
>> I don't know if Alexey ever improved the patch to actually work (at least
>> there was some talk about it), but even if so, the end result was never
>> contributed back to us.
>> Also please be advised that this is the kind of change that you'll need to
>> have fully functional Lustre setup to verify it works, please let me know
>> if you have any problems setting this up.
> I'm currently trying to understand the concerned parts of the lustre
> code. I will try to setup the lustre file system and take a look at
> Alexey's implementation this week.
I think it makes sense to split this change into a couple of smaller chunks than
what was in the original patch, if the original patch is used at all. The proposals
I made above are relatively simple to implement and change the existing code to use
the new dynamic array (or list) handling without introducing a lot of complexity.
Then, separately, it would be possible to move the code over from linear list
walking to hash-based device name/UUID lookups, which is where things got more
complex and error-prone.
Having the changes done in a series of smaller and easily understood patches makes
the code much easier to review, easier to test in case a bug is introduced, and
often provides value at each step (i.e. dynamic device count) even if the later
patches are held up or rejected for one reason or another.
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