Re: [Lsf-pc] [LSF/MM ATTEND] Online Logical Head Depop and SMR disks chunked writepages

[Damien Le Moal <Damien.LeMoal@hgst.com>]

>On 02/29/2016 10:02 AM, Damien Le Moal wrote:
>> 
>>> On Wed 24-02-16 01:53:24, Damien Le Moal wrote:
>>>>
>>>>> On Tue 23-02-16 05:31:13, Damien Le Moal wrote:
>>>>>>
>>>>>>> On 02/22/16 18:56, Damien Le Moal wrote:
>>>>>>>> 2) Write back of dirty pages to SMR block devices:
>>>>>>>>
>>>>>>>> Dirty pages of a block device inode are currently processed using the
>>>>>>>> generic_writepages function, which can be executed simultaneously
>>>>>>>> by multiple contexts (e.g sync, fsync, msync, sync_file_range, etc).
>>>>>>>> Mutual exclusion of the dirty page processing being achieved only at
>>>>>>>> the page level (page lock & page writeback flag), multiple processes
>>>>>>>> executing a "sync" of overlapping block ranges over the same zone of
>>>>>>>> an SMR disk can cause an out-of-LBA-order sequence of write requests
>>>>>>>> being sent to the underlying device. On a host managed SMR disk, where
>>>>>>>> sequential write to disk zones is mandatory, this result in errors and
>>>>>>>> the impossibility for an application using raw sequential disk write
>>>>>>>> accesses to be guaranteed successful completion of its write or fsync
>>>>>>>> requests.
>>>>>>>>
>>>>>>>> Using the zone information attached to the SMR block device queue
>>>>>>>> (introduced by Hannes), calls to the generic_writepages function can
>>>>>>>> be made mutually exclusive on a per zone basis by locking the zones.
>>>>>>>> This guarantees sequential request generation for each zone and avoid
>>>>>>>> write errors without any modification to the generic code implementing
>>>>>>>> generic_writepages.
>>>>>>>>
>>>>>>>> This is but one possible solution for supporting SMR host-managed
>>>>>>>> devices without any major rewrite of page cache management and
>>>>>>>> write-back processing. The opinion of the audience regarding this
>>>>>>>> solution and discussing other potential solutions would be greatly
>>>>>>>> appreciated.
>>>>>>>
>>>>>>> Hello Damien,
>>>>>>>
>>>>>>> Is it sufficient to support filesystems like BTRFS on top of SMR drives 
>>>>>>> or would you also like to see that filesystems like ext4 can use SMR 
>>>>>>> drives ? In the latter case: the behavior of SMR drives differs so 
>>>>>>> significantly from that of other block devices that I'm not sure that we 
>>>>>>> should try to support these directly from infrastructure like the page 
>>>>>>> cache. If we look e.g. at NAND SSDs then we see that the characteristics 
>>>>>>> of NAND do not match what filesystems expect (e.g. large erase blocks). 
>>>>>>> That is why every SSD vendor provides an FTL (Flash Translation Layer), 
>>>>>>> either inside the SSD or as a separate software driver. An FTL 
>>>>>>> implements a so-called LFS (log-structured filesystem). With what I know 
>>>>>>> about SMR this technology looks also suitable for implementation of a 
>>>>>>> LFS. Has it already been considered to implement an LFS driver for SMR 
>>>>>>> drives ? That would make it possible for any filesystem to access an SMR 
>>>>>>> drive as any other block device. I'm not sure of this but maybe it will 
>>>>>>> be possible to share some infrastructure with the LightNVM driver 
>>>>>>> (directory drivers/lightnvm in the Linux kernel tree). This driver 
>>>>>>> namely implements an FTL.
>>>>>>
>>>>>> I totally agree with you that trying to support SMR disks by only modifying
>>>>>> the page cache so that unmodified standard file systems like BTRFS or ext4
>>>>>> remain operational is not realistic at best, and more likely simply impossible.
>>>>>> For this kind of use case, as you said, an FTL or a device mapper driver are
>>>>>> much more suitable.
>>>>>>
>>>>>> The case I am considering for this discussion is for raw block device accesses
>>>>>> by an application (writes from user space to /dev/sdxx). This is a very likely
>>>>>> use case scenario for high capacity SMR disks with applications like distributed
>>>>>> object stores / key value stores.
>>>>>>
>>>>>> In this case, write-back of dirty pages in the block device file inode mapping
>>>>>> is handled in fs/block_dev.c using the generic helper function generic_writepages.
>>>>>> This does not guarantee the generation of the required sequential write pattern
>>>>>> per zone necessary for host-managed disks. As I explained, aligning calls of this
>>>>>> function to zone boundaries while locking the zones under write-back solves
>>>>>> simply the problem (implemented and tested). This is of course only one possible
>>>>>> solution. Pushing modifications deeper in the code or providing a
>>>>>> "generic_sequential_writepages" helper function are other potential solutions
>>>>>> that in my opinion are worth discussing as other types of devices may benefit also
>>>>>> in terms of performance (e.g. regular disk drives prefer sequential writes, and
>>>>>> SSDs as well) and/or lighten the overhead on an underlying FTL or device mapper
>>>>>> driver.
>>>>>>
>>>>>> For a file system, an SMR compliant implementation of a file inode mapping
>>>>>> writepages method should be provided by the file system itself as the sequentiality
>>>>>> of the write pattern depends further on the block allocation mechanism of the file
>>>>>> system.
>>>>>>
>>>>>> Note that the goal here is not to hide to applications the sequential write
>>>>>> constraint of SMR disks. The page cache itself (the mapping of the block
>>>>>> device inode) remains unchanged. But the modification proposed guarantees that
>>>>>> a well behaved application writing sequentially to zones through the page cache
>>>>>> will see successful sync operations.
>>>>>
>>>>> So the easiest solution for the OS, when the application is already aware
>>>>> of the storage constraints, would be for an application to use direct IO.
>>>>> Because when using page-cache and writeback there are all sorts of
>>>>> unexpected things that can happen (e.g. writeback decides to skip a page
>>>>> because someone else locked it temporarily). So it will work in 99.9% of
>>>>> cases but sometimes things will be out of order for hard-to-track down
>>>>> reasons. And for ordinary drives this is not an issue because we just slow
>>>>> down writeback a bit but rareness of this makes it non-issue. But for host
>>>>> managed SMR the IO fails and that is something the application does not
>>>>> expect.
>>>>>
>>>>> So I would really say just avoid using page-cache when you are using SMR
>>>>> drives directly without a translation layer. For writes your throughput
>>>>> won't suffer anyway since you have to do big sequential writes. Using
>>>>> page-cache for reads may still be beneficial and if you are careful enough
>>>>> not to do direct IO writes to the same range as you do buffered reads, this
>>>>> will work fine.
>>>>>
>>>>> Thinking some more - if you want to make it foolproof, you could implement
>>>>> something like read-only page cache for block devices. Any write will be in
>>>>> fact direct IO write, writeable mmaps will be disallowed, reads will honor
>>>>> O_DIRECT flag.
>>>>
>>>> Hi Jan,
>>>>
>>>> Indeed, using O_DIRECT for raw block device write is an obvious solution to
>>>> guarantee the application successful sequential writes within a zone. However,
>>>> host-managed SMR disks (and to a lesser extent host-aware drives too) already
>>>> put on applications the constraint of ensuring sequential writes. Adding to this
>>>> further mandatory rewrite to support direct I/Os is in my opinion asking a lot,
>>>> if not too much.
>>>
>>> So I don't think adding O_DIRECT to open flags is such a burden -
>>> sequential writes are IMO much harder to do :). And furthermore this could
>>> happen magically inside the kernel in which case app needn't be aware about
>>> this at all (similarly to how we handle writes to persistent memory).
>>>
>>>> The example you mention above of writeback skipping a locked page and resulting
>>>> in I/O errors is precisely what the proposed patch avoids by first locking the
>>>> zone the page belongs to. In the same spirit as the writeback page locking, if
>>>> the zone is already locked, it is skipped. That is, zones are treated in a sense
>>>> as gigantic pages, ensuring that the actual dirty pages within each one are
>>>> processed in one go, sequentially.
>>>
>>> But you cannot rule out mm subsystem locking a page to do something (e.g.
>>> migrate the page to help with compaction of large order pages). These other
>>> places accessing and locking pages are what I'm worried about. Furthermore
>>> kswapd can decide to writeback particular page under memory pressure and
>>> that will just make SMR disk freak out.
>>>
>>>> This allows preserving all possible application level accesses (buffered,
>>>> direct or mmapped). The only constraint is the one the disk imposes:
>>>> writes must be sequential.
>>>>
>>>> Granted, this view may be too simplistic and may be overlooking some hard
>>>> to track page locking paths which will compete with this. But I think
>>>> that this can be easily solved by forcing the zone-aligned
>>>> generic_writepages calls to not skip any page (a flag in struct
>>>> writeback_control would do the trick). And no modification is necessary
>>>> on the read side (i.e. page locking only is enough) since reading an SMR
>>>> disks blocks after a zone write-pointer position does not make sense (in
>>>> Hannes code, this is possible, but the request does not go to the disk
>>>> and returns garbage data).
>>>>
>>>> Bottom line: no fundamental change to the page caching mechanism, only
>>>> how it is being used/controlled for writeback makes this work.
>>>> Considering the benefits on the application side, it is in my opinion a
>>>> valid modification to have.
>>>
>>> See above, there are quite a few places which will break your assumptions.
>>> And I don't think changing them all to handle SMR is worth it. IMO caching
>>> sequential writes to SMR disks has low effect (if any) anyway so I would
>>> just avoid that. We can talk about how to make this as seamless to
>>> applications as possible. The only thing which I don't think is reasonably
>>> doable without dirtying pagecache are writeable mmaps of an SMR device so
>>> applications would have to avoid that.
>> 
>> Jan,
>> 
>> Thank you for your insight.
>> These "few places" breaking sequential write sequences are indeed problematic
>> for SMR drives. At the same time, I wonder how these paths would react to an I/O
>> error generated by the check "write at write pointer" in the request submission
>> path at the SCSI level. Could these be ignored in the case of an "unaligned write
>> error" ? That is, the page is left dirty and hopefully the regular writeback path
>> catches them later in the proper sequence. This may however be dangerous as there
>> is no way to determine if the unaligned error is due to kswapd or other kernel
>> threads trying to write back the "wrong" page, or the application having submitted
>> an out of sequence write.
>> 
>> Until now, the discussion has focused on avoiding unaligned write errors for cached
>> writes. But this happens only on host-managed SMR disks. Another aspect of the SMR
>> support should also be to avoid random write to zones on host-aware disks. These will
>> not return an error on unaligned writes and silently process them as a regular disk.
>> However, this can over time degrade performance as the disk FW has to handle more and
>> more internal zone defragmentation.
>> 
>To chime in here, we _might_ be able to fix this via a totally different
>route.
>If we were allow to pass _linked_ bios to ->make_request_fn (ie bios
>where the ->bi_next field was already populated) we would have an easy
>marker for merging those requests. At the same time we would be able to
>process these linked bios as a single unit, allowing other bios only to
>be added to the front or the back of these linked bios.
>That would guarantee in-order delivery for SMR, and at the same time
>allow us to get merging running for block-mq.
>
>Alternatively one could try to use plugging here, but I'm not sure if
>that would be sufficient; will need to test.

Hannes,

I like also the idea of linked BIOs as it may simplify fixing a lot of the ordering
problems we have throughout the stack. However, in the case of writeback of
buffered writes, as Jan pointed out, the problem first comes from potential out-of-order
dirty page writeback from different paths, which generates a non-sequential BIO ordering.
I do not see how linking BIOs can cover all possible cases.
Or are you suggesting to basically move the "write pointer position check" upward in
the stack to within the ->make_request_fn function ? This indeed would ensure that
out-of-order page writeback selection fails early, always within the writeback BIO
issuing context. If so, I am afraid however that error handling may be tricky as some
failed BIO submission could be retried but not others (e.g. those that correspond to a
non sequential selection of a dirty page within a correct sequence can be, selection of
a completely randomly written page cannot).

>> If possible, I look forward to more discussions about this at LSF/MM.
>> 
>Same here.
>Btw, I do like the idea of Online logical head depop.
>No idea how we could implement that, but the idea is nice.

Right now, I am exploring extending the SMR zone management code, reusing
the zone condition/state to reflect the state of LBAs of a disk (the disk is
"chunked" so that regularly sized LBA ranges correspond to logical zones).
For instance, the ZBC defined "read-only" and "offline" zone conditions for
LBAs respectively under a bad head and under a depopped head. New conditions
can be added for other states as required.
FSes can access that information through the zone management functions, but
interfacing all this with applications may be very tricky.
All very fuzzy for now. I would like to start a discussion at LSF/MM, including

also standard aspects of the feature.

Best regards.

------------------------
Damien Le Moal, Ph.D.
Sr. Manager, System Software Group, HGST Research,
HGST, a Western Digital company
Damien.LeMoal@hgst.com
(+81) 0466-98-3593 (ext. 513593)
1 kirihara-cho, Fujisawa, 
Kanagawa, 252-0888 Japan
www.hgst.com
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