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

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

From:  Jan Kara <jack@suse.cz>
Date:  Monday, February 29, 2016 at 22:40
To:  Damien Le Moal <Damien.LeMoal@hgst.com>
Cc:  Jan Kara <jack@suse.cz>, "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>, Bart Van Assche <bart.vanassche@sandisk.com>, Matias Bjorling <m@bjorling.me>, "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>, "lsf-pc@lists.linuxfoundation.org" <lsf-pc@lists.linuxfoundation.org>
Subject:  Re: [Lsf-pc] [LSF/MM ATTEND] Online Logical Head Depop and SMR disks chunked writepages


>On Mon 29-02-16 02:02:16, 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.
>
>You'd hope ;) But in fact what happens is that the page ends
>up being clean, marked as having error, and buffers will not be uptodate =>
>you have just lost one page worth of data. See what happens in
>end_buffer_async_write(). Now our behavior in presence of IO errors needs
>improvement for a long time so you are certainly welcome to improve on this
>but what I described is what happens now.


Jan,

Got it. Thanks for the pointers.
I will work a little more on identifying this. In any case, the first problem
to tackle I guess is to get more information than just a -EIO on error. Without
that, no chance to ever be able to retry recoverable errors (unaligned writes).

Thanks !

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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