Messages in this thread | | | From | "" <> | Date | Fri, 26 Jun 2020 09:03:45 +0200 | Subject | Re: [PATCH v2 0/2] zone-append support in io-uring and aio |
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On 26.06.2020 06:56, Damien Le Moal wrote: >On 2020/06/26 15:37, javier.gonz@samsung.com wrote: >> On 26.06.2020 03:11, Damien Le Moal wrote: >>> On 2020/06/26 2:18, Kanchan Joshi wrote: >>>> [Revised as per feedback from Damien, Pavel, Jens, Christoph, Matias, Wilcox] >>>> >>>> This patchset enables zone-append using io-uring/linux-aio, on block IO path. >>>> Purpose is to provide zone-append consumption ability to applications which are >>>> using zoned-block-device directly. >>>> >>>> The application may specify RWF_ZONE_APPEND flag with write when it wants to >>>> send zone-append. RWF_* flags work with a certain subset of APIs e.g. uring, >>>> aio, and pwritev2. An error is reported if zone-append is requested using >>>> pwritev2. It is not in the scope of this patchset to support pwritev2 or any >>>> other sync write API for reasons described later. >>>> >>>> Zone-append completion result ---> >>>> With zone-append, where write took place can only be known after completion. >>>> So apart from usual return value of write, additional mean is needed to obtain >>>> the actual written location. >>>> >>>> In aio, this is returned to application using res2 field of io_event - >>>> >>>> struct io_event { >>>> __u64 data; /* the data field from the iocb */ >>>> __u64 obj; /* what iocb this event came from */ >>>> __s64 res; /* result code for this event */ >>>> __s64 res2; /* secondary result */ >>>> }; >>>> >>>> In io-uring, cqe->flags is repurposed for zone-append result. >>>> >>>> struct io_uring_cqe { >>>> __u64 user_data; /* sqe->data submission passed back */ >>>> __s32 res; /* result code for this event */ >>>> __u32 flags; >>>> }; >>>> >>>> Since 32 bit flags is not sufficient, we choose to return zone-relative offset >>>> in sector/512b units. This can cover zone-size represented by chunk_sectors. >>>> Applications will have the trouble to combine this with zone start to know >>>> disk-relative offset. But if more bits are obtained by pulling from res field >>>> that too would compel application to interpret res field differently, and it >>>> seems more painstaking than the former option. >>>> To keep uniformity, even with aio, zone-relative offset is returned. >>> >>> I am really not a fan of this, to say the least. The input is byte offset, the >>> output is 512B relative sector count... Arg... We really cannot do better than >>> that ? >>> >>> At the very least, byte relative offset ? The main reason is that this is >>> _somewhat_ acceptable for raw block device accesses since the "sector" >>> abstraction has a clear meaning, but once we add iomap/zonefs async zone append >>> support, we really will want to have byte unit as the interface is regular >>> files, not block device file. We could argue that 512B sector unit is still >>> around even for files (e.g. block counts in file stat). Bu the different unit >>> for input and output of one operation is really ugly. This is not nice for the user. >>> >> >> You can refer to the discussion with Jens, Pavel and Alex on the uring >> interface. With the bits we have and considering the maximun zone size >> supported, there is no space for a byte relative offset. We can take >> some bits from cqe->res, but we were afraid this is not very >> future-proof. Do you have a better idea? > >If you can take 8 bits, that gives you 40 bits, enough to support byte relative >offsets for any zone size defined as a number of 512B sectors using an unsigned >int. Max zone size is 2^31 sectors in that case, so 2^40 bytes. Unless I am >already too tired and my math is failing me...
Yes, the match is correct. I was thinking more of the bits being needed for other use-case that could collide with append. We considered this and discard it for being messy - when Pavel brought up the 512B alignment we saw it as a good alternative.
Note too that we would be able to translate to a byte offset in iouring.h too so the user would not need to think of this.
I do not feel strongly on this, so the one that better fits the current and near-future for uring, that is the one we will send on V3. Will give it until next week for others to comment too.
> >zone size is defined by chunk_sectors, which is used for raid and software raids >too. This has been an unsigned int forever. I do not see the need for changing >this to a 64bit anytime soon, if ever. A raid with a stripe size larger than 1TB >does not really make any sense. Same for zone size...
Yes. I think already max zone sizes are pretty huge. But yes, this might change, so we will take it when it happens.
[...]
Javier
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