2017-06-27 21:40 GMT+08:00 Jason Dillaman <jdillama@xxxxxxxxxx>:
> This is definitely an optimization we can test post-Luminous release
> once bluestore is the defacto OSD object store. Of course, even
> bluestore won't track holes down to 8KiB -- only 16KiB or 64KiB
> depending on your backing device and settings. I am pretty sure
> Luminous already has an optimization to not copy-up if the full parent
> object is zeroed.
you mean if the full parent object is zeroed, then it will not
copy-up? But what about a 4M object only with several 16 KiB or 64KiB
holes in Bluestore, It seems those objects still read to rbd-client
side and send copy-up request to osd-side and I do not find bluestone
will treat the whole 64KiB allocated extents as holes if its data is
all zeros.
> I do remember a presentation about surprising results when
> implementing NFS v4.2 READ_PLUS sparse support where it actually
> degraded performance due to the need to seek the file holes. There
> might be a performance trade-off to consider when objects have lots of
> holes due to increased metadata plus decreased data locality.
Yeah, but I think if we can send a single MOSDOp containing several
OSDOps. So it will be treat as a single transaction in osd-side and
deal with much more efficiently. If we send several MOSDOps, then it
will become bad since each transaction on osd-side will be queued and
processed serially because the pg_lock and rw_lock for object.
Actually, we face the same issue when vm flush in-memory data on disk
and lots of adjacent but not continue writeOps will submit to osd-side
with Each MOSDOp simultaneously so that the single pg will process
each transaction one by one, which leads to a bad latency for those
ops at the end of the pg_wq queue.
> On Tue, Jun 27, 2017 at 4:22 AM, Ning Yao <zay11022@xxxxxxxxx> wrote:
>> Hi, all
>>
>> currently I find that when do copy on write for a clone image. librbd
>> call the cls copyup function to write the data, reading from its
>> parent, to the child.
>>
>> However, there is a issue here: if an object in the parent image -->
>> [0, 8192] with data and [8192, end] without data, then after COW
>> operation, it will filling the whole object [0, end] to the children
>> object with [8192, end] all zeros. This phenomenon also occurs in
>> flatten images.
>>
>> Actually, we already have sparse_read to just read data without holes.
>> However, copyup function does not support to write serveral fragments
>> such as {[0, 8192], [16384,20480]}.
>>
>> So it that possible to direct send OSDOp {[cow write], [cow write],
>> [user write]} instead of OSDOp {[copyup], [user write]} ?
>>
>>
>>
>> Regards
>> Ning Yao
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>
>
> --
> Jason
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