Subject: Re: [boost] NuDB: A fast key/value insert-only database for SSD drives in C++11
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-03-27 13:24:24
>> Your hash function (which runs per thread) only needs to be as fast as
>> your storage device is at a queue depth of 1. So, taking a top of the
>> range NVM SSD, the Samsung 960 Pro, it can write at QD1 about 50k IOPS.
>> That's around 200Mb/sec/thread. Blake2b runs at 1Gb/sec, so it should
>> comfortably fit with a bit of room to spare on a single thread.
> I'm probably being dense, but I don't get your argument. NuDB hashes the
> value to use as a key, from what I understood. If you have a large
> value, the write will be much faster than with random 4k IOPS, no?
> For reference, on my i7 4770k running Win 8.1 a 1TB 960 Pro has about
> 1700MB/s sequential write speed and 150MB/s 4k random write, both at QD1.
Obviously enough, if you write 16Kb to storage, the OS will issue 4x 4Kb
writes (why 4Kb? That's the PCIe DMA maximum). Your 960 Pro very likely
will issue all four of those in parallel, so the write of 16Kb will
complete in the same time as writing a single 4Kb block.
Taking just your figures above, dividing 1700 by 150 yields about 11, so
in your test and/or your filing system and/or your OS, sequential writes
are being issued on average at 11 at a time.
If you are using the kernel page cache, all that stuff is taken care of
for you. NuDB uses the kernel page cache, so all that is out of your
But for me when I was designing AFIO, I was starting on the premise that
many if not most would be running with O_DIRECT|O_SYNC and they will be
doing their own multiplexing of 4Kb block i/o onto storage. So the async
backend AFIO v2 uses is specifically a non-multithreaded implementation
- per thread you run an afio::io_service, and you issue an i/o, while
that completes you prepare the next i/o, upon completion you issue the
next i/o and so on. You keep your thread always writing data, when not
writing then preparing the next data to write etc.
Thus becomes the challenge of how much work to pack onto each kernel
thread, and that's where calculating hash overheads etc comes in. All I
was saying earlier was that modern CPUs are fast enough to do crypto
strength hashing on i/o even to the very fastest SSDs available right
now. If you want durability, you won't give up write performance to
achieve it so long as your CPU is as top end as your SSD.
If you don't want durability, then none of this matters. Crypto hashes
are very expensive, and say SpookyHash is pretty amazing for its
performance. So use SpookyHash, but don't claim durability unless you've
implemented a hash chain of history where one can wind arbitrarily
backwards and forwards in time and thus have the hashes checking one
another's correctness such that a collision in one would be spotted by
-- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/
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