Video is available https://www.youtube.com/watch?v=ljrpXVlkQ84
Seems to be missing first part of talk and also had no sound. Hopefully these will be fixed in future.
How to Build a Non-Volatile Memory DBMS
21–30 of 37 posts
Re: How to Build a Non-Volatile Memory DBMS
#22Since machines can crash, possibly corrupting data, it seems like you still need replication? The network might be the new bottleneck.
Journal files. You can begin a transaction, crash, and lose the transaction. But the thing itself will be consistent. Perhaps there's something funny about the failure modes of non-volatile memory that's different than a disk. Replication can improve things, sure. there's always a chance of a cosmic ray doing something weird to your stuff. That's (so far) a lot less common than a regular old crash.
I was thinking along the lines of the machine being on the wrong side of a network partition and then disappearing entirely. Cloud services are supposed to survive the loss of any single machine.
Re: How to Build a Non-Volatile Memory DBMS
#23Earlier quoted context omitted.
Yes but in byte-addressable mode you can trust the map is fixed, which doesn't seem to be the case here.
Not exactly. I'm not a user of this, but http://www.lmdb.tech/doc/group__mdb__env.html#ga492952277c48... Ctrl+f through the doc for "MDB_FIXEDMAP" for more details. So there is _some_ support, where you can expect to have the whole file at a fixed address. What is not handled is that your data may be moved within the file, and there's nothing you can do about it, short of keeping a long-lived read only transaction: h…
You have it right.
In practice, nobody has wanted support for this so it has remained unimplemented. Note that if your data items are large enough (more than 1/2 a page) they'll go into their own overflow pages, and then won't shift around at all. (But if you delete/modify a value, the new version will of course be in a new overflow page.)
Granted, this could be a case of "if you build it they will come" - before LMDB existed, nobody cared about mmap'd transactional DBs either. And personally, I still see good use cases for it.
Re: How to Build a Non-Volatile Memory DBMS
#24Eh. None of this is new, and we already anticipated this with LMDB back in 2009.
The fact that NVRAM is directly addressable (and thus can bypass the page cache) will eventually play out as irrelevant. It will always be a fact that slower mass-storage will exist, more cheaply than fast in-core storage, persistent or not. The page cache will still be needed even for NVRAM, and the "page" will still be the necessary atomic unit of memory interchange. (Direct access is of course a great thing, but it will be direct access to virtual addresses. Virtual memory, and paging in/out between primary and secondary storage, is never going away. Every commodity system that has tried to do without PMMUs has failed, for numerous good reasons.)
The continual allusions to "frequent writes can destroy memory cells" seems to mainly relate to the extremely short lifetime of Intel's 3DXpoint memory, which is from every measure a total failure.
http://semiaccurate.com/2017/03/10/intel-mislead-press-xpoin...
It would be best to ignore 3DXpoint and just focus on STT-MRAM, which is already at parity with DRAM for endurance. (But still lacking in density.)
Much of the other stuff in those slides is still off the mark. E.g., LMDB today has perfect crash reliability with zero recovery time. The "write behind logging" they propose still has logging overhead and non-zero crash recovery time - which equals wasted work. Anything that requires logging or any form of compaction or garbage collection is wasted work. It's completely unnecessary, and that has nothing to do with NVM. Treating NVM DB design as if it's an entirely new and different animal is frankly ignorant. The right design works in all scenarios - as LMDB does.
Re: How to Build a Non-Volatile Memory DBMS
#25I am currently thinking about stating graduate school and this dude "Andy Pavlo" is my hero, the amount of information he pumps (I don't know any other world) to his listeners is outlandish while staying fun and serious and practical at the same time. I watched most of his lectures. I have done some work in industrial reaseach lab with cuda. I did OS a lot and it was my first choice. But after watching his lectures i…
You cannot write a good database without a solid understanding of operating systems and computer architecture.
Re: How to Build a Non-Volatile Memory DBMS
#26Eh. None of this is new, and we already anticipated this with LMDB back in 2009. The fact that NVRAM is directly addressable (and thus can bypass the page cache) will eventually play out as irrelevant. It will always be a fact that slower mass-storage will exist, more cheaply than fast in-core storage, persistent or not. The page cache will still be needed even for NVRAM, and the "page" will still be the necessary at…
I'm not sure that using mmap(2) is enough. You're still writing pages, so you're writing more than you have to, so writes will not go as fast on NVM as they could.
LMDB does logging since it does copy-on-write -- it just reuses free pages as soon as possible, which means that over time the log disappears. (LMDB is not an append-only DB.)
I agree as to GCs. Having to GC is not just wasted work, it's a performance disaster, and any study that hand-waves about GC without actually measuring its impact on performance is fatally flawed. Assume petabytes of data, and assume performance between SSDs and DRAM: GC will still take enormous amounts of time and I/O that could have been used for something else. Write-behind logging is terrible if it means you need a GC. Why even bother with write-behind logging if a failure means that you must GC anyways.
Only write-ahead logging helps you avoid a GC. A write-ahead log could be optimized to log only (address, length) tuples for each transaction so as to minimize WAL writes.
However, the insights about smaller-than-page I/Os, particularly as to writes, seem likely to be correct. Though having anything like a b-tree on NVM with smaller-than-page writes seems to me to imply in-place writes, but I'm not ready to give up on COW.
Re: How to Build a Non-Volatile Memory DBMS
#27Earlier quoted context omitted.
Yes but in byte-addressable mode you can trust the map is fixed, which doesn't seem to be the case here.
Not exactly. I'm not a user of this, but http://www.lmdb.tech/doc/group__mdb__env.html#ga492952277c48... Ctrl+f through the doc for "MDB_FIXEDMAP" for more details. So there is _some_ support, where you can expect to have the whole file at a fixed address. What is not handled is that your data may be moved within the file, and there's nothing you can do about it, short of keeping a long-lived read only transaction: h…
Note that the LMDB approach, and especially if you want MDB_FIXEDMAP, limits DB size to the largest mmap()ing you can get at a fixed location. That's not good in a world of 48-bit address spaces.
Re: How to Build a Non-Volatile Memory DBMS
#28Earlier quoted context omitted.
Journal files. You can begin a transaction, crash, and lose the transaction. But the thing itself will be consistent. Perhaps there's something funny about the failure modes of non-volatile memory that's different than a disk. Replication can improve things, sure. there's always a chance of a cosmic ray doing something weird to your stuff. That's (so far) a lot less common than a regular old crash.
I was thinking along the lines of the machine being on the wrong side of a network partition and then disappearing entirely. Cloud services are supposed to survive the loss of any single machine.
Ah, i think you're right, but that's a different problem. The Jepsen talks are enlightening, as so many errors get highlighted. Ultimately, if your algorithm is right, the hardware won't matter at all. Each node could be a person with pencil and paper, and the messages are lossy paper airplanes.
The NVM stuff (i think) just changes parameters to cache size, journal log size, stuff like that. I mean, it's cool tech, and it'll probably make stuff faster. But the reliability comes from the architecture and faithful implementation.
Re: How to Build a Non-Volatile Memory DBMS
#29Eh. None of this is new, and we already anticipated this with LMDB back in 2009. The fact that NVRAM is directly addressable (and thus can bypass the page cache) will eventually play out as irrelevant. It will always be a fact that slower mass-storage will exist, more cheaply than fast in-core storage, persistent or not. The page cache will still be needed even for NVRAM, and the "page" will still be the necessary at…
I'm not sure that using mmap(2) is enough. You're still writing pages, so you're writing more than you have to, so writes will not go as fast on NVM as they could. LMDB does logging since it does copy-on-write -- it just reuses free pages as soon as possible, which means that over time the log disappears. (LMDB is not an append-only DB.) I agree as to GCs. Having to GC is not just wasted work, it's a performance disa…
> I'm not sure that using mmap(2) is enough. You're still writing pages, so you're writing more than you have to, so writes will not go as fast on NVM as they could.
If you're using the new fangled memory there's a few components in play.
First in Linux there's DAX. DAX lets you mmap in the devices pages directly without going through the page cache. And the processor can handle this like a memory mmaped device (without interactions from the OS) including subpage read/writes.
Second, Intel chose to reuse previously existing operations that you would normally use for flushing cache lines (to main memory). Previously there were going to use additional pcommit operation. https://software.intel.com/en-us/blogs/2016/09/12/deprecate-...
Re: How to Build a Non-Volatile Memory DBMS
#30Earlier quoted context omitted.
Not exactly. I'm not a user of this, but http://www.lmdb.tech/doc/group__mdb__env.html#ga492952277c48... Ctrl+f through the doc for "MDB_FIXEDMAP" for more details. So there is _some_ support, where you can expect to have the whole file at a fixed address. What is not handled is that your data may be moved within the file, and there's nothing you can do about it, short of keeping a long-lived read only transaction: h…
Note that the LMDB approach, and especially if you want MDB_FIXEDMAP, limits DB size to the largest mmap()ing you can get at a fixed location. That's not good in a world of 48-bit address spaces.
Practically it's even less than 47 bits. :) But sure, know your limits.