Re: Regarding newstore performance

[Mark Nelson <mnelson@redhat.com>]

Hi Xioxi,

I may not be understanding correctly, but doesn't this just control how 
long the archive of old logs are kept around for rather than how long 
writes live in the log?

Mark

On 04/17/2015 09:40 AM, Chen, Xiaoxi wrote:
> Hi Mark,
>
>       These two tunings should help on keeping the WAL log live long enough. By default the value is 0/0, that means the WAL log file will be deleted ASAP, this is definitely not the way we want. Sadly these two is not exposed by RocksDB store, need hand writing to  os/RocksDBStore.cc:: do_open.
>
>       Seems all the problem now is focusing on KV-DB, is that make sense for us to have a small benchmark tool that simulate newstore workload to RocksDB? The pattern seems like 1WAP item(4KB or something) per commit , in the 4KB random write case. then we can play with the tuning out of Ceph.
>
>         // The following two fields affect how archived logs will be deleted.
>    // 1. If both set to 0, logs will be deleted asap and will not get into
>    //    the archive.
>    // 2. If WAL_ttl_seconds is 0 and WAL_size_limit_MB is not 0,
>    //    WAL files will be checked every 10 min and if total size is greater
>    //    then WAL_size_limit_MB, they will be deleted starting with the
>    //    earliest until size_limit is met. All empty files will be deleted.
>    // 3. If WAL_ttl_seconds is not 0 and WAL_size_limit_MB is 0, then
>    //    WAL files will be checked every WAL_ttl_secondsi / 2 and those that
>    //    are older than WAL_ttl_seconds will be deleted.
>    // 4. If both are not 0, WAL files will be checked every 10 min and both
>    //    checks will be performed with ttl being first.
>    uint64_t WAL_ttl_seconds;
>    uint64_t WAL_size_limit_MB;
>
> 							Xiaoxi
>
> -----Original Message-----
> From: Mark Nelson [mailto:mnelson@redhat.com]
> Sent: Friday, April 17, 2015 8:11 PM
> To: Sage Weil
> Cc: Somnath Roy; Chen, Xiaoxi; Haomai Wang; ceph-devel
> Subject: Re: Regarding newstore performance
>
>
>
> On 04/16/2015 07:38 PM, Sage Weil wrote:
>> On Thu, 16 Apr 2015, Mark Nelson wrote:
>>> On 04/16/2015 01:17 AM, Somnath Roy wrote:
>>>> Here is the data with omap separated to another SSD and after 1000GB
>>>> of fio writes (same profile)..
>>>>
>>>> omap writes:
>>>> -------------
>>>>
>>>> Total host writes in this period = 551020111 ------ ~2101 GB
>>>>
>>>> Total flash writes in this period = 1150679336
>>>>
>>>> data writes:
>>>> -----------
>>>>
>>>> Total host writes in this period = 302550388 --- ~1154 GB
>>>>
>>>> Total flash writes in this period = 600238328
>>>>
>>>> So, actual data write WA is ~1.1 but omap overhead is ~2.1 and
>>>> adding those getting ~3.2 WA overall.
>>
>> This all suggests that getting rocksdb to not rewrite the wal entries
>> at all will be the big win.  I think Xiaoxi had tunable suggestions
>> for that?  I didn't grok the rocksdb terms immediately so they didn't
>> make a lot of sense at the time.. this is probably a good place to
>> focus, though.  The rocksdb compaction stats should help out there.
>>
>> But... today I ignored this entirely and put rocksdb in tmpfs and
>> focused just on the actual wal IOs done to the fragments files after the fact.
>> For simplicity I focused just on 128k random writes into 4mb objects.
>>
>> fio can get ~18 mb/sec on my disk with iodepth=1.  Interestingly,
>> setting
>> iodepth=16 makes no different *until* I also set thinktime=10 (us, or
>> almost any value really) and thinktime_blocks=16, at which point it
>> goes up with the iodepth.  I'm not quite sure what is going on there
>> but it seems to be preventing the elevator and/or disk from reordering
>> writes and make more efficient sweeps across the disk.  In any case,
>> though, with that tweaked I can get up to ~30mb/sec with qd 16, ~40mb/sec with qd 64.
>> Similarly, with qa 1 and thinktime of 250us, it drops to like
>> 15mb/sec, which is basically what I was getting from newstore.  Here's
>> my fio
>> config:
>>
>> 	http://fpaste.org/212110/42923089/
>
>
> Yikes!  That is a great observation Sage!
>
>>
>> Conclusion: we need multiple threads (or libaio) to get lots of IOs in
>> flight so that the block layer and/or disk can reorder and be efficient.
>> I added a threadpool for doing wal work (newstore wal threads = 8 by
>> default) and it makes a big difference.  Now I am getting more like
>> 19mb/sec w/ 4 threads and client (smalliobench) qd 16.  It's not going
>> up much from there as I scale threads or qd, strangely; not sure why yet.
>>
>> But... that's a big improvement over a few days ago (~8mb/sec).  And
>> on this drive filestore with journal on ssd gets ~8.5mb/sec.  So we're
>> winning, yay!
>>
>> I tabled the libaio patch for now since it was getting spurious EINVAL
>> and would consistently SIGBUG from io_getevents() when ceph-osd did
>> dlopen() on the rados plugins (weird!).
>>
>> Mark, at this point it is probably worth checking that you can
>> reproduce these results?  If so, we can redo the io size sweep.  I
>> picked 8 wal threads since that was enough to help and going higher
>> didn't seem to make much difference, but at some point we'll want to
>> be more careful about picking that number.  We could also use libaio
>> here, but I'm not sure it's worth it.  And this approach is somewhat
>> orthogonal to the idea of efficiently passing the kernel things to fdatasync.
>
> Absolutely!  I'll get some tests running now.  Looks like everyone is jumping on the libaio bandwagon which naively seems like the right way to me too.  Can you talk a little bit more about how you'd see fdatasync work in this case though vs the threaded implementation?
>
>>
>> Anyway, next up is probably wrangling rocksdb's log!
>
> I jumped on #rocksdb on freenode yesterday to ask about it, but I think we'll probably just need to hit the mailing list.
>
>>
>> sage
>>