Need some time to digest the BOB and see if it can simplify the reasoning of how fsync is implemented in hbase.
hdfs was evaluated by the paper where I noticed the following: bq. both HDFS and ZooKeeper respondents lament that such an fsync() is not easily achievable with Java Cheers On Sun, Apr 2, 2017 at 1:53 PM, 杨苏立 Yang Su Li <[email protected]> wrote: > Regarding HBASE-5954 specifically, have you thought about using BOB (block > order breaker, > https://www.usenix.org/system/files/conference/osdi14/ > osdi14-paper-pillai.pdf) > to verify if a change is correct. > > It allows you to explore many different crash scenarios. > > > > On Sun, Apr 2, 2017 at 1:35 PM, 杨苏立 Yang Su Li <[email protected]> wrote: > > > I understand why HBase by default does not use hsync -- it does come with > > big performance cost (though for FSYNC_WAL which is not the default > option, > > you should probably do it because the documentation explicitly promised > > it). > > > > > > I just want to make sure my description about HBase is accurate, > including > > the durability aspect. > > > > On Sun, Apr 2, 2017 at 12:19 PM, Ted Yu <[email protected]> wrote: > > > >> Suli: > >> Have you looked at HBASE-5954 ? > >> > >> It gives some background on why hbase code is formulated the way it > >> currently is. > >> > >> Cheers > >> > >> On Sun, Apr 2, 2017 at 9:36 AM, 杨苏立 Yang Su Li <[email protected]> > >> wrote: > >> > >> > Don't your second paragraph just prove my point? -- If data is not > >> > persisted to disk, then it is not durable. That is the definition of > >> > durability. > >> > > >> > If you want the data to be durable, then you need to call hsync() > >> instead > >> > of hflush(), and that would be the correct behavior if you use > FSYNC_WAL > >> > flag (per HBase documentation). > >> > > >> > However, HBase does not do that. > >> > > >> > Suli > >> > > >> > On Sun, Apr 2, 2017 at 11:26 AM, Josh Elser <[email protected]> > >> wrote: > >> > > >> > > No, that's not correct. HBase would, by definition, not be a > >> > > consistent database if a write was not durable when a client sees a > >> > > successful write. > >> > > > >> > > The point that I will concede to you is that the hflush call may, in > >> > > extenuating circumstances, may not be completely durable. For > example, > >> > > HFlush does not actually force the data to disk. If an abrupt power > >> > > failure happens before this data is pushed to disk, HBase may think > >> > > that data was durable when it actually wasn't (at the HDFS level). > >> > > > >> > > On Thu, Mar 30, 2017 at 4:26 PM, 杨苏立 Yang Su Li <[email protected] > > > >> > > wrote: > >> > > > Also, please correct me if I am wrong, but I don't think a put is > >> > durable > >> > > > when an RPC returns to the client. Just its corresponding WAL > entry > >> is > >> > > > pushed to the memory of all three data nodes, so it has a low > >> > probability > >> > > > of being lost. But nothing is persisted at this point. > >> > > > > >> > > > And this is true no mater you use SYNC_WAL or FSYNC_WAL flag. > >> > > > > >> > > > On Tue, Mar 28, 2017 at 12:11 PM, Josh Elser <[email protected]> > >> > wrote: > >> > > > > >> > > >> 1.1 -> 2: don't forget about the block cache which can invalidate > >> the > >> > > need > >> > > >> for any HDFS read. > >> > > >> > >> > > >> I think you're over-simplifying the write-path quite a bit. I'm > not > >> > sure > >> > > >> what you mean by an 'asynchronous write', but that doesn't exist > at > >> > the > >> > > >> HBase RPC layer as that would invalidate the consistency > guarantees > >> > (if > >> > > an > >> > > >> RPC returns to the client that data was "put", then it is > durable). > >> > > >> > >> > > >> Going off of memory (sorry in advance if I misstate something): > the > >> > > >> general way that data is written to the WAL is a "group commit". > >> You > >> > > have > >> > > >> many threads all trying to append data to the WAL -- performance > >> would > >> > > be > >> > > >> terrible if you serially applied all of these writes. Instead, > many > >> > > writes > >> > > >> can be accepted and a the caller receives a Future. The caller > must > >> > wait > >> > > >> for the Future to complete. What's happening behind the scene is > >> that > >> > > the > >> > > >> writes are being bundled together to reduce the number of syncs > to > >> the > >> > > WAL > >> > > >> ("grouping" the writes together). When one caller's future would > >> > > complete, > >> > > >> what really happened is that the write/sync which included the > >> > caller's > >> > > >> update was committed (along with others). All of this is > happening > >> > > inside > >> > > >> the RS's implementation of accepting an update. > >> > > >> > >> > > >> https://github.com/apache/hbase/blob/55d6dcaf877cc5223e67973 > >> > > >> 6eb613173229c18be/hbase-server/src/main/java/org/apache/ > >> hadoop/hbase/ > >> > > >> regionserver/wal/FSHLog.java#L74-L106 > >> > > >> > >> > > >> > >> > > >> 杨苏立 Yang Su Li wrote: > >> > > >> > >> > > >>> The attachment can be found in the following URL: > >> > > >>> http://pages.cs.wisc.edu/~suli/hbase.pdf > >> > > >>> > >> > > >>> Sorry for the inconvenience... > >> > > >>> > >> > > >>> > >> > > >>> On Mon, Mar 27, 2017 at 8:25 PM, Ted Yu<[email protected]> > >> wrote: > >> > > >>> > >> > > >>> Again, attachment didn't come thru. > >> > > >>>> > >> > > >>>> Is it possible to formulate as google doc ? > >> > > >>>> > >> > > >>>> Thanks > >> > > >>>> > >> > > >>>> On Mon, Mar 27, 2017 at 6:19 PM, 杨苏立 Yang Su Li< > >> [email protected]> > >> > > >>>> wrote: > >> > > >>>> > >> > > >>>> Hi, > >> > > >>>>> > >> > > >>>>> I am a graduate student working on scheduling on storage > >> systems, > >> > > and we > >> > > >>>>> are interested in how different threads in HBase interact with > >> each > >> > > >>>>> other > >> > > >>>>> and how it might affect scheduling. > >> > > >>>>> > >> > > >>>>> I have written down my understanding on how HBase/HDFS works > >> based > >> > on > >> > > >>>>> its > >> > > >>>>> current thread architecture (attached). I am wondering if the > >> > > developers > >> > > >>>>> > >> > > >>>> of > >> > > >>>> > >> > > >>>>> HBase could take a look at it and let me know if anything is > >> > > incorrect > >> > > >>>>> or > >> > > >>>>> inaccurate, or if I have missed anything. > >> > > >>>>> > >> > > >>>>> Thanks a lot for your help! > >> > > >>>>> > >> > > >>>>> On Wed, Mar 22, 2017 at 3:39 PM, 杨苏立 Yang Su Li< > >> [email protected] > >> > > > >> > > >>>>> wrote: > >> > > >>>>> > >> > > >>>>> Hi, > >> > > >>>>>> > >> > > >>>>>> I am a graduate student working on scheduling on storage > >> systems, > >> > > and > >> > > >>>>>> we > >> > > >>>>>> are interested in how different threads in HBase interact > with > >> > each > >> > > >>>>>> > >> > > >>>>> other > >> > > >>>> > >> > > >>>>> and how it might affect scheduling. > >> > > >>>>>> > >> > > >>>>>> I have written down my understanding on how HBase/HDFS works > >> based > >> > > on > >> > > >>>>>> > >> > > >>>>> its > >> > > >>>> > >> > > >>>>> current thread architecture (attached). I am wondering if the > >> > > >>>>>> > >> > > >>>>> developers of > >> > > >>>> > >> > > >>>>> HBase could take a look at it and let me know if anything is > >> > > incorrect > >> > > >>>>>> > >> > > >>>>> or > >> > > >>>> > >> > > >>>>> inaccurate, or if I have missed anything. > >> > > >>>>>> > >> > > >>>>>> Thanks a lot for your help! > >> > > >>>>>> > >> > > >>>>>> -- > >> > > >>>>>> Suli Yang > >> > > >>>>>> > >> > > >>>>>> Department of Physics > >> > > >>>>>> University of Wisconsin Madison > >> > > >>>>>> > >> > > >>>>>> 4257 Chamberlin Hall > >> > > >>>>>> Madison WI 53703 > >> > > >>>>>> > >> > > >>>>>> > >> > > >>>>>> > >> > > >>>>> -- > >> > > >>>>> Suli Yang > >> > > >>>>> > >> > > >>>>> Department of Physics > >> > > >>>>> University of Wisconsin Madison > >> > > >>>>> > >> > > >>>>> 4257 Chamberlin Hall > >> > > >>>>> Madison WI 53703 > >> > > >>>>> > >> > > >>>>> > >> > > >>>>> > >> > > >>> > >> > > >>> > >> > > >>> > >> > > > > >> > > > > >> > > > -- > >> > > > Suli Yang > >> > > > > >> > > > Department of Physics > >> > > > University of Wisconsin Madison > >> > > > > >> > > > 4257 Chamberlin Hall > >> > > > Madison WI 53703 > >> > > > >> > > >> > > >> > > >> > -- > >> > Suli Yang > >> > > >> > Department of Physics > >> > University of Wisconsin Madison > >> > > >> > 4257 Chamberlin Hall > >> > Madison WI 53703 > >> > > >> > > > > > > > > -- > > Suli Yang > > > > Department of Physics > > University of Wisconsin Madison > > > > 4257 Chamberlin Hall > > Madison WI 53703 > > > > > > > -- > Suli Yang > > Department of Physics > University of Wisconsin Madison > > 4257 Chamberlin Hall > Madison WI 53703 >
