It's syncing the checksum file, so the disk head very likely has to move. There are rotational seek delays too.
On Mon, Aug 26, 2013 at 7:30 AM, lei liu <liulei...@gmail.com> wrote: > Hi all, > > DataNode sequential write file, so I think the disk seek time should be > very small.Why is disk seek time 10ms? I think that is too long. Whether we > can optimize the linux system configuration, reduce disk seek time. > > > 2013/8/26 haosdent <haosd...@gmail.com> > > > haha, thank you very much, I get it now. > > > > -- > > Best Regards, > > Haosong Huang > > Sent with Sparrow (http://www.sparrowmailapp.com/?sig) > > > > > > On Monday, August 26, 2013 at 11:18 AM, Andrew Wang wrote: > > > > > Ah, I forgot the checksum fsync, so two seeks. Even with 4k writes, > 50ms > > > still feels in the right ballpark. Best case it's ~20ms, still way > slower > > > than hflush. > > > > > > It's also worth asking if there's other dirty data waiting for > writeback, > > > since I believe it can also get written out on an fsync. > > > > > > hflush doesn't durably write to disk, so you're still in danger of > losing > > > data if there's a cluster-wide power outage. Because HDFS writes to two > > > different racks, hflush still protects you from single-rack outages. > Most > > > people think this is good enough (I believe HBase by default runs with > > just > > > hflush), but if you *really* want to be sure, pay the cost of hsync and > > do > > > durable writes. > > > > > > > > > On Sun, Aug 25, 2013 at 7:44 PM, haosdent <haosd...@gmail.com (mailto: > > haosd...@gmail.com)> wrote: > > > > > > > In fact, I just write 4k in every hsync. Datenode would write > checksum > > > > file and data file when I hsync data to datanode. Each of them would > > spent > > > > nearly 25ms, so a hsync call would spent nearly 50ms. But hflush is > > very > > > > fast, which spent both 1ms in write checksum and data. If a hsync > would > > > > spent 50ms, what meanings we use it? Or my test way is wrong? > > > > > > > > -- > > > > Best Regards, > > > > Haosong Huang > > > > Sent with Sparrow (http://www.sparrowmailapp.com/?sig) > > > > > > > > > > > > On Monday, August 26, 2013 at 7:07 AM, Andrew Wang wrote: > > > > > > > > > 50ms is believable. hsync makes each DN call fsync and wait for > > acks, so > > > > > you'd expect at least a disk seek time (~10ms) with some extra time > > > > > depending on how much unsync'd data is being written. > > > > > > > > > > So, just as some back of the envelope math, assuming a disk that > can > > > > write > > > > > at 100MB/s: > > > > > > > > > > 50ms - 10ms seek = 40ms writing time > > > > > 100 MB/s * 40ms = 4MB > > > > > > > > > > If you're hsync'ing every 4MB, 50ms would be exactly what I'd > expect. > > > > > > > > > > Best, > > > > > Andrew > > > > > > > > > > > > > > > On Sat, Aug 24, 2013 at 10:11 PM, haosdent <haosd...@gmail.com > (mailto: > > haosd...@gmail.com) (mailto: > > > > haosd...@gmail.com (mailto:haosd...@gmail.com))> wrote: > > > > > > > > > > > Hi, all. Hadoop support hsync which would call fsync of system > > after > > > > > > 2.0.2. I have tested the performance of hsync() and hflush() > again > > and > > > > > > again, but I found that the hsync call() everytime would spent > > nearly > > > > > > > > > > > > > > > > > > > > > > > > 50ms > > > > > > while the hflush call() just spent 2ms. In this slide( > > > > > > > > > > > > > > > > http://www.slideshare.net/enissoz/hbase-and-hdfs-understanding-filesystem-usagePage18 > ), > > the author mentions that hsync() is 2x slower than hflush(). So, > > > > > > is anything wrong? Thank you very much and looking forward to > your > > > > > > > > > > > > > help. > > > > > > > > > > > > -- > > > > > > Best Regards, > > > > > > Haosong Huang > > > > > > Sent with Sparrow (http://www.sparrowmailapp.com/?sig) > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
