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> 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)> 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) > > > > > > > > > > > >
