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HDFS >> mail # dev >> hsync is too slower than hflush

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Re: hsync is too slower than hflush
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 <[EMAIL PROTECTED] (mailto:[EMAIL PROTECTED])> 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-usagePage 18), 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)
> >