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HDFS, mail # dev - hsync is too slower than hflush


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Re: hsync is too slower than hflush
Andrew Wang 2013-08-26, 03:18
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 <[EMAIL PROTECTED]> 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 <[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-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)
> > >
> >
> >
> >
>
>
>