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MapReduce >> mail # user >> Using a hard drive instead of


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Re: Using a hard drive instead of
Thank you, everybody, for your input - it was very useful. I need to do my
homework now, and I will be back with the update. The device really exists.
It is not cheap, but it may make sense as the NN of a serious cluster.

Sincerely,
Mark

On Fri, Oct 12, 2012 at 10:46 PM, Ravi Prakash <[EMAIL PROTECTED]> wrote:

> Maybe at a slight tangent, but for each write operation on HDFS (e.g.
> create a file, delete a file, create a directory), the NN waits until the
> edit has been *flushed* to disk. So I can imagine such a hypothetical(?)
> disk would tremendously speed up the NN even as it is. Mark, can you please
> please please send me 5 of these disks? :-P
> To answer your question, you probably want to change BlockManager and
> FSNamesystem, both basically being the crux of HDFS NN. Its going to be a
> pretty significant undertaking.
> @memory-mapped files would lose data in case of failure (unless ofcourse
> you use special hardware, thinking of which, really its not soooo special,
> so maybe worth trying). Has anyone tried this before?
>
>   ------------------------------
> *From:* Lance Norskog <[EMAIL PROTECTED]>
> *To:* [EMAIL PROTECTED]
> *Sent:* Friday, October 12, 2012 12:01 AM
>
> *Subject:* Re: Using a hard drive instead of
>
> This is why memory-mapped files were invented.
>
> On Thu, Oct 11, 2012 at 9:34 PM, Gaurav Sharma
> <[EMAIL PROTECTED]> wrote:
> > If you don't mind sharing, what hard drive do you have with these
> > properties:
> > -"performance of RAM"
> > -"can accommodate very many threads"
> >
> >
> > On Oct 11, 2012, at 21:27, Mark Kerzner <[EMAIL PROTECTED]>
> wrote:
> >
> > Harsh,
> >
> > I agree with you about many small files, and I was giving this only in
> way
> > of example. However, the hard drive I am talking about can be 1-2 TB in
> > size, and that's pretty good, you can't easily get that much memory. In
> > addition, it would be more resistant to power failures than RAM. And
> yes, it
> > has the performance of RAM, and can accommodate very many threads.
> >
> > Mark
> >
> > On Thu, Oct 11, 2012 at 11:16 PM, Harsh J <[EMAIL PROTECTED]> wrote:
> >>
> >> Hi Mark,
> >>
> >> Note that the NameNode does random memory access to serve back any
> >> information or mutate request you send to it, and that there can be
> >> several number of concurrent clients. So do you mean a 'very fast hard
> >> drive' thats faster than the RAM for random access itself? The
> >> NameNode does persist its block information onto disk for various
> >> purposes, but to actually make the NameNode use disk storage
> >> completely (and not specific parts of it disk-cached instead) wouldn't
> >> make too much sense to me. That'd feel like trying to communicate with
> >> a process thats swapping, performance-wise.
> >>
> >> The too many files issue is bloated up to sound like its a NameNode
> >> issue but it isn't in reality. HDFS allows you to process lots of
> >> files really fast, aside of helping store them for long periods, and a
> >> lot of tiny files only gets you down in such operations with overheads
> >> of opening and closing files in the way of reading them all at a time.
> >> With a single or a few large files, all you do is block (data) reads,
> >> and very few NameNode communications - ending up going much faster.
> >> This is the same for local filesystems as well, but not many think of
> >> that.
> >>
> >> On Fri, Oct 12, 2012 at 9:29 AM, Mark Kerzner <[EMAIL PROTECTED]
> >
> >> wrote:
> >> > Hi,
> >> >
> >> > Imagine I have a very fast hard drive that I want to use for the
> >> > NameNode.
> >> > That is, I want the NameNode to store its blocks information on this
> >> > hard
> >> > drive instead of in memory.
> >> >
> >> > Why would I do it? Scalability (no federation needed), many files are
> >> > not a
> >> > problem, and warm fail-over is automatic. What would I need to change
> in
> >> > the
> >> > NameNode to tell it to use the hard drive?
> >> >
> >> > Thank you,
> >> > Mark