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From Ravi Prakash <ravi...@ymail.com>
Subject Re: Using a hard drive instead of
Date Sat, 13 Oct 2012 03:46:02 GMT
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 <goksron@gmail.com>
To: user@hadoop.apache.org 
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
<gaurav.gs.sharma@gmail.com> 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 <mark.kerzner@shmsoft.com> 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 <harsh@cloudera.com> 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 <mark.kerzner@shmsoft.com>
>> 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
>> --
>> Harsh J

Lance Norskog
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