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HDFS, mail # user - Re: distributed cache


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Re: distributed cache
Lin Ma 2012-12-28, 10:02
Thanks Harsh,

(1) "Thankfully, due to block sizes the latter isn't a problem for large
files on a proper DN, as the blocks are spread over the disks and across
the nodes." -- What do you mean DN?

(2) So, you mean concurrent read for small block will not degrade
performance, but concurrent read for large block will degrade performance
compared to single thread read for large block? Please feel free to correct
me if I am wrong. The results are interesting. Appreciate if you could
elaborate a bit more details why.

regards,
Lin

On Wed, Dec 26, 2012 at 8:19 PM, Harsh J <[EMAIL PROTECTED]> wrote:

> Hi,
>
> Sorry for having been ambiguous. For (1) I meant a large block (if the
> block size is large). For (2) I meant multiple, concurrent threads.
>
> On Wed, Dec 26, 2012 at 5:36 PM, Lin Ma <[EMAIL PROTECTED]> wrote:
> > Thanks Harsh,
> >
> > For long read, you mean read a large continuous part of a file, other
> than a
> > small chunk of a file?
> > "gradually decreasing performance for long reads" -- you mean parallel
> > multiple threads long read degrade performance? Or single thread
> exclusive
> > long read degrade performance?
> >
> > regards,
> > Lin
> >
> >
> > On Wed, Dec 26, 2012 at 7:48 PM, Harsh J <[EMAIL PROTECTED]> wrote:
> >>
> >> Hi Lin,
> >>
> >> It is comparable (and is also logically similar) to reading a file
> >> multiple times in parallel in a local filesystem - not too much of a
> >> performance hit for small reads (by virtue of OS caches, and quick
> >> completion per read, as is usually the case for distributed cache
> >> files), and gradually decreasing performance for long reads (due to
> >> frequent disk physical movement)? Thankfully, due to block sizes the
> >> latter isn't a problem for large files on a proper DN, as the blocks
> >> are spread over the disks and across the nodes.
> >>
> >> On Wed, Dec 26, 2012 at 4:13 PM, Lin Ma <[EMAIL PROTECTED]> wrote:
> >> > Thanks Harsh, multiple concurrent read is generally faster or?
> >> >
> >> > regards,
> >> > Lin
> >> >
> >> >
> >> > On Wed, Dec 26, 2012 at 6:21 PM, Harsh J <[EMAIL PROTECTED]> wrote:
> >> >>
> >> >> There is no limitation in HDFS that limits reads of a block to a
> >> >> single client at a time (no reason to do so) - so downloads can be as
> >> >> concurrent as possible.
> >> >>
> >> >> On Wed, Dec 26, 2012 at 3:41 PM, Lin Ma <[EMAIL PROTECTED]> wrote:
> >> >> > Thanks Harsh,
> >> >> >
> >> >> > Supposing DistributedCache is uploaded by client, for each replica,
> >> >> > in
> >> >> > Hadoop design, it could only serve one download session (download
> >> >> > from a
> >> >> > mapper or a reducer which requires the DistributedCache) at a time
> >> >> > until
> >> >> > DistributedCache file download is completed, or it could serve
> >> >> > multiple
> >> >> > concurrent parallel download session (download from multiple
> mappers
> >> >> > or
> >> >> > reducers which requires the DistributedCache).
> >> >> >
> >> >> > regards,
> >> >> > Lin
> >> >> >
> >> >> >
> >> >> > On Wed, Dec 26, 2012 at 4:51 PM, Harsh J <[EMAIL PROTECTED]>
> wrote:
> >> >> >>
> >> >> >> Hi Lin,
> >> >> >>
> >> >> >> DistributedCache files are stored onto the HDFS by the client
> first.
> >> >> >> The TaskTrackers download and localize it. Therefore, as with any
> >> >> >> other file on HDFS, "downloads" can be efficiently parallel with
> >> >> >> higher replicas.
> >> >> >>
> >> >> >> The point of having higher replication for these files is also
> tied
> >> >> >> to
> >> >> >> the concept of racks in a cluster - you would want more replicas
> >> >> >> spread across racks such that on task bootup the downloads happen
> >> >> >> with
> >> >> >> rack locality.
> >> >> >>
> >> >> >> On Sat, Dec 22, 2012 at 6:54 PM, Lin Ma <[EMAIL PROTECTED]> wrote:
> >> >> >> > Hi Kai,
> >> >> >> >
> >> >> >> > Smart answer! :-)
> >> >> >> >
> >> >> >> > The assumption you have is one distributed cache replica could
> >> >> >> > only
> >> >> >> > serve
> >> >> >> > one download session for tasktracker node (this is why you get