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From Eli Collins <...@cloudera.com>
Subject Re: [DISCUSS] Remove append?
Date Wed, 21 Mar 2012 18:52:18 GMT
Good point. I thought I'd start with devs first. If you can't get it
past devs there's no reason to go further.

Also, users will tell you they want everything. I'd like to root cause
this, eg if they want append to solve the small files problem I'd like
to know if solving the latter means we don't have to do the former.

ps - fwiw the cdh-user@ mailing list has 800 people on it and it's
rarely requested. Ditto in customer conversations. However the user
base continues to grow rapidly and change in makeup so the past isn't
necessarily a good predictor.


On Wed, Mar 21, 2012 at 11:31 AM, Tim Broberg <Tim.Broberg@exar.com> wrote:
> No specific advice on this particular issue, but in general, I learned the hard way to
stop asking the question, "Feature X is hard to support, is anybody really going to use this?"
*Every time* I have asked this question, I get the answer I want to hear. *Every time*, they
come back and ask for the feature back later and it's more work than it would have been if
I had just planned for it from the beginning.
> YMMV, and I'm always asking marketing guys whereas you're asking developers.
> Ok, there's one piece of specific advice: Go find the people that will tell you what
you don't want to hear. Ask hdfs-user's whether they need the feature rather than hdfs-dev's.
> We all have too much empathy for your position here to make you suffer.
>    - Tim.
> -----Original Message-----
> From: Eli Collins [mailto:eli@cloudera.com]
> Sent: Tuesday, March 20, 2012 8:38 PM
> To: hdfs-dev@hadoop.apache.org
> Subject: [DISCUSS] Remove append?
> Hey gang,
> I'd like to get people's thoughts on the following proposal. I think we should consider
removing append from HDFS.
> Where we are today.. append was added in the 0.17-19 releases
> (HADOOP-1700) and subsequently disabled (HADOOP-5224) due to quality issues. It and sync
were re-designed, re-implemented, and shipped in
> 21.0 (HDFS-265). To my knowledge, there has been no real production use. Anecdotally
people who worked on branch-20-append have told me they think the new trunk code is substantially
less well-tested than the branch-20-append code (at least for sync, append was never well
tested). It has certainly gotten way less pounding from HBase users.
> The design however, is much improved, and people think we can get hsync (and append)
stabilized in trunk (mostly testing and bug fixing).
> Rationale follows..
> Append does not seem to be an important requirement, hflush was. There has not been much
demand for append, from users or downstream projects. Because Hadoop 1.x does not have a working
append implementation (see HDFS-3120, the branch-20-append work was focused on sync not getting
append working) which is not enabled by default and downstream projects will want to support
Hadoop 1.x releases for years, most will not introduce dependencies on append anyway. This
is not to say demand does not exist, just that if it does, it's been much smaller than security,
sync, HA, backwards compatbile RPC, etc. This probably explains why, over 5 years after the
original implementation started, we don't have a stable release with append.
> Append introduces non-trivial design and code complexity, which is not worth the cost
if we don't have real users. Removing append means we have the property that HDFS blocks,
when finalized, are immutable.
> This significantly simplifies the design and code, which significantly simplifies the
implementation of other features like snapshots, HDFS-level caching, dedupe, etc.
> The vast majority of the HDFS-265 effort is still leveraged w/o append. The new data
durability and read consistency behavior was the key part.
> GFS, which HDFS' design is based on, has append (and atomic record
> append) so obviously a workable design does not preclude append.
> However we also should not ape the GFS feature set simply because it exists. I've had
conversations with people who worked on GFS that regret adding record append (see also http://queue.acm.org/detail.cfm?id=1594206).
In short, unless append is a real priority for our users I think we should focus our energy
> Thanks,
> Eli
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