Hola Ben,
Yes I am saying since there are no writes there are no copies, and that
is why you get just the grouping of blocks.
Do you want to group blocks of one file only or different files?
What is a potential use of that?
Are you planning to implement operations like taking a subset_of_blocks
of one file, intersection, union?
If we had hard-links then grouping could be performed on one-block files
by creating directories.
This is low level, but works.
--Konstantin
Benjamin Reed wrote:
I think you are saying that the copy never happens because there is never a
write that will cause the copy. Copy on write is probably not the correct
term both because there is not a write but also in the API I propose you
don't really need to make a copy of another file, you are just basically
grouping a set of existing blocks into a new file.
The reason I call it copy-on-write is because the data structures used are the
same: you have a single block used by a separate file and you have a two
files that may both reference a single block but another block in one file is
not referenced by another file.
Anyone know a better name for this kind of semantics.
Creating one-block files doesn't help at all. Both because we don't want to
deal with a bunch of one-block files and because even if the files are one
block you still cannot have that one block referenced by two separate files.
ben
On Lunes 09 Julio 2007, Konstantin Shvachko wrote:
Copy on write was discussed in HADOOP-334 in the context of periodic
checkpointing of the name-space.
Other than that I remember a discussion about file clone() operation,
which makes a new inode, but uses the
same blocks as the original file, which are copied once they are
modified or appended.
But this functionality would be possible only if we had at least
appends. Since hdfs does not support
modifications the purpose of COW is going to be only to support grouping
of blocks from different
(or is it just one?) files.
I think it is possible, but very non-possix.
And you can always create one-block files and group them in directories
instead.
--Konstantin
Benjamin Reed wrote:
I need to implement COW for HDFS for a project I'm working on. I
vaguely remember it being discussed before, but I can't find any
threads about it. I wanted to at least check for interest/previous
work before proceeding. Hard links would work for me as well, but they
are harder to implement. I was thinking of adding the following to the
client protocol:
public void cow(String src, String clientName, boolean overwrite,
LocatedBlocks blocks) throws IOException;
The call would simply create a new file and populate its contents with
the blocks contained in the LocatedBlocks.
Apart from fast copies, it also allows fast truncations and extensions
of existing files.
(This is not a hard link because it is possible that the set of blocks
may not correspond to any other file.)
Has such a thing been discussed before?
thanx
ben
