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https://issues.apache.org/jira/browse/HBASE-10216?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14313284#comment-14313284
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Andrew Purtell commented on HBASE-10216:
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We could propose a new HDFS API that "would merge files so that the merging and
deleting can be performed on local data nodes with no file contents moving over
the network", but does this not only push something implemented today in the
HBase regionserver down into the HDFS datanodes? Could a merge as described be
safely executed in parallel on multiple datanodes without coordination? No,
because the result is not a 1:1 map of input block to output block. Therefore a
single datanode would handle the merge procedure. From a block device and
network perspective nothing would change.
Set the above aside. We can't push something as critical to HBase as compaction
down into HDFS. First, the HDFS project is unlikely to accept the idea or
implement it in the first place. Even in the unlikely event that happens, we
would need reimplement compaction using the new HDFS facility to take advantage
of it, yet we will need to support older versions of HDFS without the new API
for a while, and if the new HDFS API ever doesn't perfectly address the
minutiae of HBase compaction then or going forward we would be back where we
started.
Let's look at the read and write aspects with an eye toward what we have today,
and assuming no new HDFS API.
Reads: With short circuit reads enabled, recommended for all deployments, if
file blocks are available on the local datanode then block reads are fully
local via a file descriptor passed over a unix domain socket, we never touch a
TCP/IP socket. The probability that a block read for an HFile is local can be
made very high by taking care to align region placement with block placement
and/or fix up where block locality has dropped below a threshold using an
existing HDFS API, see HBASE-4755 and HDFS-4606
Writes: Writers like regionservers always contact the local datanode, assuming
colocation of datanode and regionserver, as the first hop in the write
pipeline. The datanode will then pipeline the write over the network to
replicas, but only the second hop in the pipeline (from local datanode to first
remote replica) will add contention on the local NIC, the third (from remote
replica to other remote replica) will be pipelined from the remote. It's true
we can initially avoid second-replica network IO initially by writing to a
local file. Or we can have the equivalent in HDFS by setting the initial
replication factor of the new file to 1. In either case after closing the
file, to make the result robust against node loss, we need to replicate all
blocks of the newly written file immediately afterward. So now we are waiting
for network IO and contending the NIC anyway, we have just deferred network IO
until the file was completely written. We are not saving a single byte in
transmission on the local NIC. We would have to add housekeeping that insures
we don't delete older HFiles until the new/merged HFile is completely
replicated; this makes something our business that today HDFS handles
transparently.
For us to see any significant impact, I think the proposal on this issue must
be replaced with one where we flush from memstore to local files and then at
some point merge locally flushed files to a compacted file on disk. Only then
are we really saving on IO. All of those locally flushed files represent data
that never leaves the local node, never crosses the network, never causes reads
or writes beyond the local node. This is the benefit *and* the nature of the
data availability problem that follows: We can't consider locally flushed files
as persisted data. If a node crashes before they are compacted they are lost
(until the node comes back online... maybe), or if a local file is corrupted
before compaction the data inside is also lost. We can only consider flushed
data persisted after a completed compaction, only after the compaction result
is fully replicated in HDFS. We somehow have to track all of the data in local
flush files and insure it has all been compacted before deleting the WALs that
contain those edits. We somehow need to detect when local flush files after
node recovery are stale. Etc etc. Will the savings be worth the added
complexity and additional failure modes? Maybe, but I believe Facebook
published a paper on this that was inconclusive.
> Change HBase to support local compactions
> -----------------------------------------
>
> Key: HBASE-10216
> URL: https://issues.apache.org/jira/browse/HBASE-10216
> Project: HBase
> Issue Type: New Feature
> Components: Compaction
> Environment: All
> Reporter: David Witten
>
> As I understand it compactions will read data from DFS and write to DFS.
> This means that even when the reading occurs on the local host (because
> region server has a local copy) all the writing must go over the network to
> the other replicas. This proposal suggests that HBase would perform much
> better if all the reading and writing occurred locally and did not go over
> the network.
> I propose that the DFS interface be extended to provide method that would
> merge files so that the merging and deleting can be performed on local data
> nodes with no file contents moving over the network. The method would take a
> list of paths to be merged and deleted and the merged file path and an
> indication of a file-format-aware class that would be run on each data node
> to perform the merge. The merge method provided by this merging class would
> be passed files open for reading for all the files to be merged and one file
> open for writing. The custom class provided merge method would read all the
> input files and append to the output file using some standard API that would
> work across all DFS implementations. The DFS would ensure that the merge had
> happened properly on all replicas before returning to the caller. It could
> be that greater resiliency could be achieved by implementing the deletion as
> a separate phase that is only done after enough of the replicas had completed
> the merge.
> HBase would be changed to use the new merge method for compactions, and would
> provide an implementation of the merging class that works with HFiles.
> This proposal would require a custom code that understands the file format to
> be runnable by the data nodes to manage the merge. So there would need to be
> a facility to load classes into DFS if there isn't such a facility already.
> Or, less generally, HDFS could build in support for HFile merging.
> The merge method might be optional. If the DFS implementation did not
> provide it a generic version that performed the merge on top of the regular
> DFS interfaces would be used.
> It may be that this method needs to be tweaked or ignored when the region
> server does not have a local copy data so that, as happens currently, one
> copy of the data moves to the region server.
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