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https://issues.apache.org/jira/browse/HBASE-4608?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13131940#comment-13131940
]
Todd Lipcon commented on HBASE-4608:
------------------------------------
One quick sketch of how this might work:
{code}
interface CompressionDictionary {
public byte[] getEntry(int idx);
public int findEntry(byte[] data);
public int addEntry(byte[] data);
}
{code}
while writing:
start each HLog with an empty CompressionDictionary:
{code}
void writeString(byte[] data) {
int dictIdx = dict.findEntry(data);
if (dictIdx == -1) {
// not in dict
writeByte(0x00);
WritableUtils.writeString(data); // current implementation
} else {
writeInt((1 << 31) | dictIdx);
}
}
{code}
while reading:
{code}
byte[] readString(in) {
in.mark();
byte firstbyte = in.read();
if (firstbyte & (1 << 31)) {
in.reset();
int dictidx = in.readInt() & ~(1 << 31);
return dict.getEntry(dictidx);
} else {
assert firstbyte == 0;
byte[] ret = WritableUtils.readString();
dict.addEntry(ret);
}
}
{code}
then the dictionary could be implemented as a fixed size associative hash...
maybe a cuckoo hash or something exotic (they're on my mind since reading the
SILT paper last week)
> HLog Compression
> ----------------
>
> Key: HBASE-4608
> URL: https://issues.apache.org/jira/browse/HBASE-4608
> Project: HBase
> Issue Type: New Feature
> Reporter: Li Pi
> Assignee: Li Pi
>
> The current bottleneck to HBase write speed is replicating the WAL appends
> across different datanodes. We can speed up this process by compressing the
> HLog. Current plan involves using a dictionary to compress table name, region
> id, cf name, and possibly other bits of repeated data. Also, HLog format may
> be changed in other ways to produce a smaller HLog.
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