Duo Xu created HADOOP-12403:
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Summary: Enable multiple writes in flight for HBase WAL writing
Key: HADOOP-12403
URL: https://issues.apache.org/jira/browse/HADOOP-12403
Project: Hadoop Common
Issue Type: Improvement
Components: tools
Reporter: Duo Xu
Assignee: Duo Xu
Azure HDI HBase clusters use Azure blob storage as file system. We found that
the bottle neck was during writing to write ahead log (WAL). The latest HBase
WAL write model (HBASE-8755) uses multiple AsyncSyncer threads to sync data to
HDFS. However, our WASB driver is still based on a single thread model. Thus
when the sync threads call into WASB layer, every time only one thread will be
allowed to send data to Azure storage.This jira is to introduce a new write
model in WASB layer to allow multiple writes in parallel.
1. Since We use page blob for WAL, this will cause "holes" in the page blob as
every write starts on a new page. We use the first two bytes of every page to
record the actual data size of the current page.
2. When reading WAL, we need to know the actual size of the WAL. This should be
the sum of the number represented by the first two bytes of every page. However
looping over every page to get the size will be very slow. So during writing,
the writer threads will keep updating a metadata of the blob called
"total_data_uploaded".
3. Although we allow multiple writes in flight, we need to make sure the sync
threads which call into WASB layers return in order. Reading HBase source code
FSHLog.java, we find that every sync requests associated with a transaction id.
If the sync succeeds, all the transactions before this transaction id are
assumed to be in Azure Storage. We use a queue to store the sync requests and
make sure they return to HBase layer in order.
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