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HDFS-7976. Update NFS user guide for mount option 'sync' to minimize or avoid reordered writes. Contributed by Brandon Li

This commit is contained in:
Brandon Li 2015-03-24 10:28:38 -07:00
parent 6413d34986
commit 51f1f49374
2 changed files with 10 additions and 3 deletions
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3
hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
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@ -784,6 +784,9 @@ Release 2.7.0 - UNRELEASED
HDFS-7956. Improve logging for DatanodeRegistration. HDFS-7956. Improve logging for DatanodeRegistration.
(Plamen Jeliazkov via shv) (Plamen Jeliazkov via shv)
HDFS-7976. Update NFS user guide for mount option "sync" to minimize or
avoid reordered writes. (brandonli)
OPTIMIZATIONS OPTIMIZATIONS
HDFS-7454. Reduce memory footprint for AclEntries in NameNode. HDFS-7454. Reduce memory footprint for AclEntries in NameNode.

10
hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsNfsGateway.md
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@ -126,7 +126,8 @@ It's strongly recommended for the users to update a few configuration properties
</property> </property>
* Users are expected to update the file dump directory. NFS client often * Users are expected to update the file dump directory. NFS client often
reorders writes. Sequential writes can arrive at the NFS gateway at random reorders writes, especially when the export is not mounted with "sync" option.
Sequential writes can arrive at the NFS gateway at random
order. This directory is used to temporarily save out-of-order writes order. This directory is used to temporarily save out-of-order writes
before writing to HDFS. For each file, the out-of-order writes are dumped after before writing to HDFS. For each file, the out-of-order writes are dumped after
they are accumulated to exceed certain threshold (e.g., 1MB) in memory. they are accumulated to exceed certain threshold (e.g., 1MB) in memory.
@ -254,13 +255,16 @@ Verify validity of NFS related services
Mount the export "/" Mount the export "/"
-------------------- --------------------
Currently NFS v3 only uses TCP as the transportation protocol. NLM is not supported so mount option "nolock" is needed. It's recommended to use hard mount. This is because, even after the client sends all data to NFS gateway, it may take NFS gateway some extra time to transfer data to HDFS when writes were reorderd by NFS client Kernel. Currently NFS v3 only uses TCP as the transportation protocol. NLM is not supported so mount option "nolock" is needed.
Mount option "sync" is strongly recommended since it can minimize or avoid reordered writes, which results in more predictable throughput.
Not specifying the sync option may cause unreliable behavior when uploading large files.
It's recommended to use hard mount. This is because, even after the client sends all data to NFS gateway, it may take NFS gateway some extra time to transfer data to HDFS when writes were reorderd by NFS client Kernel.
If soft mount has to be used, the user should give it a relatively long timeout (at least no less than the default timeout on the host) . If soft mount has to be used, the user should give it a relatively long timeout (at least no less than the default timeout on the host) .
The users can mount the HDFS namespace as shown below: The users can mount the HDFS namespace as shown below:
[root]>mount -t nfs -o vers=3,proto=tcp,nolock,noacl $server:/ $mount_point [root]>mount -t nfs -o vers=3,proto=tcp,nolock,noacl,sync $server:/ $mount_point
Then the users can access HDFS as part of the local file system except that, hard link and random write are not supported yet. To optimize the performance of large file I/O, one can increase the NFS transfer size(rsize and wsize) during mount. By default, NFS gateway supports 1MB as the maximum transfer size. For larger data transfer size, one needs to update "nfs.rtmax" and "nfs.rtmax" in hdfs-site.xml. Then the users can access HDFS as part of the local file system except that, hard link and random write are not supported yet. To optimize the performance of large file I/O, one can increase the NFS transfer size(rsize and wsize) during mount. By default, NFS gateway supports 1MB as the maximum transfer size. For larger data transfer size, one needs to update "nfs.rtmax" and "nfs.rtmax" in hdfs-site.xml.