doc: Adding documentation for DHT lookup-optimize option

This is intended to be admin facing documentation, with some granular
details.

Change-Id: I379f8cea5375649fba915e907648e3a71344092d
BUG: 1228818
Signed-off-by: Shyam <srangana@redhat.com>
Reviewed-on: http://review.gluster.org/11109
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Tested-by: Raghavendra G <rgowdapp@redhat.com>

1 files changed, 145 insertions, 0 deletions

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+# DHT lookup optimization
+
+Distribute xlator (or DHT) has a performance penalty when dealing with negative
+lookups. This document explains the problem and optimization provided for
+alleviating the same in GlusterFS.
+
+## Negative lookups and issues surrounding them
+
+Negative lookups are lookup operations for entries that are not present in the
+volume. IOW, a lookup for a file/directory that does not exist is a negative
+lookup.
+
+DHT normally looks up an entry in the hashed subvolume first (based on the
+layout), if not found in the hashed location, it fans out a lookup across all
+subvolumes to DHT, to ensure that the entry is not present in another subvolume.
+This behavior comes from the perspective that if a rebalance is in progress,
+and the layout on disk is temporarily out of alignment with the actual location
+of the file, the entry is still found by the fan out lookup.
+
+Such fan out lookups are costly and typically slow down file creates. This
+especially impacts small file performance, where a large number of files are
+being added/created in quick succession to the volume.
+
+## Optimizing lookups in DHT
+
+A balanced volume is either, a new volume that is created, and no bricks are
+added to, or removed from the same, or a volume that has undergone expansion
+(or reduction) of bricks and a full rebalance has been run on the volume.
+
+In such volumes, the fan out lookup behavior can be turned off to speed up
+negative lookups, as files are in their respective hashed locations (or at
+least their DHT link-to entries are present in the hashed location).
+
+With GlusterFS 3.7.2 negative lookup fan-out behavior is optimized, by not
+performing the same, in an balanced volume.
+
+The optimization provided, further detects a cluster out of balance (when a
+fix-layout is done, or a brick is removed) to automatically turn **on** the
+fan out negative lookup behavior, thereby preventing duplicate entry creation
+in the volume, till the volume is brought into balance again.
+
+## Configuration options to enable optimized lookups
+
+With Gluster 3.7.2 the following options are provided to enable DHT lookup
+optimization,
+
+Option: cluster.lookup-optimize
+Description: "This option if set to ON enables the optimization of -ve lookups,
+by not doing a lookup on non-hashed subvolumes for files, in case the hashed
+subvolume does not return any result. This option disregards the
+lookup-unhashed setting, when enabled."
+Default: OFF
+
+CLI command to enable this option:
+  gluster volume set <volname> cluster.lookup-optimize <on/off>
+
+### Client compatibility support
+
+As DHT xlator runs on the client stack of gluster (i.e on the machine where the
+FUSE/NFS Server/SAMBA Server are running), this configuration requires that the
+cluster and the clients are upgraded to 3.7.2 version, at the minimum.
+
+When setting this option, if any Gluster brick node or connected clients are of
+an older version, the option will error out stating incompatible version
+detected in the cluster and not allow the configuration change.
+
+Older clients connecting to the cluster post this configuration option is set,
+would also error out and not be able to mount the volume due to the version
+incompatibility.
+
+### Compatability with lookup-unhashed setting
+
+In older DHT versions, the configuration option lookup-unhashed emulated a
+similar behavior for a balanced cluster. The downside of this option is that
+if the cluster grows or becomes unbalanced, there is a risk of losing entry
+consistency. The current changes to gluster and specifically in DHT, prevent
+this inconsistency from occurring when using the new option (lookup-optimize).
+
+Additionally, if the lookup-optimize option is set, the older lookup-unhashed
+setting is ignored by DHT.
+
+##  Requirements for the optimization to function
+
+When the lookup-optimize option is enabled, there are a few prerequisites
+before which the option is honored by DHT. The following list provides some of
+these conditions and ways to meet the same.
+
+1. New volume
+  A new volume is a volume that has just been created and is unused or not
+  started
+  - For a volume that is just created
+    - Prerequisite: Before starting and accessing this volume, set the lookup
+    optimization to ON
+    - Gotchas: All directories that are created post the above setting, will
+    leverage the negative lookup optimization, except entries in the root of
+    the volume.
+      NOTE: The root of the volume, or the brick root on each brick of the
+      volume, is already created prior to the start of the volume, or the
+      ability to set this option. As a result the root of the volume gains this
+      optimization only post the first full rebalance, or is treated equivalent
+      to an existing directory (see (2)-(1) below).
+
+2. Existing volume
+  An existing volume is one which is under use, and may have had bricks added
+  or removed in its lifetime. In this scenario there are 2 cases where the
+  lookup optimization behavior changes,
+  Prerequisite: Enable the lookup-optimize option
+  1. New directory creation
+    - All directories created beyond this point will gain the negative lookup
+    optimization
+  2. Existing directories
+    - Existing directories will not gain the lookup optimization
+    - To enable existing directories to also gain the lookup optimization a
+    full rebalance on the volume needs to be performed
+
+The optimization is also bypassed by the code automatically in the following
+conditions,
+
+1. Brick removed
+  - When a remove-brick is executed for a volume, it immediately triggers a
+  rebalance to move data out of the removed bricks. In these circumstances the
+  optimization is bypassed and a fan out lookup is performed for negative
+  lookups.
+  - Post removal of the brick, the lookup optimization would automatically kick
+  in
+2. Brick added and only fix-layout is executed
+  - When a brick is added and a fix-layout only is executed, the files are
+  still not present in the correct hashed locations. As a result under these
+  conditions the lookup optimization is bypassed by DHT.
+  - A full rebalance post fix-layout would get the optimization enabled
+    NOTE: Although fix-layout is deprecated, it is still present and honored,
+    as a result this distinction is presented in this document. This is not an
+    endorsement of fix-layout still being supported.
+
+## FAQ
+<< TBD >>
+1. How do I verify that I have a problem with negative lookups? OR
+  When should I use this option?
+2. Can I roll back to an older client post enabling this optimization?
+3. How do I verify this option is working for me?
+4. What additional meta-data does this option add to the bricks?
+5. I see duplicate entries after enabling this option, what should I do?
+6. I see the following error in my client logs, help!
+7. My create performance is still poor, help!
+8. <Other suggestions welcome>