[Gluster-devel] [Gluster-users] Feedback on DHT option "cluster.readdir-optimize"

[Kyle Johnson]

Hey there,

We have a number of processes which daily walk our entire directory tree 
and perform operations on the found files.

Pre-gluster, this processes was able to complete within 24 hours of 
starting.  After outgrowing that single server and moving to a gluster 
setup (two bricks, two servers, distribute, 10gig uplink), the processes 
became unusable.

After turning this option on, we were back to normal run times, with the 
process completing within 24 hours.

Our data is heavy nested in a large number of subfolders under /media/ftp.

A subset of our data:

15T of files in 48163 directories under /media/ftp/dig_dis.

Without readdir-optimize:

[root at colossus dig_dis]# time ls|wc -l
48163

real    13m1.582s
user    0m0.294s
sys     0m0.205s


With readdir-optimize:

[root at colossus dig_dis]# time ls | wc -l
48163

real    0m23.785s
user    0m0.296s
sys     0m0.108s


Long story short - this option is super important to me as it resolved 
an issue that would have otherwise made me move my data off of gluster.


Thank you for all of your work,

Kyle




On 11/07/2016 10:07 PM, Raghavendra Gowdappa wrote:
> Hi all,
>
> We have an option in called "cluster.readdir-optimize" which alters the behavior of readdirp in DHT. This value affects how storage/posix treats dentries corresponding to directories (not for files).
>
> When this value is on,
> * DHT asks only one subvol/brick to return dentries corresponding to directories.
> * Other subvols/bricks filter dentries corresponding to directories and send only dentries corresponding to files.
>
> When this value is off (this is the default value),
> * All subvols return all dentries stored on them. IOW, bricks don't filter any dentries.
> * Since a directory has one dentry representing it on each subvol, dht (loaded on client) picks up dentry only from hashed subvol.
>
> Note that irrespective of value of this option, _all_ subvols return dentries corresponding to files which are stored on them.
>
> This option was introduced to boost performance of readdir as (when set on), filtering of dentries happens on bricks and hence there is reduced:
> 1. network traffic (with filtering all the redundant dentry information)
> 2. number of readdir calls between client and server for the same number of dentries returned to application (If filtering happens on client, lesser number of dentries in result and hence more number of readdir calls. IOW, result buffer is not filled to maximum capacity).
>
> We want to hear from you Whether you've used this option and if yes,
> 1. Did it really boost readdir performance?
> 2. Do you've any performance data to find out what was the percentage of improvement (or deterioration)?
> 3. Data set you had (Number of files, directories and organisation of directories).
>
> If we find out that this option is really helping you, we can spend our energies on fixing issues that will arise when this option is set to on. One common issue with turning this option on is that when this option is set, some directories might not show up in directory listing [1]. The reason for this is that:
> 1. If a directory can be created on a hashed subvol, mkdir (result to application) will be successful, irrespective of result of mkdir on rest of the subvols.
> 2. So, any subvol we pick to give us dentries corresponding to directory need not contain all the directories and we might miss out those directories in listing.
>
> Your feedback is important for us and will help us to prioritize and improve things.
>
> [1] https://www.gluster.org/pipermail/gluster-users/2016-October/028703.html
>
> regards,
> Raghavendra
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