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@colossus dig_dis]# time ls|wc -l
48163
real 13m1.582s
user 0m0.294s
sys 0m0.205s
With readdir-optimize:
[root@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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