Re: Gluster distributed replicated setup does not serve read from all bricks belonging to the same replica

[Ravishankar N <ravishankar@xxxxxxxxxx>]

On 11/24/2018 01:03 PM, Anh Vo wrote:

Looking at the source (afr-common.c) even in the case of using hashed mode and the hashed brick doesn't have a good copy it will try the next brick am I correct?

That is correct, no matter which brick the policy chooses,  if that brick is not readable for a given file (i.e. a heal is pending on it from the other good bricks), we just iterate from brick-0, and pick the first one that is good (i.e. readable).
-Ravi

I'm curious because your first reply seemed to place some significance on the part about pending self-heal. Is there anything about pending self-heal that would have made hashed mode worse, or is it about as bad as any brick selection policy?

Thanks

On Thu, Nov 22, 2018 at 7:59 PM Ravishankar N <ravishankar@xxxxxxxxxx> wrote:

On 11/22/2018 07:07 PM, Anh Vo wrote:

Thanks Ravi, I will try that option.

One question:

Let's say there are self heal pending, how would the default of "0" have worked? I understand 0 means "first responder" What if first responder doesn't have good copy? (and it failed in such a way that the dirty attribute wasn't set on its copy - but there are index heal pending from the other two sources)

0 = first readable child of AFR, starting from 1st child. So if 1st brick doesn't have the good copy, it will try the 2nd brick and so on. 
The default value seems to be '1' not '0'. You can look at afr_read_subvol_select_by_policy() in the source code to understand the preference of selection.

Regards,
Ravi

On Wed, Nov 21, 2018 at 9:57 PM Ravishankar N <ravishankar@xxxxxxxxxx> wrote:

Hi,
If there are multiple clients , you can change the 'cluster.read-hash-mode' volume option's value to 2. Then different reads should be served from different bricks for different clients. The meaning of various values for 'cluster.read-hash-mode' can be got from `gluster volume set help`. gluster-4.1 also has added a new value[1] to this option. Of course, the assumption is that all bricks host good copies (i.e. there are no self-heals pending).

Hope this helps,
Ravi

[1]  https://review.gluster.org/#/c/glusterfs/+/19698/

On 11/22/2018 10:20 AM, Anh Vo wrote:

Hi,

Our setup: We have a distributed replicated setup of 3 replica. The total number of servers varies between clusters, in some cases we have a total of 36 (12 x 3) servers, in some of them we have 12 servers (4 x 3). We're using gluster 3.12.15

In all instances what I am noticing is that only one member of the replica is serving read for a particular file, even when all the members of the replica set is online. We have many large input files (for example: 150GB zip file) and when there are 50 clients reading from one single server the performance degrades by several magnitude for reading that file only. Shouldn't all members of the replica participate in serving the read requests?

Our options

cluster.shd-max-threads: 1

cluster.heal-timeout: 900

network.inode-lru-limit: 50000

performance.md-cache-timeout: 600

performance.cache-invalidation: on

performance.stat-prefetch: on

features.cache-invalidation-timeout: 600

features.cache-invalidation: on

cluster.metadata-self-heal: off

cluster.entry-self-heal: off

cluster.data-self-heal: off

features.inode-quota: off

features.quota: off

transport.listen-backlog: 100

transport.address-family: inet

performance.readdir-ahead: on

nfs.disable: on

performance.strict-o-direct: on

network.remote-dio: off

server.allow-insecure: on

performance.write-behind: off

cluster.nufa: disable

diagnostics.latency-measurement: on

diagnostics.count-fop-hits: on

cluster.ensure-durability: off

cluster.self-heal-window-size: 32

cluster.favorite-child-policy: mtime

performance.io-thread-count: 32

cluster.eager-lock: off

server.outstanding-rpc-limit: 128

cluster.rebal-throttle: aggressive

server.event-threads: 3

client.event-threads: 3

performance.cache-size: 6GB

cluster.readdir-optimize: on

storage.build-pgfid: on

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