Re: Question on file reads from a distributed replicated volume

[Krutika Dhananjay <kdhananj@xxxxxxxxxx>]

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From: "Bharathram Sivasubramanian" <Bharathram.Sivasubramanian@xxxxxxxxxxx>
To: "Krutika Dhananjay" <kdhananj@xxxxxxxxxx>
Cc: Gluster-users@xxxxxxxxxxx
Sent: Saturday, April 11, 2015 7:26:27 AM
Subject: Re: Question on file reads from a distributed        replicated volume

Thanks, Krutika!

So, if I understand you correctly...if both Brick 1 and Brick 2 have a good copy of the file, then we can't tell for sure if the file will be read only from Brick 1 all the time and not Brick 2? in other words, for read#1, the file could be read from brick 2 and for another read which could happen a few minutes later, the file could be read from brick 1?

Nope. For a given file, read requests will always be served from the same brick. This means that in your example, both read #1 and read #2 will served from the same brick, which is the read child of the file. In 3.6.0 and above, the read child of a file is computed by applying a hash function on its GFID, which means it is always guaranteed to give the same value (because the gfid of a file will remain the same throughout its lifetime, needless to say).

-Krutika

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From: Krutika Dhananjay <kdhananj@xxxxxxxxxx>
Sent: Friday, April 10, 2015 9:30 PM
To: Bharathram Sivasubramanian
Cc: Gluster-users@xxxxxxxxxxx
Subject: Re: Question on file reads from a distributed replicated volume

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From: "Bharathram Sivasubramanian" <Bharathram.Sivasubramanian@xxxxxxxxxxx>
To: Gluster-users@xxxxxxxxxxx
Sent: Saturday, April 11, 2015 3:03:48 AM
Subject: [Gluster-users] Question on file reads from a distributed        replicated volume

Let us assume that we have a distributed replicated gluster volume with 4 bricks. Brick 1 is replicating its data with Brick 2 and Brick 3 is replicating its data with Brick 4. Suppose a file called aa.txt is present and replicated on Bricks 1 & 2. A gluster client would like to read this file. Which brick will the file come from? In other words:

-        Will the file always be fetched from brick 1?

-        Will the file always be fetched from brick 2?

-        Will the file be fetched from brick 1 or 2 depending on the attempt? i.e. is there some load balancing where sometimes the file comes from brick 1 and at other times the file will come from brick 2?

A given file will consistently be "read" from one of the N (in this case 2) replicas, as long as it holds a good copy of the file (that which does not need heal), even with client restart. We call it the "read child" of the file. Load balancing within a volume is done across different files wherein different files would have different but constant read children.

-Krutika

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