Russell,
The maximum number of replicas, as specified via
mmcrfs -R MaxDataReplicas
causes space to be reserved in the inode and indirect block for the replicated disk address for each data block.
For files which are small enough to fit in the inode, it should not make a difference.
For files with indirection level "direct" then -R 2 or -R3 will result in fewer disk addresses to fit in the inode, and for the larger files force the use of indirect blocks. Then each indirect block will have space for half (-R 2) or a third (-R 3) of the number of disk addresses, as compared with -R 1. Which means that additional indirect blocks will be needed to accommodate a file of a given size.
For even larger files requiring higher indirection levels, the statement above applies to the leaf indirect blocks.
>>> This information was written prior to GPFSv5 when subblocks changed from only 32. Does it still hold true that unused data replica’s use metadata space with v5?
The same remains true for v5 file systems with more than 32 subblocks per block.
Felipe
----
Felipe Knop [email protected]
GPFS Development and Security
IBM Systems
IBM Building 008
2455 South Rd, Poughkeepsie, NY 12601
(845) 433-9314 T/L 293-9314
Felipe Knop [email protected]
GPFS Development and Security
IBM Systems
IBM Building 008
2455 South Rd, Poughkeepsie, NY 12601
(845) 433-9314 T/L 293-9314
----- Original message -----
From: Russell Nordquist <[email protected]>
Sent by: [email protected]
To: [email protected]
Cc:
Subject: [EXTERNAL] [gpfsug-discuss] data replicas and metadata space used
Date: Wed, Sep 2, 2020 10:55 AM
I was reading this slide deck on GPFS metadata sizing and I ran across somethingOn slide 51 it says"Max replicas for Data, multiplies the MD capacity used
• Reserves space in MD for the replicas even if no files replicated!”This is something I did not realize - setting data replicas to 2 or even 3 consumes metadata space even if you are not using the data replicas. For metadata replicas it says unused replica’s have little impact - great. I like to set data and metadata replica’s to 3 when I make a filesystem even when the initial replicas used are 1 since you never know what will change down the road. However this makes me wonder about that idea for the data replica’s - it’s really expensive metadata spacewise.This information was written prior to GPFSv5 when subblocks changed from only 32. Does it still hold true that unused data replica’s use metadata space with v5?thanksRussell_______________________________________________
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