Hello Jacob,
Thank you for sharing your case at Kingston. :)
The customer plans to replace the machine on the Informix server from IBM E850 to E950. Disk storage does not change. In preparing for work, I discovered that some chunks of Informix were using block raw devices. My client is using Informix server on AIX 7.1 and each raw device chunk has an offset.
If possible, I would like to migrate the data after removing the chunks using the block raw device, but due to the size of the data, it will be difficult to have that much downtime.
Of course, changing the symbolic link will be done after the Informix process has stopped. If I am not lucky, I may have to do a full restore.
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SangGyu Jeong
Software Engineer
Infrasoft
Seoul Korea, Republic of
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Original Message:
Sent: Wed October 07, 2020 12:53 PM
From: Jacob Salomon
Subject: Replacing block device to character device
Oh, the memories!
While I was still at Informix (a long, long time ago), I was sent to Kingston overnight to help a client. Their problem: periodically - several times each hour - there would be a dramatic slowdown on all processing. Some monitoring tool (AIX was pretty good with those) showed that the buses were being deluged with data and preventing memory access at the hardware level.
It turned out that all their chunks' symlinks were referencing block raw devices instead of the character devices. What the difference? I/O to the block device would go through the Unix cache and, of course, character would not. The crazy use of the buses was a checkpoint flushing the buffer pool to the disk but that I/O going through the much smaller Unix cache and the OS frantically trying to flush the OS cache to the disk. It was probably leaving some buffers unflushed, which is, of course, quite bad.
I think their solution was a bit more complicated than just swapping the symlink to the character device; I think there was an offset (in that release of AIX) at the start of each raw character device where the block access started. It's a pretty good bet AIX has outgrown that requirement by now.
Also, even using flat files as the chunks, our engines now routinely skip the cache for access to the chunk data. But I don't think that option was available under release 7 or that release of AIX.
-- A grizzled veteran reminiscing... Please humor. :-)
-- Jacob S.
Original Message:
Sent: 10/6/2020 4:04:00 AM
From: SangGyu Jeong
Subject: Replacing block device to character device
Hello All,
I found that a block raw device is being used in chunks on a customer's Informix server.
Is it okay to use the device by replacing it with the character raw device path?
The chunk is using symbolic links, so we plan to create a new symbolic link pointing to the character raw device path.
Thanks,
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SangGyu Jeong
Software Engineer
Infrasoft
Seoul Korea, Republic of
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#Informix