Fix the ZFS checksum error histograms with larger record sizes #15049
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My analysis in PR openzfs#14716 was incorrect. Each histogram bucket contains the number of incorrect bits, by position in a 64-bit word, over the entire record. 8-bit buckets can overflow for record sizes above 2k. To forestall that, saturate each bucket at 255. That should still get the point across: either all bits are equally wrong, or just a couple are. Sponsored-by: Axcient Signed-off-by: Alan Somers <[email protected]>
behlendorf
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Jul 13, 2023
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Jul 21, 2023
My analysis in PR openzfs#14716 was incorrect. Each histogram bucket contains the number of incorrect bits, by position in a 64-bit word, over the entire record. 8-bit buckets can overflow for record sizes above 2k. To forestall that, saturate each bucket at 255. That should still get the point across: either all bits are equally wrong, or just a couple are. Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Alan Somers <[email protected]> Sponsored-by: Axcient Closes openzfs#15049
behlendorf
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Jul 21, 2023
My analysis in PR #14716 was incorrect. Each histogram bucket contains the number of incorrect bits, by position in a 64-bit word, over the entire record. 8-bit buckets can overflow for record sizes above 2k. To forestall that, saturate each bucket at 255. That should still get the point across: either all bits are equally wrong, or just a couple are. Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Alan Somers <[email protected]> Sponsored-by: Axcient Closes #15049
lundman
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Dec 12, 2023
My analysis in PR openzfs#14716 was incorrect. Each histogram bucket contains the number of incorrect bits, by position in a 64-bit word, over the entire record. 8-bit buckets can overflow for record sizes above 2k. To forestall that, saturate each bucket at 255. That should still get the point across: either all bits are equally wrong, or just a couple are. Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Alan Somers <[email protected]> Sponsored-by: Axcient Closes openzfs#15049
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My analysis in PR #14716 was incorrect. Each histogram bucket contains the number of incorrect bits, by position in a 64-bit word, over the entire record. 8-bit buckets can overflow for record sizes above 2k. To forestall that, saturate each bucket at 255. That should still get the point across: either all bits are equally wrong, or just a couple are.
Sponsored-by: Axcient
Signed-off-by: Alan Somers [email protected]
Motivation and Context
After PR #14716, the bad_cleared_histogram and bad_set_histogram fields of an ereport.fs.zfs.checksum event could contain incorrect values for record sizes of 2kB and above, due to an integer overflow.
Description
Rather than overflow, saturate the histogram buckets at 255. That will still be sufficient for diagnostic purposes.
How Has This Been Tested?
Tested on FreeBSD 14 using the zfsd_degrade_001_pos test case from FreeBSD's zfsd test suite.
Types of changes
Checklist:
Signed-off-by.