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Slightly improve dnode hash

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As I understand just for being less predictable dnode hash includes
8 bits of objset pointer, starting at 6.  But since objset_t is
more than 1KB in size, its allocations are likely aligned to 2KB,
that means 11 lower bits provide no entropy. Just take the 8 bits
starting from 11.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes #16131
This commit is contained in:
Alexander Motin authored and GitHub committed 2024-05-01 10:59:32 -07:00
1 parent 051460b8b2
commit 8fd3a5d02f
1 file changed
+3 -3
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module/zfs/dmu_objset.c
+3 -3
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@@ -400,10 +400,10 @@ dnode_hash(const objset_t *os, uint64_t obj)
ASSERT(zfs_crc64_table[128] == ZFS_CRC64_POLY);
/*
* The low 6 bits of the pointer don't have much entropy, because
* the objset_t is larger than 2^6 bytes long.
* The lower 11 bits of the pointer don't have much entropy, because
* the objset_t is more than 1KB long and so likely aligned to 2KB.
*/
crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (osv >> 6)) & 0xFF];
crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (osv >> 11)) & 0xFF];
crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 0)) & 0xFF];
crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 8)) & 0xFF];
crc = (crc >> 8) ^ zfs_crc64_table[(crc ^ (obj >> 16)) & 0xFF];